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  • Long‐term survival probability, not current habitat quality, predicts dispersal distance in a stream salamander
    Ecology (IF 4.285) Pub Date : 2020-01-20
    Brett R. Addis; Winsor H. Lowe

    Dispersal evolves as an adaptive mechanism to optimize individual fitness across the landscape. Specifically, dispersal represents a mechanism to escape fitness costs resulting from changes in environmental conditions. Decades of empirical work suggest that individuals use local habitat cues to make movement decisions, but theory predicts that dispersal can also evolve as a fixed trait – independent of local conditions – in environments characterized by a history of stochastic spatiotemporal variation. Until now, however, both conditional and fixed models of dispersal evolution have primarily been evaluated using emigration data (stay vs. leave), and not dispersal distances – a more comprehensive measure of dispersal. Our goal was to test whether conditional or fixed models of dispersal evolution predict variation in dispersal distance in the stream salamander Gyrinophilus porphyriticus. We quantified variation in habitat conditions using measures of salamander performance from 4 years of spatially explicit, capture‐mark‐recapture (CMR) data across 3 headwater streams in the Hubbard Brook Experimental Forest in central New Hampshire, USA. We used body condition as an index of local habitat quality that individuals may use to make dispersal decisions, and survival probability estimated from multistate CMR models as an index of mortality risk resulting from the long‐term history of environmental variation. We found that dispersal distances increased with declining survival probability, indicating that salamanders disperse further in risky environments. Dispersal distances were unrelated to spatial variation in body condition, suggesting that salamanders do not base dispersal distance decisions on local habitat quality. Our study provides the first empirical support for fixed models of dispersal evolution which predict that dispersal evolves in response to a history of spatiotemporal environmental variation, rather than as a conditional response to current habitat conditions. More broadly, this study underscores the value of assessing alternative scales of environmental variation to gain a more complete and balanced understanding of dispersal evolution.

    更新日期:2020-01-21
  • Disturbance legacies increase and synchronize nutrient concentrations and bacterial productivity in coastal ecosystems
    Ecology (IF 4.285) Pub Date : 2020-01-20
    John S. Kominoski; Evelyn E. Gaiser; Edward Castañeda‐Moya; Stephen E. Davis; Shimelis Dessu; Paul Julian; Dong Yoon Lee; Luca Marazzi; Victor H. Rivera‐Monroy; Andres Sola; Ulrich Stingl; Sandro Stumpf; Donatto Surratt; Rafael Travieso; Tiffany G. Troxler

    Long‐term ecological research can resolve effects of disturbance on ecosystem dynamics by capturing the scale of disturbance and interactions with environmental changes. To quantify how disturbances interact with long‐term directional changes (sea‐level rise, freshwater restoration), we studied 17 years of monthly dissolved organic carbon (DOC), total nitrogen (TN) and phosphorus (TP) concentrations and bacterioplankton productivity across freshwater‐to‐marine estuary gradients exposed to multiple disturbance events (e.g., droughts, fire, hurricanes, and low‐temperature anomalies) and long‐term increases in water levels. By studying two neighboring drainages that differ in hydrologic connectivity, we additionally tested how disturbance legacies are shaped by hydrologic connectivity. We predicted that disturbance events would interact with long‐term increases in water levels in freshwater and marine ecosystems to increase spatiotemporal similarity (i.e., synchrony) of organic matter, nutrients, and microbial activities. Wetlands along the larger, deeper, and tidally influenced Shark River Slough (SRS) drainage had higher and more variable DOC, TN, and TP concentrations than wetlands along the smaller, shallower, tidally restricted Taylor River Slough/Panhandle (TS/Ph) drainage. Along SRS, DOC concentrations declined with proximity to coast, and increased in magnitude and variability following drought and flooding in 2015 and a hurricane in 2017. Along TS/Ph, DOC concentrations varied by site (higher in marine than freshwater wetlands) but not year. In both drainages, increases in TN from upstream freshwater marshes occurred following fire in 2008 and droughts in 2010 and 2015, whereas downstream increases in TP occurred with coastal storm surge from hurricanes in 2005 and 2017. Decreases in DOC:TN and DOC:TP were explained by increased TN and TP. Increases in bacterioplankton productivity occurred throughout both drainages following low‐temperature events (2010 and 2011) and a hurricane (2017). Long‐term TN and TP concentrations and bacterioplankton productivity were correlated (r > 0.5) across a range of sampling distances (1‐50 km), indicating spatiotemporal synchrony. DOC concentrations were not synchronized across space or time. Our study advances disturbance ecology theory by illustrating how disturbance events interact with long‐term environmental changes and hydrologic connectivity to determine the magnitude and extent of disturbance legacies. Understanding disturbance legacies will enhance prediction and enable more effective management of rapidly changing ecosystems.

    更新日期:2020-01-21
  • Ecological response to altered rainfall differs across the Neotropics
    Ecology (IF 4.285) Pub Date : 2020-01-20
    Diane S. Srivastava; Régis Céréghino; M. Kurtis Trzcinski; A. Andrew M. MacDonald; Nicholas A.C. Marino; Dimaris Acosta Mercado; Céline Leroy; Bruno Corbara; Gustavo Q. Romero; Vinicius F. Farjalla; Ignacio M. Barberis; Olivier Dézerald; Edd Hammill; Trisha B. Atwood; Gustavo C.O. Piccoli; Fabiola Ospina Bautista; Jean‐François Carrias; Juliana S. Leal; Guillermo Montero; Pablo A.P. Antiqueira; Rodrigo Freire; Emilio Realpe; Sarah L. Amundrud; Paula M. de Omena; Alice B.A. Campos

    There is growing recognition that ecosystems may be more impacted by infrequent extreme climatic events than by changes in mean climatic conditions. This has led to calls for experiments that explore the sensitivity of ecosystems over broad ranges of climatic parameter space. However, because such response surface experiments have so far been limited in geographic and biological scope, it is not clear if differences between studies reflect geographic location or the ecosystem component considered. In this study, we manipulated rainfall entering tank bromeliads in seven sites across the Neotropics, and characterized the response of the aquatic ecosystem in terms of invertebrate functional composition, biological stocks (total invertebrate biomass, bacterial density) and ecosystem fluxes (decomposition, carbon, nitrogen). Of these response types, invertebrate functional composition was the most sensitive, even though in some sites the species pool had a high proportion of drought‐tolerant families. Total invertebrate biomass was universally insensitive to rainfall change because of statistical averaging of divergent responses between functional groups. The response of invertebrate functional composition to rain differed between geographical locations because (1) the effect of rainfall on bromeliad hydrology differed between sites, and invertebrates directly experience hydrology not rainfall and (2) the taxonomic composition of some functional groups differed between sites, and families differed in their response to bromeliad hydrology. These findings suggest that it will be difficult to establish thresholds of “safe ecosystem functioning” when ecosystem components differ in their sensitivity to climatic variables, and such thresholds may not be broadly applicable over geographic space. In particular, ecological forecast horizons for climate change may be spatially restricted in systems where habitat properties mediate climatic impacts, and those – like the tropics ‐ with high spatial turnover in species composition.

    更新日期:2020-01-21
  • Spatial and temporal turnover of soil microbial communities is not linked to function in a primary tropical forest
    Ecology (IF 4.285) Pub Date : 2020-01-20
    Stephanie N. Kivlin; Christine V. Hawkes

    The spatial and temporal linkages between turnover of soil microbial communities and their associated functions remain largely unexplored in terrestrial ecosystems. Yet defining these relationships and how they vary across ecosystems and microbial lineages is key to incorporating microbial communities into ecological forecasts and ecosystem models. To define linkages between turnover of soil bacterial and fungal communities and their function we sampled fungal and bacterial composition, abundance, and enzyme activities across a 3‐ha area of wet tropical primary forest over two years. We show that fungal and bacterial communities both exhibited temporal turnover, but turnover of both groups was much lower than in temperate ecosystems. Turnover over time was driven by gain and loss of microbial taxa and not changes in abundance of individual species present in multiple samples. Only fungi varied over space with idiosyncratic variation that did not increase linearly with distance among sampling locations. Only phosphorus‐acquiring enzyme activities were linked to shifts in septate, decomposer fungal abundance; no enzymes were affected by composition or diversity of fungi or bacteria. While temporal and spatial variation in composition was appreciable, because turnover of microbial communities did not alter the functional repertoire of decomposing enzymes, functional redundancy among taxa may be high in this ecosystem. Slow temporal turnover of tropical soil microbial communities and large functional redundancy suggests that shifts in abundance of particular functional groups may more accurately capture ecosystem function than composition in these heterogeneous ecosystems.

    更新日期:2020-01-21
  • Climate change influences mycorrhizal fungal–plant interactions, but conclusions are limited by geographical study bias
    Ecology (IF 4.285) Pub Date : 2020-01-18
    Alison E. Bennett; Aimée T. Classen

    Climate change is altering the interactions among plants and soil organisms in ways that will alter the structure and function of ecosystems. We reviewed the literature and developed a map of studies focused on how the three most common types of mycorrhizal fungi (arbuscular mycorrhizal (AM), ectomycorrhizal (EcM), and ericoid mycorrhizal (ErM) fungi) respond to elevated atmospheric carbon dioxide concentrations (eCO2), climatic warming, and changes in the distribution of precipitation. Broadly, we ask how do mycorrhizal fungi respond to climate change, how do these responses vary by fungal type, and how do mycorrhizal traits influence plant adaptation, movement, or extinction in response to climatic change? First, we found that 92% of studies were conducted in the northern hemisphere, and plant host, ecosystem type and study location were only correlated with each other in the northern hemisphere because studies across all mycorrhizal fungal types were only common in the northern hemisphere. Second, we show that temperature and rainfall variability had more variable effects than eCO2 on mycorrhizal fungal structures, but these effects were context dependent. Third, while mycorrhizal fungal types vary in their responses to climate change, it appears that warming lead to more variable responses in ectomycorrhizal than in arbuscular mycorrhizal fungi. Finally, we discuss common traits of mycorrhizal fungi that could aid in fungal and plant adaption to climate change. We posit that mycorrhizal fungi can buffer plant hosts against extinction risk, they can facilitate or retard the dispersal success of plants moving away from poor environments, and, by buffering host plants, they can enable host plant adaptation to new climates. All of these influences are, however, context dependent a finding that reflects the complex traits of mycorrhizal fungi as a group, the diversity of plant species they associate with and the variation in ecosystems in which they reside. Overall, while we point out many gaps in our understanding of the influence of climate changes on mycorrhizal fungi, we also highlight the large number of opportunities for researching plant and mycorrhizal fungal responses to and mitigation of climate changes.

    更新日期:2020-01-21
  • Variations in seasonal (not mean) temperatures drive rapid adaptations to novel environments at a continent‐scale
    Ecology (IF 4.285) Pub Date : 2020-01-16
    Jean‐Claude Tourneur; Joël Meunier

    The recent development of human societies has led to major, rapid and often inexorable changes in the environment of most animal species. Over the last decades, a growing number of studies formulated predictions on the modalities of animal adaptation to novel or changing environments, questioning how and at what speed animals should adapt to such changes, discussing the levels of risks imposed by changes in the mean and/or variance of temperatures on animal performance, and exploring the underlying roles of phenotypic plasticity and genetic inheritance. These fundamental predictions, however, remain poorly tested using field data. Here, we tested these predictions using a unique continental‐scale data set in the European earwig Forficula auricularia L, a univoltine insect introduced in North America one century ago. We conducted a common garden experiment, in which we measured 13 life‐history traits in 4158 field‐sampled earwigs originating from 19 populations across North America. Our results first demonstrate that 10 of the 13 measured life‐history traits are associated with two sets of variations in seasonal temperatures, i.e. winter‐summer and autumn‐spring. We found, however, no association with the overall mean monthly temperatures of the invaded locations. Furthermore, our use of a common garden setup reveals that the observed patterns of variation in earwigs’ life‐history traits are not mere plastic responses to their current environment, but are either due to their genetic background and/or to the environmental conditions they experienced during early life development. Overall, these findings provide continent‐scale support to the claims that adaptation to thermal changes can occur quickly (in less than 100 generations), even in insects with long life cycles, and emphasize the importance of variation in seasonal temperature over mean population temperatures in climate adaptation.

    更新日期:2020-01-17
  • Analyzing snapshot diversity patterns with the Neutral Theory can show functional groups' effects on community assembly
    Ecology (IF 4.285) Pub Date : 2020-01-16
    Fabien Laroche; Cyrille Violle; Adrien Taudière; François Munoz

    A central question of community ecology is to understand how the interplay between processes of the Neutral Theory (e.g., immigration and ecological drift) and niche‐based processes (e.g., environmental filtering, intra‐ and interspecific density‐dependence) shape species diversity in competitive communities. The articulation between these two categories of mechanisms can be studied through the lens of the intermediate organizational level of “functional groups” (FGs), defined as clusters of species with similar traits. Indeed, FGs stress ecological differences among species and are thus likely to unravel non‐neutral interactions within communities. Here we presented a novel approach to explore how FGs affect species coexistence by comparing species and functional diversity patterns. Our framework considers the Neutral Theory as a mechanistic null hypothesis. It assesses how much the functional diversity deviates from species diversity in communities, and compares this deviation, called the “average functional deviation”, to a neutral baseline. We showed that the average functional deviation can indicate reduced negative density‐dependence or environmental filtering among FGs. We validated our framework using simulations illustrating the two situations. We further analyzed tropical tree communities in Western Ghats, India. Our analysis of the average functional deviation revealed environmental filtering between deciduous and evergreen FGs along a broad rainfall gradient. By contrast, we did not find clear evidence for reduced density‐dependence among FGs. We predict that applying our approach to new case studies where environmental gradients are milder and FGs are more clearly associated to resource partitioning should reveal the missing pattern of reduced density‐dependence among FGs.

    更新日期:2020-01-16
  • Functional responses are maximized at intermediate temperatures
    Ecology (IF 4.285) Pub Date : 2020-01-16
    Stella F. Uiterwaal; John P. DeLong

    Functional responses describe how consumer foraging rates change with resource density. Despite extensive research looking at the factors underlying foraging interactions, there remains ongoing controversy about how temperature and body size control the functional response parameters space clearance (or attack) rate and handling time. Here, we investigate the effects of temperature, consumer mass, and resource mass using the largest compilation of functional responses yet assembled. This compilation contains 2,083 functional response curves covering a wide range of foragers and prey types, environmental conditions, and habitats. After accounting for experimental arena size, dimensionality of the foraging interaction, and consumer taxon, we find that both space clearance rate and handling time are optimized at intermediate temperatures (a unimodal rather than monotonic response), suggesting that the response to global climate change depends on the location of the consumer's current temperature relative to the optimum. We further confirm that functional responses are higher and steeper for large consumers and small resources, and models using consumer and resource masses separately outperformed models using consumer:resource mass ratios, suggesting that consumer and resource body mass act independently to set interaction strengths. Lastly, we show that the extent to which foraging is affected by temperature or mass depends on the taxonomic identity of the consumer and the dimensionality of the consumer‐resource interaction. We thus argue that although overall body size and temperature effects can be identified, they are not universal, and therefore food web and community modeling approaches could be improved by considering taxonomic identity along with body size and unimodal temperature effects.

    更新日期:2020-01-16
  • Seeing beyond the trees: a comparison of tropical and temperate plant growth‐forms and their vertical distribution
    Ecology (IF 4.285) Pub Date : 2020-01-16
    Michelle Elise Spicer; Hannah Mellor; Walter P. Carson

    Forests are the most diverse and productive terrestrial ecosystems on Earth, so sustainably managing them for the future is a major global challenge. Yet, our understanding of forest diversity relies almost exclusively on the study of trees. Here, we demonstrate unequivocally that other growth‐forms (shrubs, lianas, herbs, epiphytes) make up the majority of vascular plant species in both tropical and temperate forests. By comparing the relative distribution of species richness among plant growth‐forms for over 3,400 species in 18 forests in the Americas, we construct the first high‐resolution quantification of plant growth‐form diversity across two ecologically important regions at a near‐continental scale. We also quantify the physical distribution of plant species among forest layers—that is, where among the vertical strata plants ultimately live their adult lives—and show that plants are strongly downshifted in temperate forests versus tropical forests. Our data illustrate a previously unquantified fundamental difference between tropical and temperate forests: what plant growth‐forms are most speciose, and where they ultimately live in the forest. Recognizing these differences requires that we re‐focus ecological research and forest management plans to encompass a broader suite of plant growth‐forms. This more holistic perspective is essential to conserve global biodiversity.

    更新日期:2020-01-16
  • The influence of host‐plant connectivity on fungal assemblages in the root microbiota of Brachypodium pinnatum
    Ecology (IF 4.285) Pub Date : 2020-01-16
    C Mony; N Vannier; P Brunellière; M. Biget; S. Coudouel; P. Vandenkoornhuyse

    Dispersal limitation may drive the structure of fungal microbiota of plant roots at small spatial scales. Fungal root microorganisms disperse through the plant rooting systems from hosts to hosts. Due to a pronounced host‐preference effect, the composition of endophytic root microbiota may follow plant distribution. A given plant community may hence include a matrix of host‐plant species that represent various habitat permeabilities to fungal dispersal in the floristic landscape. We experimentally tested the effect of host‐plant isolation on endophytic fungal assemblages (Ascomycota, Basidiomycota, Glomeromycotina) inhabiting Brachypodium pinnatum roots. We calculated host‐plant isolation using Euclidean distance (distance‐based dispersal limitation) and Resistance distance (functional‐based dispersal limitation), based on host presences. All fungal groups were more influenced by the resistance distance between B. pinnatum than by the Euclidean distance. Fungal dispersal was hence strongly related to the spatial distribution of the host‐plants. The fungal groups displayed however different responses (in richness, abundance, and composition) to host isolation. Additionally, fungal assemblages were more strongly controlled by the degree of connectivity between host‐plants during the prior year than by current connectivity. This discrepancy may be due to changes in plant species coverage in a year and/or to the delay of dispersal response of fungi. This study it the first to demonstrate how small‐scale host‐plant distributions mediate connectivity in microorganisms. The consequences of plant distributions for the permeability of the floristic landscape to fungi dispersal appear to control fungal assemblages, but with possibly different mechanisms for the different fungal groups.

    更新日期:2020-01-16
  • Trait–environment interactions affect plant establishment success during restoration
    Ecology (IF 4.285) Pub Date : 2020-01-13
    Chad R. Zirbel; Lars A. Brudvig

    Establishment and persistence are central to community assembly and are determined by how traits interact with the environment to determine performance ('trait–environment interactions'). Community assembly studies have rarely considered such trait–environment interactions, however, which can lead to incorrect inferences about how traits affect assembly. We evaluated how functional traits, environmental conditions, and trait–environment interactions structure plant establishment, as a measure of performance. Within 12 prairie restorations created by sowing 70 species, we quantified environmental conditions and counted individuals of each seeded species to quantify first year establishment. Three trait–environment interactions structured establishment. Leaf nitrogen interacted with herbivore pressure, as low leaf nitrogen species established relatively better under higher herbivory than species with high leaf nitrogen. Soil moisture interacted with root mass fraction (RMF), with low RMF species establishing better with low soil moisture and higher RMF species better on wetter soils. Specific leaf area (SLA) interacted with light availability, as low SLA species established better under high light conditions and high SLA species under low light conditions. Our work illustrates how community assembly can be better described by trait–environment interactions than correlating traits or environment with performance. This knowledge can assist species selection to maximize restoration success.

    更新日期:2020-01-14
  • The underappreciated role of rodent generalists in fungal spore‐dispersal networks
    Ecology (IF 4.285) Pub Date : 2020-01-13
    Ryan B. Stephens; Rebecca J. Rowe

    Animals are often the primary dispersers of seeds and fungal spores. Specialist species, that consume fruits or fungal fruiting bodies (sporocarps) as their main food source, are thought to play a more important role in dispersal networks compared to generalist species. However, dispersal networks are often based on occurrence data, overlooking the influence of animal abundance and dispersal effectiveness on network interactions. Using rodent‐mycorrhizal fungi networks, we determined how diet specialization and abundance influence the role of rodent species in dispersing fungal spores in temperate forests of northern New Hampshire, USA. We tracked the interactions of five rodent species and 34 fungal taxa over a three‐year period across hardwood, mixed, and softwood forest stands. We accounted for fluctuations in rodent abundance and differences in the number of spores dispersed in rodent scat. Myodes gapperi, a fungal specialist, dispersed a more diverse spore community than rodent generalists and was consistently the most important disperser in forest types with high fungal availability. Nevertheless, during years when generalist species such as Tamias striatus and Peromyscus maniculatus reached high abundance, their relative importance (species strength) in networks approached or even surpassed that of M. gapperi, particularly in forest types where M. gapperi was less common and fungal availability was low. Increased numbers of generalists enhanced network interaction diversity and the number of fungal taxa dispersed, the timing of which was coincident with seedling establishment following masting, a stage when inoculation by mycorrhizal fungi is critical for growth and survival. Our findings suggest that although specialists play key roles in dispersing mycorrhizal fungal spores, generalists play a heretofore underappreciated role.

    更新日期:2020-01-14
  • Wandering woodpeckers: foray behavior in a social bird
    Ecology (IF 4.285) Pub Date : 2020-01-12
    Sahas Barve; Natasha D. G. Hagemeyer; Russell E. Winter; Samuel D. Chamberlain; Walter D. Koenig; David W. Winkler; Eric L. Walters

    In many cooperatively breeding taxa, nonbreeding subordinates, or helpers, use extra‐territorial forays to discover dispersal opportunities. Such forays are considered energetically costly and foraying birds face aggression from conspecific members of the territories they visit. In contrast, breeders in cooperatively breeding taxa are expected to foray seldomly. We used novel tracking technologies to follow 62 acorn woodpeckers (Melanerpes formicivorus), a cooperatively breeding bird, to study extra‐territorial foray behavior. Both helpers and breeders engaged in extra‐territorial forays routinely and often several times per day. Helpers forayed earlier in the day and invested more time when foraying to high‐quality territories. Unexpectedly, breeders forayed as often and as far as helpers. Breeders from high‐quality territories forayed closer to their home territories than breeders from low‐quality territories, reflecting a potential trade‐off between foraying and territory defense. Such a routine pattern of extra‐territorial forays in both helpers and breeders suggests that the motives behind forays differ by sex and social status and involve more than simply searching for dispersal opportunities.

    更新日期:2020-01-13
  • Insights from excrement: invasive gastropods shift diet to consume the coffee leaf rust and its mycoparasite
    Ecology (IF 4.285) Pub Date : 2020-01-12
    Zachary Hajian‐Forooshani; John Vandermeer; Ivette Perfecto

    Agroecosystems are almost always by definition composed of novel assemblages of organisms from various parts of the world (Perfecto and Vandermeer 2015). As ecologists, we have little ability to predict a priori how interactions within these novel assemblages will organize themselves and what their impacts will be within and adjacent to agricultural production. While it may be possible to make coarse predictions about well‐studied organisms, as with natural enemy release in non‐native ranges, it is less often the case that we are able to predict the development of novel interactions which result from host shifts in new ecological contexts (Agosta 2006; Nylin et al. 2018).

    更新日期:2020-01-13
  • Ocean currents and the population genetic signature of fish migrations
    Ecology (IF 4.285) Pub Date : 2020-01-11
    Nils C. Krueck; Eric A. Treml; David J. Innes; Jennifer R. Ovenden

    Animal migrations are a fascinating and global phenomenon, yet they are often difficult to study and sometimes poorly understood. Here, we build on classic ecological theory by hypothesizing that some enigmatic spawning migrations across ocean habitats can be inferred from a population genetic signature of larval dispersal by ocean currents. We test this assumption by integrating spatially‐realistic simulations of alternative spawning migration routes, associated patterns of larval dispersal, and associated variation in the population genetic structure of eastern Australian sea mullet (Mugil cephalus). We then use simulation results to assess the implications of alternative spawning destinations for larval replenishment, and we contrast simulated against measured population genetic variation. Both analyses suggest that the spawning migrations of M. cephalus in eastern Australia are likely to be localized (approximately 100 km along the shore), and that spawning is likely to occur in inshore waters. Our conclusions are supported by multiple lines of evidence available through independent studies, but they challenge the more traditional assumption of a single, long‐distance migration event with subsequent offshore spawning in the East Australian Current. More generally, our study operationalizes classic theory on the relationship between fish migrations, ocean currents and reproductive success. However, rather than confirming the traditionally assumed adaptation of migratory behaviour to dominant ocean current flow, our findings support the concept of a genetically measurable link between fish migrations and local oceanographic conditions, specifically water temperature and coastal retention of larvae. We believe that future studies using similar approaches for high resolution and spatially‐realistic ecological‐genetic scenario testing can help rapidly advance our understanding of key ecological processes in many other marine species.

    更新日期:2020-01-13
  • Metabolic insight into bacterial community assembly across ecosystem boundaries
    Ecology (IF 4.285) Pub Date : 2020-01-10
    Nathan I. Wisnoski; Mario E. Muscarella; Megan L. Larsen; Ariane L. Peralta; Jay T. Lennon

    The movement of organisms across habitat boundaries has important consequences for populations, communities, and ecosystems. However, because most species are not well adapted to all habitat types, dispersal into suboptimal habitats could induce physiological changes associated with persistence strategies that influence community assembly. For example, high rates of cross‐boundary dispersal are thought to maintain sink populations of terrestrial bacteria in aquatic habitats, but these bacteria may also persist by lowering their metabolic activity, introducing metabolic heterogeneity that buffers the population against species sorting. To differentiate between these assembly processes, we analyzed bacterial composition along a hydrological flow path from terrestrial soils through an aquatic reservoir by sequencing the active and total (active + inactive) portions of the community. When metabolic heterogeneity was ignored, our data were consistent with views that cross‐boundary dispersal is important for structuring aquatic bacterial communities. In contrast, we found evidence for strong species sorting in the active portion of the aquatic community, suggesting that dispersal may have a weaker effect than persistence strategies on aquatic community assembly. By accounting for metabolic heterogeneity in complex communities, our findings clarify the roles of local‐ and regional‐scale assembly processes in terrestrial‐aquatic meta‐ecosystems.

    更新日期:2020-01-13
  • Demography of snowshoe hare population cycles
    Ecology (IF 4.285) Pub Date : 2020-01-10
    Madan K. Oli; Charles J. Krebs; Alice J. Kenney; Rudy Boonstra; Stan Boutin; James E. Hines

    Cyclic fluctuations in abundance exhibited by some mammalian populations in northern habitats (“population cycles”) are key processes in the functioning of many boreal and tundra ecosystems. Understanding population cycles, essentially demographic processes, necessitates discerning the demographic mechanisms that underlie numerical changes. Using mark‐recapture data spanning 5 population cycles (1977‐2017), we examined demographic mechanisms underlying the 9‐10‐year cycles exhibited by snowshoe hares (Lepus americanus Erxleben) in southwestern Yukon, Canada. Snowshoe hare populations always decreased during winter and increased during summer; the balance between winter declines and summer increases characterized the four, multiyear cyclic phases: increase, peak, decline, and low. Little or no recruitment occurred during winter, but summer recruitment varied markedly across the 4 phases with the highest and lowest recruitment observed during the increase and decline phase, respectively. Population crashes during the decline were triggered by a substantial decline in winter survival and by a lack of subsequent summer recruitment. In contrast, initiation of the increase phase was triggered by a 2‐fold increase in summer recruitment abetted secondarily by improvements in subsequent winter survival. We show that differences in peak density across cycles is explained by differences in overall population growth rate, amount of time available for population growth to occur, and starting population density. Demographic mechanisms underlying snowshoe hare population cycles were consistent across cycles in our study site but we do not yet know if similar demographic processes underlie population cycles in other northern snowshoe hare populations.

    更新日期:2020-01-10
  • A scale‐dependent framework for trade‐offs, syndromes, and specialization in organismal biology
    Ecology (IF 4.285) Pub Date : 2020-01-08
    Anurag A. Agrawal

    Biodiversity is defined by trait differences between organisms, and biologists have long sought to predict associations among ecologically important traits. Why do some traits trade off but others are coexpressed? Why might some trait associations hold across levels of organization, from individuals and genotypes to populations and species, whereas others only occur at one level? Understanding such scaling is a core biological problem, bearing on the evolution of ecological strategies as well as forecasting responses to environmental change. Explicitly considering the hierarchy of biodiversity and expectations at each scale (individual change, evolution within and among populations, and species turnover) is necessary as we work toward a predictive framework in evolutionary ecology. Within species, a trait may have an association with another trait because of phenotypic plasticity, genetic correlation, or population‐level local adaptation. Plastic responses are often adaptive and yet individuals have a fixed pool of resources; thus, positive and negative trait associations can be generated by immediate environmental needs and energetic demands. Genetic variation and covariation for traits within a population are typically shaped by varying natural selection in space and time. Although genetic correlations are infrequently long‐term constraints, they may indicate competing organismal demands. Traits are often quantitatively differentiated among populations (local adaptation), although selection rarely favors qualitatively different strategies until populations become reproductively isolated. Across species, niche specialization to particular habitats or biotic interactions may determine trait correlations, a subset of which are termed “strategic trade‐offs” because they are a consequence of adaptive specialization. Across scales, constraints within species often do not apply as new species evolve, and conversely, trait correlations observed across populations or species may not be reflected within populations. I give examples of such scale‐dependent trait associations and their causes across taxonomic groups and ecosystems, and in the final section of the paper, I specifically evaluate leaf economics spectrum traits and their associations with plant defense against herbivory. Scale‐dependent predictions emerge for understanding plant ecology holistically, and this approach can be fruitfully applied more generally in evolutionary ecology. Adaptive specialization and community context are two of the primary drivers of trade‐offs and syndromes across biological scales.

    更新日期:2020-01-08
  • Do differences in developmental mode shape the potential for local adaptation?
    Ecology (IF 4.285) Pub Date : 2020-01-08
    L. L. Jupe; D. T. Bilton; A. M. Knights

    Future climate change is leading to the redistribution of life on Earth as species struggle to cope with rising temperatures. Local adaptation allows species to become locally optimized and persist despite environmental selection, but the extent to which this occurs in nature may be limited by dispersal and gene flow. Congeneric marine gastropod species (Littorina littorea and L. saxatilis) with markedly different developmental modes were collected from across a latitudinal thermal gradient to explore the prevalence of local adaptation to temperature. The acute response of metabolic rate (using oxygen consumption as a proxy) to up‐ramping and down‐ramping temperature regimes between 6°C and 36°C was quantified for five populations of each species. The highly dispersive L. littorea exhibited minimal evidence of local adaptation to the thermal gradient, with no change in thermal optimum (Topt) or thermal breadth (Tbr) and a decline in maximal performance (max) with increasing latitude. In contrast, the direct developing L. saxatilis displayed evidence of local optimization, although these varied idiosyncratically with latitude, suggesting a suite of selective pressures may be involved in shaping thermal physiology in this relatively sedentary species. Our results show that the biogeography of thermal traits can differ significantly between related species, and show that interpopulation differences in thermal performance do not necessarily follow simple patterns that may be predicted based on latitudinal changes in environmental temperatures. Further research is clearly required to understand the mechanisms that can lead to the emergence of local adaptation in marine systems better and allow improved predictions of species redistribution in response to climate change.

    更新日期:2020-01-08
  • Soil chemistry turned upside down: a meta‐analysis of invasive earthworm effects on soil chemical properties
    Ecology (IF 4.285) Pub Date : 2020-01-08
    Olga Ferlian; Madhav P. Thakur; Alejandra Castañeda González; Layla M. San Emeterio; Susanne Marr; Barbbara da Silva Rocha; Nico Eisenhauer

    Recent studies have shown that invasive earthworms can dramatically reduce native biodiversity, both above and below the ground. However, we still lack a synthetic understanding of the underlying mechanisms behind these changes, such as whether earthworm effects on soil chemical properties drive such relationships. Here, we investigated the effects of invasive earthworms on soil chemical properties (pH, water content, and the stocks and fluxes of carbon, nitrogen, and phosphorus) by conducting a meta‐analysis. Invasive earthworms generally increased soil pH, indicating that the removal of organic layers and the upward transport of more base‐rich mineral soil caused a shift in soil pH. Moreover, earthworms significantly decreased soil water content, suggesting that the burrowing activities of earthworms may have increased water infiltration of and/or increased evapotranspiration from soil. Notably, invasive earthworms had opposing effects on organic and mineral soil for carbon and nitrogen stocks, with decreases in organic, and increases in mineral soil. Nitrogen fluxes were higher in mineral soil, whereas fluxes in organic soil were not significantly affected by the presence of invasive earthworms, indicating that earthworms mobilize and redistribute nutrients among soil layers and increase overall nitrogen loss from the soil. Invasive earthworm effects on element stocks increased with ecological group richness only in organic soil. Earthworms further decreased ammonium stocks with negligible effects on nitrate stocks in organic soil, whereas they increased nitrate stocks but not ammonium stocks in mineral soil. Notably, all of these results were consistent across forest and grassland ecosystems underlining the generality of our findings. However, we found some significant differences between studies that were conducted in the field (observational and experimental settings) and in the lab, such as that the effects on soil pH decreased from field to lab settings, calling for a careful interpretation of lab findings. Our meta‐analysis provides strong empirical evidence that earthworm invasion may lead to substantial changes in soil chemical properties and element cycling in soil. Furthermore, our results can help explain the dramatic effects of invasive earthworms on native biodiversity, for example, shifts towards the dominance of grass species over herbaceous ones, as shown by recent meta‐analyses.

    更新日期:2020-01-08
  • Upwelling buffers climate change impacts on coral reefs of the eastern tropical Pacific
    Ecology (IF 4.285) Pub Date : 2020-01-06
    Carly J. Randall; Lauren T. Toth; James J. Leichter; Juan L. Maté; Richard B. Aronson

    Corals of the eastern tropical Pacific live in a marginal and oceanographically dynamic environment. Along the Pacific coast of Panamá, stronger seasonal upwelling in the Gulf of Panamá in the east transitions to weaker upwelling in the Gulf of Chiriquí in the west, resulting in complex regional oceanographic conditions that drive differential coral‐reef growth. Over millennial timescales, reefs in the Gulf of Chiriquí recovered more quickly from climatic disturbances compared with reefs in the Gulf of Panamá. In recent decades, corals in the Gulf of Chiriquí have also had higher growth rates than in the Gulf of Panamá. As the ocean continues to warm, however, conditions could shift to favor the growth of corals in the Gulf of Panamá, where upwelling may confer protection from high‐temperature anomalies. Here we describe the recent spatial and temporal variability in surface oceanography of nearshore environments in Pacific Panamá and compare those conditions with the dynamics of contemporary coral‐reef communities during and after the 2016 coral‐bleaching event. Although both gulfs have warmed significantly over the last 150 yr, the annual thermal maximum in the Gulf of Chiriquí is increasing faster, and ocean temperatures there are becoming more variable than in the recent past. In contrast to historical trends, we found that coral cover, coral survival, and coral growth rates were all significantly higher in the Gulf of Panamá. Corals bleached extensively in the Gulf of Chiriquí following the 2015–2016 El Niño event, whereas upwelling in the Gulf of Panamá moderated the high temperatures caused by El Niño, allowing the corals largely to escape thermal stress. As the climate continues to warm, upwelling zones may offer a temporary and localized refuge from the thermal impacts of climate change, while reef growth in the rest of the eastern tropical Pacific continues to decline.

    更新日期:2020-01-06
  • Defensive fruit metabolites obstruct seed dispersal by altering bat behavior and physiology at multiple temporal scales
    Ecology (IF 4.285) Pub Date : 2020-01-02
    Justin W. Baldwin; Dina K. N. Dechmann; Wibke Thies; Susan R. Whitehead

    The paradoxical presence of toxic chemical compounds in ripe fruits represents a balance between plant enemies and allies: chemical traits can defend seeds against antagonistic herbivores, seed predators, or fungal pathogens, but also can impose costs by repelling mutualistic seed dispersers, although the costs are often difficult to quantify. Seeds gain fitness benefits from traveling far from the parent plant, as they can escape from parental competition and elude specialized herbivores as well as pathogens that accumulate on adult plants. However, seeds are difficult to follow from their parent plant to their final destination. Thus, little is known about the factors that determine seed dispersal distance. We investigated this potential cost of fruit secondary compounds, reduced seed dispersal distance, by combining two data sets from previous work on a Neotropical bat‐plant dispersal system (bats in the genus Carollia and plants in the genus Piper). We used data from captive behavioral experiments, which show how amides in ripe fruits of Piper decrease the retention time of seeds and alter food choices. With new analyses, we show that these defensive secondary compounds also delay the time of fruit removal. Next, with a behaviorally annotated bat telemetry data set, we quantified post‐feeding movements (i.e., seed dispersal distances). Using generalized additive mixed models we found that seed dispersal distances varied nonlinearly with gut retention times as well as with the time of fruit removal. By interrogating the model predictions, we identified two novel mechanisms by which fruit secondary compounds can impose costs in terms of decreased seed dispersal distances: (1) small‐scale reductions in gut retention time and (2) causing fruits to forgo advantageous bat activity peaks that confer high seed dispersal distances.

    更新日期:2020-01-02
  • Looking back to look ahead: a vision for soil denitrification research
    Ecology (IF 4.285) Pub Date : 2019-12-20
    Maya Almaraz; Michelle Y. Wong; Wendy H. Yang

    Denitrification plays a critical role in regulating ecosystem nutrient availability and anthropogenic reactive nitrogen (N) production. Its importance has inspired an increasing number of studies, yet it remains the most poorly constrained term in terrestrial ecosystem N budgets. We censused the peer‐reviewed soil denitrification literature (1975–2015) to identify opportunities for future studies to advance our understanding despite the inherent challenges in studying the process. We found that only one‐third of studies reported estimates of both nitrous oxide (N2O) and dinitrogen (N2) production fluxes, often the dominant end products of denitrification, while the majority of studies reported only net N2O fluxes or denitrification potential. Of the 236 studies that measured complete denitrification to N2, 49% used the acetylene inhibition method, 84% were conducted in the laboratory, 81% were performed on surface soils (0–20 cm depth), 75% were located in North America and Europe, and 78% performed treatment manipulations, mostly of N, carbon, or water. To improve understanding of soil denitrification, we recommend broadening access to technologies for new methodologies to measure soil N2 production rates, conducting more studies in the tropics and on subsoils, performing standardized experiments on unmanipulated soils, and using more precise terminology to refer to measured process rates (e.g., net N2O flux or denitrification potential). To overcome the greater challenges in studying soil denitrification, we envision coordinated research efforts based on standard reporting of metadata for all soil denitrification studies, standard protocols for studies contributing to a Global Denitrification Research Network, and a global consortium of denitrification researchers to facilitate sharing ideas, resources, and to provide mentorship for researchers new to the field.

    更新日期:2020-01-02
  • Inferring seasonal infection risk at population and regional scales from serology samples
    Ecology (IF 4.285) Pub Date : 2019-11-19
    Mark Q. Wilber; Colleen T. Webb; Fred L. Cunningham; Kerri Pedersen; Xiu‐Feng Wan; Kim M. Pepin

    Accurate estimates of seasonal infection risk can be used by animal health officials to predict future disease risk and improve understanding of the mechanisms driving disease dynamics. It can be difficult to estimate seasonal infection risk in wildlife disease systems because surveillance assays typically target antibodies (serosurveillance), which are not necessarily indicative of current infection, and serosurveillance sampling is often opportunistic. Recently developed methods estimate past time of infection from serosurveillance data using quantitative serological assays that indicate the amount of antibodies in a serology sample. However, current methods do not account for common opportunistic and uneven sampling associated with serosurveillance data. We extended the framework of survival analysis to improve estimates of seasonal infection risk from serosurveillance data across population and regional scales. We found that accounting for the right‐censored nature of quantitative serology samples greatly improved estimates of seasonal infection risk, even when sampling was uneven in time. Survival analysis can also be used to account for common challenges when estimating infection risk from serology data, such as biases induced by host demography and continually elevated antibodies following infection. The framework developed herein is widely applicable for estimating seasonal infection risk from serosurveillance data in humans, wildlife, and livestock.

    更新日期:2020-01-02
  • Joint estimation of growth and survival from mark–recapture data to improve estimates of senescence in wild populations
    Ecology (IF 4.285) Pub Date : 2019-12-26
    Beth A. Reinke; Luke Hoekstra; Anne M. Bronikowski; Fredric J. Janzen; David Miller

    Understanding age‐dependent patterns of survival is fundamental to predicting population dynamics, understanding selective pressures, and estimating rates of senescence. However, quantifying age‐specific survival in wild populations poses significant logistical and statistical challenges. Recent work has helped to alleviate these constraints by demonstrating that age‐specific survival can be estimated using mark–recapture data even when age is unknown for all or some individuals. However, previous approaches do not incorporate auxiliary information that can improve age estimates of individuals. We introduce a survival estimator that combines a von Bertalanffy growth model, age‐specific hazard functions, and a Cormack‐Jolly‐Seber mark–recapture model into a single hierarchical framework. This approach allows us to obtain information about age and its uncertainty based on size and growth for individuals of unknown age when estimating age‐specific survival. Using both simulated and real‐world data for two painted turtle (Chrysemys picta) populations, we demonstrate that this additional information substantially reduces the bias of age‐specific hazard rates, which allows for the testing of hypotheses related to aging. Estimating patterns of senescence is just one practical application of jointly estimating survival and growth; other applications include obtaining better estimates of the timing of recruitment and improved understanding of life‐history trade‐offs between growth and survival.

    更新日期:2020-01-02
  • Seasonal windows of opportunity in milkweed–monarch interactions
    Ecology (IF 4.285) Pub Date : 2019-11-25
    Louie H. Yang; Meredith L. Cenzer

    Many organisms experience seasonal windows of opportunity for growth and reproduction. These windows represent intervals in time when organisms experience improved prospects for advancing their life history objectives, constrained by the combined effects of seasonally variable biotic and abiotic conditions acting independently or in combination. Although seasonal windows of opportunity are likely to be widespread in nature, relatively few studies have conducted the repeated observations necessary to identify them or suggest the factors that structure them in time. Here, we present the results of three experimental studies conducted at different field sites in three different years in which we manipulated the phenology of monarch caterpillars (Danaus plexippus) throughout the growing season. The primary aims of these experiments were (1) to identify seasonal windows of opportunity for successful larval development on milkweed (Asclepias spp.), and (2) to suggest which factors are most likely to constrain these windows of opportunity in time. We found strong seasonal windows of opportunity in the developmental success of monarchs, with distinct periods of higher developmental prospects during each study year. We evaluated the role of seasonal variation in abiotic thermal stress, host plant density, host plant defensive traits, and natural enemy risk as potential factors that may limit seasonal windows of opportunity. By comparing the seasonal patterns of larval success and potential explanatory factors across all 3 yr, we find patterns that are consistent with seasonally variable abiotic conditions, host plant availability, host plant traits, and natural enemy risk factors. These results suggest the potential for seasonal variation in the factors that limit monarch larval development and population growth. More generally, this study also highlights the value of temporally explicit experimental studies that can identify and examine seasonal patterns in species interactions.

    更新日期:2020-01-02
  • Linking inter‐annual variation in environment, phenology, and abundance for a montane butterfly community
    Ecology (IF 4.285) Pub Date : 2019-11-29
    James E. Stewart; Javier Gutiérrez Illán; Shane A. Richards; David Gutiérrez; Robert J. Wilson

    Climate change has caused widespread shifts in species’ phenology, but the consequences for population and community dynamics remain unclear because of uncertainty regarding the species‐specific drivers of phenology and abundance, and the implications for synchrony among interacting species. Here, we develop a statistical model to quantify inter‐annual variation in phenology and abundance over an environmental gradient, and use it to identify potential drivers of phenology and abundance in co‐occurring species. We fit the model to counts of 10 butterfly species with single annual generations over a mountain elevation gradient, as an exemplar system in which temporally limited availability of biotic resources and favorable abiotic conditions impose narrow windows of seasonal activity. We estimate parameters describing changes in abundance, and the peak time and duration of the flight period, over ten years (2004–2013) and across twenty sample locations (930–2,050 m) in central Spain. We also use the model outputs to investigate relationships of phenology and abundance with temperature and rainfall. Annual shifts in phenology were remarkably consistent among species, typically showing earlier flight periods during years with warm conditions in March or May–June. In contrast, inter‐annual variation in relative abundance was more variable among species, and generally less well associated with climatic conditions. Nevertheless, warmer temperatures in June were associated with increased relative population growth in three species, and five species had increased relative population growth in years with earlier flight periods. These results suggest that broadly coherent interspecific changes to phenology could help to maintain temporal synchrony in community dynamics under climate change, but that the relative composition of communities may vary due to interspecific inconsistency in population dynamic responses to climate change. However, it may still be possible to predict abundance change for species based on a robust understanding of relationships between their population dynamics and phenology, and the environmental drivers of both.

    更新日期:2020-01-02
  • Raising young with limited resources: supplementation improves body condition and advances fledging of Canada Jays
    Ecology (IF 4.285) Pub Date : 2019-11-19
    Nikole E. Freeman; D. Ryan Norris; Alex O. Sutton; Amy E. M. Newman

    Food availability early in life can play a vital role in an individual's development and success, but experimental evidence for the direct effects of food on body condition, physiology, and survival of young animals in the wild is still relatively scarce. Food‐caching Canada Jays (Perisoreus canadensis) begin breeding in the late winter and, therefore, rely on either cached food or seemingly limited quantities of fresh food to feed nestlings in the early spring. Using a 2‐yr food supplementation experiment conducted during the nestling period and 40 yr of observational data on food supplemented by the public, we examined whether food quantity during early life influenced the physiology, body condition, timing of fledging, and survival of young Canada Jays in Algonquin Provincial Park, Ontario, Canada. Experimental food supplementation of breeding pairs advanced the fledging date of young by 24% (5.5 d) compared to controls. In 1 yr of the experiment, nestlings raised on experimentally supplemented territories had lower feather corticosterone concentrations and were in higher body condition than controls. Across treatment and control nests, young that successfully fledged had lower concentrations of feather corticosterone and were in higher body condition than those that did not fledge. Based on 40 yr of observational data, nestling body condition was positively related to the degree of food supplementation by park visitors and nestlings in higher body condition were more likely to be observed in the population in the following fall. Our results demonstrate how food availability early in life can have important downstream consequences on metrics related to individual fitness, including first year survival.

    更新日期:2020-01-02
  • Limited evidence for spatial resource partitioning across temperate grassland biodiversity experiments
    Ecology (IF 4.285) Pub Date : 2019-11-06
    Kathryn E. Barry; Jasper van Ruijven; Liesje Mommer; Yongfei Bai; Carl Beierkuhnlein; Nina Buchmann; Hans de Kroon; Anne Ebeling; Nico Eisenhauer; Claudia Guimarães‐Steinicke; Anke Hildebrandt; Forest Isbell; Alexandru Milcu; Carsten Neßhöver; Peter B. Reich; Christiane Roscher; Leopold Sauheitl; Michael Scherer‐Lorenzen; Bernhard Schmid; David Tilman; Stefanie von Felten; Alexandra Weigelt

    Locally, plant species richness supports many ecosystem functions. Yet, the mechanisms driving these often‐positive biodiversity–ecosystem functioning relationships are not well understood. Spatial resource partitioning across vertical resource gradients is one of the main hypothesized causes for enhanced ecosystem functioning in more biodiverse grasslands. Spatial resource partitioning occurs if species differ in where they acquire resources and can happen both above‐ and belowground. However, studies investigating spatial resource partitioning in grasslands provide inconsistent evidence. We present the results of a meta‐analysis of 21 data sets from experimental species‐richness gradients in grasslands. We test the hypothesis that increasing spatial resource partitioning along vertical resource gradients enhances ecosystem functioning in diverse grassland plant communities above‐ and belowground. To test this hypothesis, we asked three questions. (1) Does species richness enhance biomass production or community resource uptake across sites? (2) Is there evidence of spatial resource partitioning as indicated by resource tracer uptake and biomass allocation above‐ and belowground? (3) Is evidence of spatial resource partitioning correlated with increased biomass production or community resource uptake? Although plant species richness enhanced community nitrogen and potassium uptake and biomass production above‐ and belowground, we found that plant communities did not meet our criteria for spatial resource partitioning, though they did invest in significantly more aboveground biomass in higher canopy layers in mixture relative to monoculture. Furthermore, the extent of spatial resource partitioning across studies was not positively correlated with either biomass production or community resource uptake. Our results suggest that spatial resource partitioning across vertical resource gradients alone does not offer a general explanation for enhanced ecosystem functioning in more diverse temperate grasslands.

    更新日期:2020-01-02
  • Nutrient retention and loss during ecosystem succession: revisiting a classic model
    Ecology (IF 4.285) Pub Date : 2019-10-09
    Kate Lajtha

    In 1975, Vitousek and Reiners proposed a conceptual model relating the net retention of a limiting nutrient to the net biomass accumulation in terrestrial ecosystems, whereby terrestrial systems should be highly conservative of nutrients during ecosystem succession when plants are actively accumulating biomass, but should be relatively leakier in older stands, when net plant biomass accumulation nears zero. The model was based on measurements in the White Mountains of New Hampshire. However, recent data showing that nitrate output in streams is declining across this region even as forests are aging seem to be inconsistent with this theory. Because the more recent data do not match the predictions of the Vitousek and Reiners model, either new hypotheses, or modifications of the original hypothesis, need to be considered. I suggest that the original model can be amended by accounting for increased woody debris; an accumulation of both above and belowground high C:N coarse woody debris from tree mortality in these regenerating forests can lead to high microbial immobilization of N and can explain the recent patterns of declining stream nitrate. Few studies or models have attempted to calculate the impacts of coarse woody debris (CWD) decomposition products to the retention of C and N in forested ecosystems and their receiving streams, but evidence suggests that CWD can significantly affect stream N exports and should be considered in future models of ecosystem biogeochemical cycles.

    更新日期:2020-01-02
  • Parasites enhance resistance to drought in a coastal ecosystem
    Ecology (IF 4.285) Pub Date : 2019-10-29
    Joseph P. Morton; Brian R. Silliman

    Parasites are more diverse and numerous than their hosts and commonly control population dynamics. Whether parasites also regulate key ecosystem processes, such as resistance to climate stress, is unclear. In southern U.S. salt marshes, drought interacts synergistically with keystone grazing to generate extensive ecosystem die‐off. Field manipulations of parasite prevalence and salt stress in sediments in healthy marshes demonstrated that trematode parasites, by suppressing feeding activity of grazers that overgraze on drought‐stressed plants, have the potential to slow the rate of ecosystem loss. Surveys along 1,000 km of coastline during an intense drought event revealed parasitism is common in grazers on die‐off borders and that increasing infection prevalence along marsh die‐off borders is negatively correlated with per capita grazing. Combined, results from this field experiment and survey suggested, but did not show, that parasites could affect rates of drought‐driven salt marsh die‐off. To test whether parasites can indeed protect marshes under real drought conditions, we experimentally manipulated parasite prevalence in grazers over a month‐long period on active die‐off borders in three North Carolina marshes. Experimentally reducing parasite prevalence markedly increased the rate of plant ecosystem decline, an effect that scaled positively with prevalence. Thus parasites, by generating a trophic cascade, indirectly enhanced ecosystem resistance to overgrazing under intense drought in these North Carolina marshes. The generality of these results across the entire range of this keystone grazer in the southeastern United States needs to be tested, employing both experiments and extensive surveys that examine how the rate of ecosystem decline is mediated by parasitism. Given the ubiquity of parasites in ecosystems, our results suggest that more research effort should be invested in examining the possible roles for parasitism in regulating ecosystem function and stability.

    更新日期:2020-01-02
  • Disturbance size and frequency mediate the coexistence of benthic spatial competitors
    Ecology (IF 4.285) Pub Date : 2019-11-06
    Max C. N. Castorani; Marissa L. Baskett

    Disturbance plays a key role in structuring community dynamics and is central to conservation and natural resource management. However, ecologists continue to debate the importance of disturbance for species coexistence and biodiversity. Such disagreements may arise in part because few studies have examined variation across multiple dimensions of disturbance (e.g., size, frequency) and how the effects of disturbance may depend on species attributes (e.g., competitiveness, dispersal ability). In light of this gap in understanding and accelerating changes to disturbance regimes worldwide, we used spatial population models to explore how disturbance size and frequency interact with species attributes to affect coexistence between seagrass (Zostera marina) and colonial burrowing shrimp (Neotrypaea californiensis) that compete for benthic space in estuaries throughout the west coast of North America. By simulating population dynamics under a range of ecologically relevant disturbance regimes, we discovered that intermediate disturbance (approximately 9–23% of landscape area per year) to short‐dispersing, competitively dominant seagrass can foster long‐term stable coexistence with broad‐dispersing, competitively inferior burrowing shrimp via the spatial storage effect. When holding the total extent of disturbance constant, the individual size and annual frequency of disturbance altered landscape spatial patterns and mediated the dominance and evenness of competitors. Many small disturbances favored short‐dispersing seagrass by hastening recolonization, whereas fewer large disturbances benefited rapidly colonizing burrowing shrimp by creating temporary refugia from competition. As a result, large, infrequent disturbances generally improved the strength and stability of coexistence relative to small, frequent disturbances. Regardless of disturbance size or frequency, the dispersal ability of the superior competitor (seagrass), the competitive ability of the inferior competitor (burrowing shrimp), and the reproduction and survival of both species strongly influenced population abundances and coexistence. Our results show that disturbance size and frequency can promote or constrain coexistence by altering the duration of time over which inferior competitors can escape competitive exclusion, particularly when colonization depends on the spatial pattern of disturbance due to dispersal traits. For coastal managers and conservation practitioners, our findings indicate that reducing particularly large disturbances may help conserve globally imperiled seagrass meadows and control burrowing shrimp colonies that can threaten the viability of oyster aquaculture.

    更新日期:2020-01-02
  • The potential of zooplankton in constraining chytrid epidemics in phytoplankton hosts
    Ecology (IF 4.285) Pub Date : 2019-10-18
    Thijs Frenken; Takeshi Miki; Maiko Kagami; Dedmer B. Van de Waal; Ellen Van Donk; Thomas Rohrlack; Alena S. Gsell

    Fungal diseases threaten natural and man‐made ecosystems. Chytridiomycota (chytrids) infect a wide host range, including phytoplankton species that form the basis of aquatic food webs and produce roughly half of Earth's oxygen. However, blooms of large or toxic phytoplankton form trophic bottlenecks, as they are inedible to zooplankton. Chytrids infecting inedible phytoplankton provide a trophic link to zooplankton by producing edible zoospores of high nutritional quality. By grazing chytrid zoospores, zooplankton may induce a trophic cascade, as a decreased zoospore density will reduce new infections. Conversely, fewer infections will not produce enough zoospores to sustain long‐term zooplankton growth and reproduction. This intricate balance between zoospore density necessary for zooplankton energetic demands (growth/survival), and the loss in new infections (and thus new zoospores) because of grazing was tested empirically. To this end, we exposed a cyanobacterial host (Planktothrix rubescens) infected by a chytrid (Rizophydium megarrhizum) to a grazer density gradient (the rotifer Keratella cf. cochlearis). Rotifers survived and reproduced on a zoospore diet, but the Keratella population growth was limited by the amount of zoospores provided by chytrid infections, resulting in a situation where zooplankton survived but were restricted in their ability to control disease in the cyanobacterial host. We subsequently developed and parameterized a dynamical food‐chain model using an allometric relationship for clearance rate to assess theoretically the potential of different‐sized zooplankton groups to restrict disease in phytoplankton hosts. Our model suggests that smaller‐sized zooplankton may have a high potential to reduce chytrid infections on inedible phytoplankton. Together, our results point out the complexity of three‐way interactions between hosts, parasites, and grazers and highlight that trophic cascades are not always sustainable and may depend on the grazer's energetic demand.

    更新日期:2020-01-02
  • Spatial, interannual, and generational sources of trait variability in a marine population
    Ecology (IF 4.285) Pub Date : 2019-10-29
    Lorenzo Ciannelli; Irina Tolkova; Robert Lauth; Patricia Puerta; Thomas Helser; Alix Gitelman; Grant Thompson

    Life‐history traits of individuals in marine populations exhibit large sources of variability. In marine fish, variation of individual size at a given age has three main components: (1) spatial, correlated with the location in which individuals are caught, (2) temporal, correlated with the time when individuals are caught, and (3) generational, correlated with the year of birth of the examined individuals. These variations, if present, have practical implications for individual fitness as well as for sampling, survey design, and population assessment. Disentangling these variations and understanding their sources is hard, given the potentially correlated nature of their effects on individual traits. This study examines the size‐at‐age relationship of the Bering Sea Pacific cod, an economically and ecologically important groundfish. We used extensive records spanning 1994 to 2016 (inclusive) of 25,213 observations of both environmental variables and catch, lengths, and ages. We found that the average size of individuals of the same age could differ up to 7 cm. Notably, we found that the cohort composition of the sampled population explained >75% of the year effect and that individuals caught in the northwest and shallower portion of the sampling area were on average 5 cm smaller than individuals caught in the southern and deeper portion. We further found that northwest movement of young cod (age 1–5) as a result of warming places individuals in areas where we predict them to have smaller size at age. Smaller and less conditioned individuals are less fecund and may not be able to perform long migrations to return to their distant spawning grounds. Both the spatial distribution and water temperature experienced by Pacific cod in the Bering Sea are changing, and this study provides a mechanism for how these changes affect Pacific cod life‐history traits and individual fitness.

    更新日期:2020-01-02
  • Consumer mobility predicts impacts of herbivory across an environmental stress gradient
    Ecology (IF 4.285) Pub Date : 2019-11-19
    Robert W. Lamb; Franz Smith; Jon D. Witman

    Environmental stress impedes predation and herbivory by limiting the ability of animals to search for and consume prey. We tested the contingency of this relationship on consumer traits and specifically hypothesized that herbivore mobility relative to the return time of limiting environmental stress would predict consumer effects. We examined how wave‐induced water motion affects marine communities via herbivory by highly mobile (fish) vs. slow‐moving (pencil urchin) consumers at two wave‐sheltered and two wave‐exposed rocky subtidal locations in the Galapagos Islands. The exposed locations experienced 99th percentile flow speeds that were 2–5 times greater than sheltered locations, with mean flow speeds >33 cm/s vs. <16 cm/s, 2–7 times higher standing macroalgal cover and 2–3 times lower cover of crustose coralline algae than the sheltered locations. As predicted by the environmental stress hypothesis (ESH), there was a negative relationship between mean flow speed and urchin abundance and herbivory rates on Ulva spp. algal feeding assays. In contrast, the biomass of surgeonfishes (Acanthuridae) and parrotfishes (Labridae: Scarinae) was positively correlated with mean flow speed. Ulva assays were consumed at equal rates by fish at exposed and sheltered locations, indicating continued herbivory even when flow speeds surpassed maximum reported swimming speeds at a rate of 1–2 times per minute. Modeled variation in fish species richness revealed minimal effects of diversity on herbivory rates at flow speeds <40 cm/s, when all species were capable of foraging, and above 120 cm/s, when no species could forage, while increasing diversity maximized herbivory rates at flow speeds of 40–120 cm/s. Two‐month herbivore exclusion experiments during warm and cool seasons revealed that macroalgal biomass was positively correlated with flow speed. Fish limited macroalgal development by 65–91% at one exposed location but not the second and by 70% at the two sheltered locations. In contrast, pencil urchins did not affect algal communities at either exposed location, but reduced macroalgae by 87% relative to controls at both sheltered locations. We propose an extension of the ESH that is contingent upon mobility to explain species‐specific changes in feeding rates and consumer effects on benthic communities across environmental gradients.

    更新日期:2020-01-02
  • Better lucky than good: How savanna trees escape the fire trap in a variable world
    Ecology (IF 4.285) Pub Date : 2019-11-06
    William A. Hoffmann; R. Wyatt Sanders; Michael G. Just; Wade A. Wall; Matthew G. Hohmann

    Fire controls tree cover in many savannas by suppressing saplings through repeated topkill and resprouting, causing a demographic bottleneck. Tree cover can increase dramatically if even a small fraction of saplings escape this fire trap, so modeling and management of savanna vegetation should account for occasional individuals that escape the fire trap because they are “better” (i.e., they grow faster than average) or because they are “lucky” (they experience an occasional longer‐than‐average interval without fire or a below‐average fire severity). We quantified variation in growth rates and topkill probability in Quercus laevis (turkey oak) in longleaf pine savanna to estimate the percentage of stems expected to escape the fire trap due to variability in (1) growth rate, (2) fire severity, and (3) fire interval. For trees growing at the mean rate and exposed to the mean fire severity and the mean fire interval, no saplings are expected to become adults under typical fire frequencies. Introducing variability in any of these factors, however, allows some individuals to escape the fire trap. A variable fire interval had the greatest influence, allowing 8% of stems to become adults within a century. In contrast, introducing variation in fire severity and growth rate should allow 2.8% and 0.3% of stems to become adults, respectively. Thus, most trees that escape the fire trap do so because of luck. By chance, they experience long fire‐free intervals and/or a low‐severity fire when they are not yet large enough to resist an average fire. Fewer stems escape the fire trap by being unusually fast‐growing individuals. It is important to quantify these sources of variation and their consequences to improve understanding, prediction, and management of vegetation dynamics of fire‐maintained savannas. Here we also present a new approach to quantifying variation in fire severity utilizing a latent‐variable model of logistic regression.

    更新日期:2020-01-02
  • Climate change and invasion may synergistically affect native plant reproduction
    Ecology (IF 4.285) Pub Date : 2019-11-19
    Justyna Giejsztowt; Aimée T. Classen; Julie R. Deslippe

    Global change drivers can interact in synergistic ways, yet the interactive effect of global change drivers, such as climatic warming and species invasions, on plant pollination are poorly represented in experimental studies. We paired manipulative experiments to probe two mechanistic pathways through which plant invasion and warming may alter phenology and reproduction of native plant species. In the first, we tested how experimental warming (+1.7°C) modulated flowering phenology and how this affected flowering overlap between a native plant (Dracophyllum subulatum) and an invasive plant (Calluna vulgaris L.). In the second experiment, we explored how variation in the ratio of native to invasive flowers, and the overall quantity of resources in a floral patch, affected the reproduction of the native species. We hypothesized that the flowering overlap of native and invasive plants would be altered by warming, given that invading plants typically exhibit greater phenological plasticity than native plants. Further, we hypothesized that pollination of native plant flowers would decrease in floral patches dominated by invasive plant flowers, but that this effect would depend on total floral density in the patch. As predicted, the invasive plant had a stronger phenological response to experimental warming than the native plant, resulting in increased flowering overlap between the native the invasive plants. There was a four‐fold increase in the number of native flowers co‐flowering with high densities of invasive flowers suggesting native plant competition for pollinators with invasive plants under a warmed climate. In the second experiment, we found depressed seed masses of the native species in high density floral patches that were dominated by invasive flowers relative to high density floral patches dominated by native flowers. At low floral densities, seed mass of native plants was unaffected by invasion. Together, these results demonstrate that by increasing their phenological overlap, warming may enhance the magnitude of existing competition for pollination exerted by an invasive plant on a native plant, particularly in plant patches with high floral density. Our results illustrate a novel pathway through which global change drivers can operate synergistically to alter an important ecosystem service: pollination.

    更新日期:2020-01-02
  • Thermal niche predictors of alpine plant species
    Ecology (IF 4.285) Pub Date : 2019-10-09
    Jörg Löffler; Roland Pape

    Within the context of species distribution models, scrutiny arises from the choice of meaningful environmental predictors. Thermal conditions are not the sole driver, but are the most widely acknowledged abiotic driver of plant life within alpine ecosystems. We linked long‐term measurements of direct, plant‐relevant, near‐surface temperatures to plant species frequency. Across 47 sites located along environmental gradients within the Scandinavian mountain chain, the thermal preferences of 26 focal species of vascular plants, lichens, and bryophytes were explored. Based on partial least‐squares regression, we applied a relative importance analysis to derive inductively the thermal variables that were best related to a species’ frequency. To discover potential seasonal variability of thermal controls, analyses were both differentiated according to meteorological season and integrated across the entire year. The pronounced interspecies and temporal variability of thermal constraints revealed the thermal niches were much more nuanced and variable than they have commonly been represented. This finding challenges us to present, interrogate, and interpret data representing these thermal niches, which seems to be required in order to move beyond purely probabilistic and correlative descriptions of species’ range limits. Thus, this information will help improve predictions of species distributions in complex arctic‐alpine landscapes.

    更新日期:2020-01-02
  • Glucose triggers strong taxon‐specific responses in microbial growth and activity: insights from DNA and RNA qSIP
    Ecology (IF 4.285) Pub Date : 2019-11-25
    Katerina Papp; Bruce A. Hungate; Egbert Schwartz

    Growth of soil microorganisms is often described as carbon limited, and adding labile carbon to soil often results in a transient and large increase in respiration. In contrast, soil microbial biomass changes little, suggesting that growth and respiration are decoupled in response to a carbon pulse. Alternatively, measuring bulk responses of the entire community (total respiration and biomass) could mask ecologically important variation among taxa in response to the added carbon. Here, we assessed taxon‐specific variation in cellular growth (measured as DNA synthesis) and metabolic activity (measured as rRNA synthesis) following glucose addition to soil using quantitative stable isotope probing with H218O. We found that glucose addition altered rates of DNA and rRNA synthesis, but the effects were strongly taxon specific: glucose stimulated growth and rRNA transcription for some taxa, and suppressed these for others. These contrasting taxon‐specific responses could explain the small and transient changes in total soil microbial biomass. Responses to glucose were not well predicted by a priori assignments of taxa into copiotrophic or oligotrophic categories. Across all taxa, rates of DNA and rRNA synthesis changed in parallel, indicating that growth and activity were coupled, and the degree of coupling was unaffected by glucose addition. This pattern argues against the idea that labile carbon addition causes a large reduction in metabolic growth efficiency; rather, the large pulse of respiration observed with labile substrate addition is more likely to be the result of rapid turnover of microbial biomass, possibly due to trophic interactions. Our results support a strong connection between rRNA synthesis and bacterial growth, and indicate that taxon‐specific responses among soil bacteria can buffer responses at the scale of the whole community.

    更新日期:2020-01-02
  • Effects of earthworms and white‐tailed deer on roots, arbuscular mycorrhizae, and forest seedling performance
    Ecology (IF 4.285) Pub Date : 2019-11-06
    Annise Dobson; Justin Richardson; Bernd Blossey

    Changes in understory plant composition and biodiversity declines in northeastern North American forests are widespread. Preserving species and ecosystem function requires appropriate identification and management of important stressors. Coexistence of stressors, among them earthworm invasions and white‐tailed deer, makes correct identification of mechanisms that cause diversity declines challenging. We used an established factorial experiment to assess survival and growth of native seedlings (Actaea pachypoda, Aquilegia canadensis, Cornus racemosa, Quercus rubra, and Prenanthes alba) in response to presence/absence of deer and earthworms. We expected deer and earthworms to reduce seedling survival and biomass, and we evaluated potential pathways to explain this impact (soil N and P concentrations and pools, root architecture, and arbuscular mycorrhizal fungi [AMF] colonization). We developed structural equation models (SEM) to identify specific pathways through which earthworms and deer were impacting plant species with different life histories. Seedling survival was not affected by our treatments nor the plant and soil variables we tested. Actaea biomass was smaller in earthworm‐invaded plots, and with larger total N pools. In contrast, both deer and earthworm treatments were associated with lower soil nutrient concentrations, and earthworm‐invaded plots had smaller N and extractable P pools. Actaea, Cornus, Prenanthes, and Quercus seedlings had a lower proportion of fine roots in earthworm‐invaded plots, while fine roots in Aquilegia made up a higher proportion of the root system. AMF colonization in Quercus was reduced in sites colonized by earthworms, but AMF in other species were unaffected. Our SEMs showed high correlation among soil variables, but because we do not know which variables are drivers of this change and which are passengers, we can only conclude that they are changing together as deer and earthworms exert their respective influence. Different plant species responded in idiosyncratic ways to earthworm and deer effects on soil fertility, root architecture and limited effects on AMF colonization. While earthworm and deer‐mediated changes to fine roots, soil nutrients, and AMF may lead to changes in plant performance over time, these changes rarely translated to lower plant performance in our seedlings.

    更新日期:2020-01-02
  • Hierarchical modeling strengthens evidence for density dependence in observational time series of population dynamics
    Ecology (IF 4.285) Pub Date : 2019-11-19
    Loïc M. Thibaut; Sean R. Connolly

    The extent to which populations in nature are regulated by density‐dependent processes is unresolved. While experiments increasingly find evidence of strong density dependence, unmanipulated population time series yield much more ambiguous evidence of regulation, especially when accounting for effects of observation error. Here, we reexamine the evidence for density dependence in time series of population sizes in nature, by conducting an aggregate analysis of the populations in the Global Population Dynamics Database (GPDD). First, following the conventional approach, we fit a density‐dependent and a density‐independent variant of the Gompertz state‐space model to each time series. Then, we conduct an aggregate analysis of the entire database by considering two random‐effects density‐dependent models that leverage information across data sets. When individual time series are tested independently, we find very little evidence for density dependence. However, in the aggregate, we find very strong evidence for density dependence, even though, paradoxically, estimated strengths of density dependence for individual time series tend to be weaker than when each individual time series is analyzed independently. Furthermore, a hierarchical model that accounts for taxonomic variation in the strength of density dependence reveals that density dependence is consistently stronger in insects and fish than in birds and mammals. Our findings resolve apparent inconsistencies between observational and experimental studies of density dependence by revealing that the observational record does indeed contain strong support for the hypothesis that density dependence is widespread in nature.

    更新日期:2020-01-02
  • Spatial covariation of competing species in a fluctuating environment
    Ecology (IF 4.285) Pub Date : 2019-11-19
    Aline Magdalena Lee; Bernt‐Erik Sæther; Steinar Engen

    Understanding how stochastic fluctuations in the environment influence population dynamics is crucial for sustainable management of biological diversity. However, because species do not live in isolation, this requires knowledge of how species interactions influence population dynamics. In addition, spatial processes play an important role in shaping population dynamics. It is therefore important to improve our understanding of how these different factors act together to shape patterns of abundance across space within and among species. Here, we present a new analytical model for understanding patterns of covariation in space between interacting species in a stochastic environment. We show that the correlation between two species in how they experience the same environmental conditions determines how correlated fluctuations in their densities would be in the absence of competition. In other words, without competition, synchrony between the species is driven by the environment, similar to the Moran effect within a species. Competition between the two species causes their abundances to become less positively or more negatively correlated. The same strength of competition has a greater negative effect on the correlation between species when one of them has a more variable growth rate than the other. In addition, dispersal or other movement weakens the effect of competition on the interspecific correlation. Finally, we show that movement increases the distance over which the species are (positively or negatively) correlated, an effect that is stronger when the species are competitors, and that there is a close connection between the spatial scaling of population synchrony within a species and between species. Our results show that the relationships between the different factors influencing interspecific correlations in abundance are not simple linear ones, but this model allows us to disentangle them and predict how they will affect population fluctuations in different situations.

    更新日期:2020-01-02
  • Comparison of distance‐based and model‐based ordinations
    Ecology (IF 4.285) Pub Date : 2019-11-06
    David W. Roberts

    Distance‐based ordinations have played a critical role in community ecology for more than half a century, but are still under active development. These methods employ a matrix of pairwise distances or dissimilarities between sample units, and map sample units from the high‐dimensional distance or dissimilarity space to a low‐dimensional representation for analysis. Distance‐ or dissimilarity‐based methods employ continuum or gradient ecological theory and a variety of statistical models to achieve the mapping. Recently, ecologists have developed model‐based ordinations based on latent vectors and individual species response models. These methods employ the individualistic perspective of Gleason as the ecological model, and Bayesian or maximum‐likelihood methods to estimate the parameters for the low dimensional space represented by the latent vectors. In this research I compared two distance‐based methods (NMDS and t‐SNE) with two model‐based methods (BORAL and REO) on five data sets to determine which methods are superior for (1) extracting meaningful ecological drivers of variability in community composition, and (2) estimating sample unit locations in ordination space that maximize the goodness‐of‐fit of individual species response models to the estimated sample unit locations. Environmental variables and species were fitted to the ordinations by generalized additive models (GAMs) with Gaussian, negative binomial, or Poisson distribution models as appropriate. Across the five data sets, 22 models of environmental variability and 449 models of species distributions were calculated for each of the ordination methods. To minimize the effects of stochasticity the entire analysis was replicated three times and results averaged across the replicates. Results were evaluated by deviance explained and AIC for environmental variables and species distributions, averaged by ordination method for each data set, and ranked from best to worst. For the four assessments distance‐based methods ranked 1 and 2 in three cases, and 1 and 3 in one case, significantly out performing the model‐based methods. t‐SNE was the top‐performing method, out performing NMDS especially on the more complex data sets. In general the gradient‐based theoretical basis and data sufficiency of distance‐based methods allowed distance‐based methods to outperform model‐based methods in every assessment.

    更新日期:2020-01-02
  • Correlative climatic niche models predict real and virtual species distributions equally well
    Ecology (IF 4.285) Pub Date : 2019-11-11
    Valentin Journé; Jean‐Yves Barnagaud; Cyril Bernard; Pierre‐André Crochet; Xavier Morin

    Climate is one of the main factors driving species distributions and global biodiversity patterns. Obtaining accurate predictions of species’ range shifts in response to ongoing climate change has thus become a key issue in ecology and conservation. Correlative species distribution models (cSDMs) have become a prominent tool to this aim in the last decade and have demonstrated good predictive abilities with current conditions, irrespective of the studied taxon. However, cSDMs rely on statistical association between species’ presence and environmental conditions and have rarely been challenged on their actual capacity to reflect causal relationships between species and climate. In this study, we question whether cSDMs can accurately identify if climate and species distributions are causally linked, a prerequisite for accurate prediction of range shift in relation to climate change. We compared the performance of cSDMs in predicting the distributions of 132 European terrestrial species, chosen randomly within five taxonomic groups (three vertebrate groups and two plant groups), and of 1,320 virtual species whose distribution is causally fully independent from climate. We found that (1) for real species, the performance of cSDMs varied principally with range size, rather than with taxonomic groups and (2) cSDMs did not predict the distributions of real species with a greater accuracy than the virtual ones. Our results unambiguously show that the high predictive power of cSDMs can be driven by spatial autocorrelation in climatic and distributional data and does not necessarily reflect causal relationships between climate and species distributions. Thus, high predictive performance of cSDMs does not ensure that they accurately depict the role of climate in shaping species distributions. Our findings therefore call for strong caution when using cSDMs to provide predictions on future range shifts in response to climate change.

    更新日期:2020-01-02
  • Relative prey abundance and predator preference predict individual diet variation in prey‐switching experiments
    Ecology (IF 4.285) Pub Date : 2019-11-11
    Kyle E. Coblentz

    Individual diet specialization appears widespread and has several ecological ramifications. Hypotheses on the causes of diet specialization generally assume prey preferences differ among predator individuals. They then predict how the magnitude of diet variation should change when ecological factors (e.g., intraspecific competition) alter prey abundances. However, the magnitude of diet variation is expected to change with prey abundances due to stochasticity in the foraging process even if all predators share the same prey preferences. Here I show that the relative prey abundance where diet variation is maximized and the magnitudes of diet variation in prey switching experiments are predicted well by a simple stochastic foraging model based only on relative prey abundances and a shared relative prey preference among predators. These results suggest that the effects of stochasticity during foraging may confound studies of individual diet specialization if these effects are not accounted for in experimental design or interpretation. Furthermore, the stochastic foraging model provides simple baseline expectations for theoretical studies on the ecological consequences of diet variation and offers a way forward on quantitative predictions of how ecological factors influence the magnitude of diet variation when stochasticity during foraging and diet specialization occur simultaneously. Last, this study highlights the continued importance of integrating stochasticity into mechanistic ecological hypotheses.

    更新日期:2020-01-02
  • Vascular plant species of the Kingdom of Tonga by vegetation type, species origin, growth form, and dispersal mechanism
    Ecology (IF 4.285) Pub Date : 2019-10-27
    Patricia L. Fall; Taly Dawn Drezner

    The aim of this research is to compile a database of vascular plants found in the Kingdom of Tonga in western Polynesia, a phyto‐geographic subregion of the South Pacific. The Tongan islands are spread over ~600,000 km2 of the Pacific Ocean between 15–23° S latitude and 173–177° W longitude. The archipelago is comprised of 171 islands with an aggregate land area of about 720 km2. Since there is no comprehensive or updated flora for Tonga, we use 143 published sources to compile a database for 1,020 plant species, of which more than 450 are indigenous to these islands. Tonga is noteworthy for its low proportion of endemics, accounting for <5% of the indigenous species and <2% of the total plant species. Our database documents species presence in Tonga as a whole, and more specifically on 11 Tongan islands or island groups. We have assembled ecological information for each plant species according to growth form, vegetation type, origin (endemic, indigenous, and introduced species), and dispersal mechanisms. We include introduced species in our database because they represent over half of the plant species growing in Tonga. Species origins reflect human alteration of Tongan ecosystems in which endemic and indigenous species represent pre‐human vegetation and introduced species indicate plants brought by either Polynesian or European settlers. For example, on Tongatapu, the largest and longest occupied island, more than half the plants are introduced, whereas on the sparsely populated, more remote islands, 70–90% of the species are indigenous. Dispersal mechanisms, which may include more than one mechanism per species, are documented in over 100 publications. Our database provides information on the whole suite of plant dispersal mechanisms over entire communities or island groups in Tonga. Plant species dispersal differs across environmental variables, including island geology, topography, vegetation type, and species origin. The older limestone islands have more bird, water, and human‐dispersed plants, while the youngest volcanic islands have the most wind‐dispersed species. Our database documents plant species endemism, introductions, vegetation types and dispersal mechanisms that reveal key biogeographic dynamics of the Tongan archipelago in the South Pacific. Please cite this Ecology Data Paper if the data are used in publication, presentation, or teaching activities. There are no copyright restrictions.

    更新日期:2020-01-02
  • Social interactions do not drive territory aggregation in a grassland songbird
    Ecology (IF 4.285) Pub Date : 2019-12-31
    S. K. Winnicki; S. M. Munguía; E. J. Williams; W. A. Boyle

    Understanding the drivers of animal distributions is a fundamental goal of ecology and informs habitat management. The costs and benefits of colonial aggregations in animals are well established, but the factors leading to aggregation in territorial animals remain unclear. Territorial animals might aggregate to facilitate social behavior such as (1) group defense from predators and/or parasites, (2) cooperative care of offspring, (3) extra‐pair mating, and/or (4) mitigating costs of extra‐pair mating through kin selection. Using experimental and observational methods, we tested predictions of all four hypotheses in a tallgrass prairie in northeast Kansas, United States. Grasshopper Sparrow (Ammodramus savannarum) males formed clumps of territories in some parts of the site while leaving other apparently suitable areas unoccupied. Despite substantial sampling effort (653 territories and 223 nests), we found no support for any hypothesized social driver of aggregation, nor evidence that aggregation increases nest success. Our results run counter to previous evidence that conspecific interactions shape territory distributions. These results suggest one of the following alternatives: (1) the benefits of aggregation accrue to different life‐history stages, or (2) the benefits of territory aggregation may be too small to detect in short‐term studies and/or the consequences of aggregation are sufficiently temporally and spatially variable that they do not always appear to be locally adaptive, perhaps exacerbated by changing landscape contexts and declining population sizes.

    更新日期:2019-12-31
  • Faster nitrogen cycling and more fungal and root biomass in cold ecosystems under experimental warming: a meta‐analysis
    Ecology (IF 4.285) Pub Date : 2019-12-31
    Alejandro Salazar; Kathrin Rousk; Ingibjörg S. Jónsdóttir; Jean‐Philippe Bellenger; Ólafur S. Andrésson

    Warming can alter the biogeochemistry and ecology of soils. These alterations can be particularly large in high northern latitude ecosystems, which are experiencing the most intense warming globally. In this meta‐analysis, we investigated global trends in how experimental warming is altering the biogeochemistry of the most common limiting nutrient for biological processes in cold ecosystems of high northern latitudes (>50°): nitrogen (N). For comparison, we also analyzed cold ecosystems at intermediate and high southern latitudes. In addition, we examined N‐relevant genes and enzymes, and the abundance of belowground organisms. Together, our findings suggest that warming in cold ecosystems increases N mineralization rates and N2O emissions and does not affect N fixation, at least not in a consistent way across biomes and conditions. Changes in belowground N fluxes caused by warming lead to an accumulation of N in the forms of dissolved organic and root N. These changes seem to be more closely linked to increases in enzyme activity that target relatively labile N sources, than to changes in the abundance of N‐relevant genes (e.g., amoA and nosZ). Finally, our analysis suggests that warming in cold ecosystems leads to an increase in plant roots, fungi, and (likely in an indirect way) fungivores, and does not affect the abundance of archaea, bacteria, or bacterivores. In summary, our findings highlight global trends in the ways warming is altering the biogeochemistry and ecology of soils in cold ecosystems, and provide information that can be valuable for prediction of changes and for management of such ecosystems.

    更新日期:2019-12-31
  • Integrating the underlying structure of stochasticity into community ecology
    Ecology (IF 4.285) Pub Date : 2019-12-26
    Lauren G. Shoemaker; Lauren L. Sullivan; Ian Donohue; Juliano S. Cabral; Ryan J. Williams; Margaret M. Mayfield; Jonathan M. Chase; Chengjin Chu; W. Stanley Harpole; Andreas Huth; Janneke HilleRisLambers; Aubrie R. M. James; Nathan J. B. Kraft; Felix May; Ranjan Muthukrishnan; Sean Satterlee; Franziska Taubert; Xugao Wang; Thorsten Wiegand; Qiang Yang; Karen C. Abbott

    Stochasticity is a core component of ecology, as it underlies key processes that structure and create variability in nature. Despite its fundamental importance in ecological systems, the concept is often treated as synonymous with unpredictability in community ecology, and studies tend to focus on single forms of stochasticity rather than taking a more holistic view. This has led to multiple narratives for how stochasticity mediates community dynamics. Here, we present a framework that describes how different forms of stochasticity (notably demographic and environmental stochasticity) combine to provide underlying and predictable structure in diverse communities. This framework builds on the deep ecological understanding of stochastic processes acting at individual and population levels and in modules of a few interacting species. We support our framework with a mathematical model that we use to synthesize key literature, demonstrating that stochasticity is more than simple uncertainty. Rather, stochasticity has profound and predictable effects on community dynamics that are critical for understanding how diversity is maintained. We propose next steps that ecologists might use to explore the role of stochasticity for structuring communities in theoretical and empirical systems, and thereby enhance our understanding of community dynamics.

    更新日期:2019-12-27
  • Browsing and fire decreases dominance of a resprouting shrub in woody encroached grassland
    Ecology (IF 4.285) Pub Date : 2019-12-26
    Rory C. O’Connor; Jeffrey H. Taylor; Jesse B. Nippert

    North American grasslands have experienced increased relative abundance of shrubs and trees over the last 150 yr. Alterations in herbivore composition, abundance, and grazing pressure along with changes in fire frequency are drivers that can regulate the transition from grassland to shrubland or woodland (a process known as woody encroachment). Historically, North American grasslands had a suite of large herbivores that grazed and/or browsed (i.e., bison, elk, pronghorn, deer), as well as frequent and intense fires. In the tallgrass prairie, many large native ungulates were extirpated by the 1860s, corresponding with increased homesteading (which led to decreased fire frequencies and intensities). Changes in the frequency and intensity of these two drivers (browsing and fire) have coincided with woody encroachment in tallgrass prairie. Within tallgrass prairie, woody encroachment can be categorized in to two groups: non‐resprouting species that can be killed with fire and resprouting species that cannot be killed with fire. Resprouting species require additional active management strategies to decrease abundance and eventually be removed from the ecosystem. In this study, we investigated plant cover, ramet density, and physiological effects of continuous simulated browsing and prescribed fire on Cornus drummondii C.A. Mey, a resprouting clonal native shrub species. Browsing reduced C. drummondii canopy cover and increased grass cover. We also observed decreased ramet density, which allowed for more infilling of grasses. Photosynthetic rates between browsed and unbrowsed control shrubs did not increase in 2015 or 2016. In 2017, photosynthetic rates for browsed shrubs were higher in the unburned site than the unbrowsed control shrubs at the end of the growing season. Additionally, after the prescribed fire, browsed shrubs had ~90% decreased cover, ~50% reduced ramet density, and grass cover increased by ~80%. In the roots of browsed shrubs after the prescribed fire, nonstructural carbohydrates (NSC) experienced a twofold reduction in glucose and a threefold reduction in both sucrose and starch. The combined effects of browsing and fire show strong potential as a successful management tool to decrease the abundance of clonal‐resprouting woody plants in mesic grasslands and illustrate the potential significance of browsers as a key driver in this ecosystem.

    更新日期:2019-12-27
  • Local environmental factors influence beta‐diversity patterns of tropical fish assemblages more than spatial factors
    Ecology (IF 4.285) Pub Date : 2019-12-26
    Edwin O. López‐Delgado; Kirk O. Winemiller; Francisco A. Villa‐Navarro

    A major goal in ecology is to understand mechanisms that influence patterns of biodiversity and community assembly at various spatial and temporal scales. Understanding how community composition is created and maintained also is critical for natural resource management and biological conservation. In this study, we investigated environmental and spatial factors influencing beta diversity of local fish assemblages along the longitudinal gradient of a nearly pristine Neotropical river in the Colombian Llanos. Standardized surveys were conducted during the low‐water season at 34 sites within the Bita River Basin. Physical, chemical, and landscape parameters were recorded at each site, and asymmetric eigenvector maps were used as spatial variables. To examine the relative influence of dispersal and environmental variables on beta diversity and its components, distance‐based redundancy analysis (db‐RDA) and variation partitioning analysis were conducted. We proposed that spatial scale of analysis and position within the river network would constrain patterns of beta diversity in different ways. However, results indicated that in this system, high beta diversity was consistent among species assemblages no matter the scale of analysis or position within the river network. Species replacement (turnover) dominated beta diversity, an indication of the importance of species sorting. These findings suggested that conservation of fish diversity in tropical rivers requires maintenance of both habitat heterogeneity (spatial variation in habitat conditions) and connectivity at the scale of entire river basins.

    更新日期:2019-12-27
  • Signaling from below: rodents select for deeper fruiting truffles with stronger volatile emissions
    Ecology (IF 4.285) Pub Date : 2019-12-24
    Ryan B. Stephens; Amy M. Trowbridge; Andrew P. Ouimette; W. Berk Knighton; Erik A. Hobbie; Paul C. Stoy; Rebecca J. Rowe

    Many plant and fungal species use volatile organic compounds (VOCs) as chemical signals to convey information about the location or quality of their fruits or fruiting bodies to animal dispersers. Identifying the environmental factors and biotic interactions that shape fruit selection by animals is key to understanding the evolutionary processes that underpin chemical signaling. Using four Elaphomyces truffle species, we explored the role of fruiting depth, VOC emissions, and protein content in selection by five rodent species. We used stable isotope analysis of nitrogen (δ15N) in truffles to estimate fruiting depth, proton‐transfer‐reaction mass spectrometry to determine volatile emission composition, and nitrogen concentrations to calculate digestible protein of truffles. We coupled field surveys of truffle availability with truffle spore loads in rodent scat to determine selection by rodents. Despite presumably easier access to the shallow fruiting species, E. americanus (0.5 cm depth) and E. verruculosus (2.5 cm depth), most rodents selected for truffles fruiting deeper in the soil, E. macrosporus (4.1 cm depth) and E. bartlettii (5.0 cm depth). The deeper fruiting species had distinct VOC profiles and produced significantly higher quantities of odiferous compounds. Myodes gapperi (southern red‐backed vole), a fungal specialist, also selected for truffles with high levels of digestible protein, E. verruculosus and E. macrosporus. Our results highlight the importance of chemical signals in truffle selection by rodents and suggest that VOCs are under strong selective pressures relative to protein rewards. Strong chemical signals likely allow detection of truffles deep within the soil and reduce foraging effort by rodents. For rodents that depend on fungi as a major food source, protein content may also be important in selecting truffles.

    更新日期:2019-12-25
  • Scientist’s guide to developing explanatory statistical models using causal analysis principles
    Ecology (IF 4.285) Pub Date : 2019-12-23
    James B. Grace; Kathryn M. Irvine

    Recent discussions of model selection and multimodel inference highlight a general challenge for researchers, which is how to clearly convey the explanatory content of a hypothesized model or set of competing models. The advice from statisticians for scientists employing multimodel inference is to develop a well‐thought‐out set of candidate models for comparison, though precise instructions for how to do that are typically not given. A coherent body of knowledge, which falls under the general term causal analysis, now exists for examining the explanatory scientific content of candidate models. Much of the literature on causal analysis has been recently developed and we suspect may not be familiar to many ecologists. This body of knowledge comprises a set of graphical tools and axiomatic principles to support scientists in their endeavors to create “well‐formed hypotheses”, as statisticians are asking them to do. Causal analysis is complementary to methods such as structural equation modeling, which provides the means for evaluation of proposed hypotheses against data. In this paper, we summarize and illustrate a set of principles that can guide scientists in their quest to develop explanatory hypotheses for evaluation. The principles presented in this paper have the capacity to close the communication gap between statisticians, who urge scientists to develop well‐thought‐out coherent models, and scientists, who would like some practical advice for exactly how to do that.

    更新日期:2019-12-25
  • Soil nitrogen cycling is determined by the competition between mycorrhiza and ammonia‐oxidizing prokaryotes
    Ecology (IF 4.285) Pub Date : 2019-12-23
    Chikae Tatsumi; Takeshi Taniguchi; Sheng Du; Norikazu Yamanaka; Ryunosuke Tateno

    Mycorrhizal fungi have considerable effects on soil carbon (C) storage, as they control the decomposition of soil organic matter (SOM), by modifying the amount of soil nitrogen (N) available for free‐living microbes. Through their access to organic N, ectomycorrhizal (ECM) fungi compete with free‐living soil microbes; this competition is thought to slow down SOM decomposition. However, arbuscular mycorrhizal (AM) fungi cannot decompose SOM, and therefore must wait for N to first be processed by free‐living microbes. It is unclear what form of N the ECM fungi and free‐living microbes compete for, or which microbial groups compete for N with ECM fungi. To investigate this, we focused on the N transformation steps (i.e., the degradation of high‐molecular‐weight organic matter, mineralization, and nitrification) and the microbes driving each step. Simple comparisons between AM forests and ECM forests are not sufficient to assert that mycorrhizal types would determine the N transformation steps in soil, because soil physiochemistry, which strongly affects N transformation steps, differs between the forests. We used an aridity gradient with large differences in soil moisture, pH, and SOM quantity and quality, to distinguish the mycorrhizal and physicochemical effects on N transformation. Soil samples (0–10 cm depth) were collected from AM‐symbiotic black locust forests under three aridity levels, and from ECM‐symbiotic oak forests under two aridity levels. Soil physicochemical properties, extractable N dynamics and abundance, composition, and function of soil microbial communities were measured. In ECM forests, the ammonia‐oxidizing prokaryotic abundance was low, whereas that of ECM fungi was high, resulting in lower nitrate N content than in AM forests. Since ECM forests did not have lower saprotrophic fungal abundance and prokaryotic decompositional activity than the AM forests, the hypothesis that ECM fungi could reduce SOM decay and ammonification by free‐living microbes, might not hold in ECM forests. However, the limitation of ECM fungi on nitrate N production would result in a feedback that will accelerate plant dependence on these fungi, thereby raising soil C storage through an increase in the ECM biomass and plant C investment in soils.

    更新日期:2019-12-25
  • Regional environmental variation and local species interactions influence biogeographic structure on oyster reefs
    Ecology (IF 4.285) Pub Date : 2019-12-20
    Jonathan H. Grabowski; Tarik C. Gouhier; James E. Byers; Luke F. Dodd; A. Randall Hughes; Michael F. Piehler; David L. Kimbro

    Although species interactions are often assumed to be strongest at small spatial scales, they can interact with regional environmental factors to modify food web dynamics across biogeographic scales. The eastern oyster (Crassostrea virginica) is a widespread foundational species of both ecological and economic importance. The oyster and its associated assemblage of fish and macroinvertebrates is an ideal system to investigate how regional differences in environmental variables influence trophic interactions and food web structure. We quantified multiple environmental factors, oyster reef properties, associated species, and trophic guilds on intertidal oyster reefs within 10 estuaries along 900 km of the southeastern United States. Geographical gradients in fall water temperature and mean water depth likely influenced regional (i.e., the northern, central and southern sections of the SAB) variation in oyster reef food web structure. Variation in the biomass of mud crabs, an intermediate predator, was mostly (84.1%) explained by reefs within each site, and did not differ substantially among regions; however, regional variation in the biomass of top predators and of juvenile oysters also contributed to biogeographic variation in food web structure. In particular, region explained almost half (40.2%) of the variation in biomass of predators of blue crab, a top predator that was prevalent only in the central region where water depth was greater. Field experiments revealed that oyster mortality due to predation was greatest in the central region, suggesting spatial variation in the importance of trophic cascades. However, high oyster recruitment in the middle region probably compensates for this enhanced predation, potentially explaining why relatively less variation (17.9%) in oyster cluster biomass was explained by region. Region also explained over half of the variation in biomass of mud crab predators (55.2%), with the southern region containing almost an order of magnitude more biomass than the other two regions. In this region, higher water temperatures in the fall corresponded with higher biomass of fish that consume mud crabs and of fish that consume juvenile and forage fish, whereas biomas of their prey (mud crabs and juvenile and forage fish, respectively) was generally low in the southern region. Collectively, these results show how environmental gradients interact with trophic cascades to structure food webs associated with foundation species across biogeographic regions.

    更新日期:2019-12-21
  • Positive interspecific associations consistent with social information use shape juvenile fish assemblages
    Ecology (IF 4.285) Pub Date : 2019-12-20
    Christopher R. Haak; Francis K. C. Hui; Geoffrey W. Cowles; Andy J. Danylchuk

    Social information obtained from heterospecifics can enhance individual fitness by reducing environmental uncertainty, making it an important driver of mixed‐species grouping behavior. Heterospecific groups are well documented among fishes, yet are notably more prevalent among juveniles than more advanced life stages, implying that the adaptive value of joining other species is greater during this developmental period. We propose this phenomenon can be explained by the heightened ecological relevance of heterospecifically produced cues pertaining to predation risk and or resources, as body‐size uniformity inherent in early ontogeny yields greater overlap in predator and prey guild membership across juveniles of disparate taxa. To evaluate the putative role of information in shaping juvenile fish assemblages, we employed a joint species distribution model (JSDM), identifying nonrandom relationships among fishes collected in 785 seine hauls within the shallow littoral zones of a subtropical island. After accounting for species–environment relationships, which explained 39% of observed covariation in the abundance of 11 taxa, we detected high rates of positive association (84% of significant correlations) predominantly between mutual foraging guild members, consistent with assemblage patterns predicted to evolve under widespread interspecific information use. Affiliations occurred primarily between species characterized by neutral (i.e., noninteracting) or negative (i.e., predator–prey) relationships in later life stages, supporting the notion that heightened niche overlap due to body size homogeneity acted to increase the pertinence of information among juveniles. Taxa exerted varying degrees of influence on assemblage structure; however Eucinostomus spp., a gregarious generalist with exceptional information‐production potential, had an effect several times that of all other species combined, further evidencing the likely role of information in motivating observed relationships. Co‐occurrence and qualitative behavioral data inferred from remote underwater video surveys reinforced these conclusions. Collectively, these results suggest that positive interactions linked to information exchange can be among the principal factors organizing juvenile fish assemblages at local scales, highlighting the role of ontogeny in mediating the relevance and exploitation of information across species.

    更新日期:2019-12-21
  • Facilitation of an invader by a native habitat‐former increases along interacting gradients of environmental stress
    Ecology (IF 4.285) Pub Date : 2019-12-20
    Marc Uyà; Fabio Bulleri; Jeffrey T. Wright; Paul E. Gribben

    Native habitat‐forming species can facilitate invasion by reducing environmental stress or consumer pressure. However, the intensity of one stressor along a local gradient may differ when expanding the scale of observation to encompass major variations in background environmental conditions. In this study, we determined how facilitation of the invasive porcelain crab, Petrolisthes elongatus, by the native tube‐forming serpulid, Galeolaria caespitosa, varied with environmental gradients at local (tidal height) and larger (wave exposure) spatial scales. G. caespitosa constructs a complex calcareous matrix on the underside of intertidal boulders and we predicted that its positive effects on P. elongatus density would increase in intensity with shore height and be stronger at wave‐sheltered than wave‐exposed locations. To test these predictions, we conducted two experiments. First, we determined the effects of serpulid presence (boulders with live or dead serpulid matrix vs bare boulders) at six shore heights that covered the intertidal distribution of P. elongatus. Second, we determined the effects of serpulid presence (present vs absent), shore height (high vs low) and wave exposure (sheltered vs exposed) on crabs across six locations within the invaded range in northern Tasmania, Australia. In Experiment 1, the presence of serpulids (either dead or alive) enhanced P. elongatus densities at all shore heights, with facilitation intensity (as determined by a Relative Interaction Index; RII) tending to increase with shore height. In Experiment 2, serpulids facilitated P. elongatus across shore heights and wave‐exposures, although crab densities were lower at high shore levels of wave‐sheltered locations. However, the intensity of crab facilitation by serpulids was greater on wave‐sheltered than on wave‐exposed shores, but only at the high shore level. This study demonstrates that local effects of native habitat‐formers on invasive species are dependent on prevailing environmental conditions at larger spatial scales and that, under more stressful conditions, invaders become increasingly reliant on positive interactions with native habitat‐formers. Increased strength of local‐scale facilitation by native species, dampening broader scale variations in environmental stressors, could enhance the ability of invasive species to establish self‐sustaining populations in the invaded range.

    更新日期:2019-12-21
  • Computationally efficient joint species distribution modeling of big spatial data
    Ecology (IF 4.285) Pub Date : 2019-12-20
    Gleb Tikhonov; Li Duan; Nerea Abrego; Graeme Newell; Matt White; David Dunson; Otso Ovaskainen

    The ongoing global change and the increased interest in macroecological processes call for the analysis of spatially extensive data on species communities to understand and forecast distributional changes of biodiversity. Recently developed joint species distribution models can deal with numerous species efficiently, while explicitly accounting for spatial structure in the data. However, their applicability is generally limited to relatively small spatial data sets because of their severe computational scaling as the number of spatial locations increases. In this work, we propose a practical alleviation of this scalability constraint for joint species modeling by exploiting two spatial‐statistics techniques that facilitate the analysis of large spatial data sets: Gaussian predictive process and nearest‐neighbor Gaussian process. We devised an efficient Gibbs posterior sampling algorithm for Bayesian model fitting that allows us to analyze community data sets consisting of hundreds of species sampled from up to hundreds of thousands of spatial units. The performance of these methods is demonstrated using an extensive plant data set of 30,955 spatial units as a case study. We provide an implementation of the presented methods as an extension to the hierarchical modeling of species communities framework.

    更新日期:2019-12-20
  • Generalized AIC and chi‐squared statistics for path models consistent with directed acyclic graphs
    Ecology (IF 4.285) Pub Date : 2019-12-19
    Bill Shipley; Jacob C. Douma

    We explain how to obtain a generalized maximum likelihood chi‐square statistic, X2ML and a full‐model Akaike Information Criterion (AIC) statistic for piecewise structural equation modeling (SEM); i.e. structural equations without latent variables whose causal topology can be represented as a directed acyclic graph (DAG). The full piecewise SEM is decomposed into submodels as a Markov network, each of which can have different distributional assumptions or functional links and that can be modeled by any method that produces maximum likelihood parameter estimates. The generalized X2ML is a function of the difference in the maximum likelihoods of the model and its saturated equivalent and the full‐model AIC is calculated by summing the AIC statistics of each of the submodels.

    更新日期:2019-12-20
  • The seasonal energetic landscape of an apex marine carnivore, the polar bear
    Ecology (IF 4.285) Pub Date : 2019-12-18
    Anthony M. Pagano; Todd C. Atwood; George M. Durner; Terrie M. Williams

    Divergent movement strategies have enabled wildlife populations to adapt to environmental change. In recent decades, the Southern Beaufort Sea subpopulation of polar bears (Ursus maritimus) has developed a divergent movement strategy in response to diminishing sea ice where the majority of the subpopulation (73–85%) stays on the sea ice in summer and the remaining bears move to land. Although declines in sea ice are generally considered a challenge to energy balance in polar bears residing in some regions of the Arctic, little quantitative data exists concerning the seasonal energy expenditures of this apex marine carnivore. We used GPS satellite collars with tri‐axial accelerometers and conductivity sensors to measure the location, behavior, and energy expenditure of five adult female polar bears in the southern Beaufort Sea across seasons of sea ice breakup and minimum extent. Using a Bayesian mixed‐effects model, we found that energy expenditure was influenced by month, ocean depth, and habitat type (sea ice or land). Total energy expenditure from May through September ranged from 37.7 – 47.2 mJ kg‐1 for individual bears. Bears that moved to land expended 7% more energy on average from May through September than bears that remained on the receding sea ice. In August, when bears were moving from the sea ice to land or moving north with the receding pack ice, bears that moved to land spent 7% more time swimming and expended 22% more energy. Meaning the immediate cost of moving to land exceeded the cost of remaining on the receding summer pack ice. These findings suggest a physiological reason why the majority of the Southern Beaufort Sea subpopulation continues to inhabit a diminishing summer ice platform. However, bears that moved to land spent 29% more time in preferred hunting habitats over the continental shelf than bears that remained on the sea ice. Bears on land also had access to subsistence‐harvested bowhead whale carcasses. Hence, our findings indicate there may be a greater overall energetic benefit to move to land in this region, which suggests that the use of the diminishing summer sea ice may be functioning as an ecological trap.

    更新日期:2019-12-19
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