Abstract
Adaptations to different environments between closely related species can be important drivers of reproductive isolation during speciation due to either habitat isolation between species or reduced fitness in hybrids that possess suboptimal adaptations. Hybrid zones are useful natural arenas to explore how ecologically divergent species compete for habitat in sympatry and how possible differences in their habitat use may contribute to the speciation process. We investigated habitat selection by sister species of tree squirrels, the Douglas squirrel and the red squirrel, that have evolved in different forest types in allopatry and hybridize in a transitional forest. We first used genome-wide SNP data and admixture analyses to classify individuals into parental or hybrid classes. Next, we estimated home ranges with radio telemetry data and then used a novel ground-based lidar system to measure forest canopy structure of squirrel home ranges, midden sites, and marginally used forest habitat. We found hybrids consisting of multiple hybrid classes were intermixed with both parental species in the same forest with varying canopy structure complexity. On average, Douglas squirrels utilized forests with slightly greater structural complexity than either red squirrels or hybrids, while marginally used forests were the least structurally complex. Interestingly, hybrid squirrels were not relegated to marginal habitat and were successful in mating among each other and with both parental species. As such, our study suggests that prezygotic-ecological isolation and postzygotic-hybrid infertility, and postzygotic ecological inviability of hybrids are not strong barriers in the speciation process between Douglas squirrels and red squirrels.
Significance statement
Closely related species that become geographically divided often encounter different environments and thus evolve different adaptations. Sometimes, these species meet again and produce hybrids. Hybrids often have lower fitness due to their inferior adaptations. We studied a pair of closely related squirrels that evolved in different environments and meet and produce hybrids in a secondary contact zone. Despite these species evolving in allopatry in very different forest types, they do not select different forest characteristics while in sympatry in the hybrid zone. Furthermore, hybrids do not show major differences in types of habitats that they choose and are capable of defending territories and reproducing among each other and with both parental species. In summary, prezygotic and postzygotic isolating mechanisms associated with habitat selection do not appear to have an important role in promoting species divergence.
Similar content being viewed by others
References
Alexander RR, Shepperd WD (1990) Picea engelmannii Parry ex Engelm. Engelmann spruce. In: Burns RM, Honkala BH (eds) Silvics of North America, vol. 1 Conifers. Forest Service United States Department of Agriculture, Washington DC, pp 187–203
Anderson EC, Thompson EA (2002) A model-based method for identifying species hybrids using multilocus genetic data. Genetics 160:1217–1229
Archibald DW, McAdam AG, Boutin S, Fletcher QE, Humphries MM (2012) Within-season synchrony of a masting conifer enhances seed escape. Am Nat 179:536–544
Atkins JW, Bohrer G, Fahey RT, Hardiman BS, Morin TH, Stovall AE, Zimmerman N, Gough CM (2018) Quantifying vegetation and canopy structural complexity from terrestrial LiDAR data using the FORESTR R package. Methods Ecol Evol 9:2057–2066
Bakker VJ (2006) Microhabitat features influence the movements of red squirrels (Tamiasciurus hudsonicus) on unfamiliar ground. J Mammal 87:124–130
Barton NH, Hewitt GM (1985) Analysis of hybrid zones. Annu Rev Ecol Syst 16:113–148
Björklund M (2019) Be careful with your principal components. Evolution 73:2151–2158
Boutin S, Wauters LA, McAdam AG, Humphries MM, Tosi G, Dhondt AA (2006) Anticipatory reproduction and population growth in seed predators. Science 314:1928–1930
Buchanan JB, Lundquist RW, Aubry KB (1990) Winter populations of Douglas’ squirrels in different-aged Douglas-fir forests. J Wildl Manag 54:577–581
Calenge C (2006) The package “adehabitat” for the R software: a tool for the analysis of space and habitat use by animals. Ecol Model 197:516–519
Chavez AS, Saltzberg CJ, Kenagy GJ (2011) Genetic and phenotypic variation across a hybrid zone between ecologically divergent tree squirrels (Tamiasciurus). Mol Ecol 20:3350–3366
Chavez AS, Maher SP, Arbogast BS, Kenagy GJ (2014) Diversification and gene flow in nascent lineages of island and mainland North American tree squirrels (Tamiasciurus). Evolution 68:1094–1109
Coops NC, Hilker T, Wulder MA, St-Onge B, Newnham G, Siggins A, Trofymow JT (2007) Estimating canopy structure of Douglas-fir forest stands from discrete-return LiDAR. Trees 21:295–310
Coyne JA, Orr HA (2004) Speciation. Sinauer Associates, Sunderland, MA
Danson FM, Morsdorf F, Koetz B (2009) Airborne and terrestrial laser scanning for measuring vegetation canopy structure. In: Heritage G, Large A (eds) Laser scanning for the environmental sciences. John Wiley & Sons, Chichester, pp 201–219
Davies AB, Asner G (2014) Advances in animal ecology from 3D-LiDAR ecosystem mapping. Trends Ecol Evol 29:681–691
Eaton D, Overcast I (2018) ipyrad: interactive assembly and analysis of RAD-seq data sets, https://github.com/dereneaton/ipyrad
Edelman AJ, Koprowski JL (2009) Introduced Abert’s squirrels in the Pinaleño Mountains: a review of their natural history and potential impacts on the red squirrel. In: Sanderson HR, Koprowski JL (eds) The last refuge of the Mt. Graham red squirrel: ecology of endangerment. University of Arizona Press, Tucson, pp 358–376
Elkins EK, Tyers DB, Frisina MR, Rossi JL, Sowell B (2018) Red squirrel (Tamiasciurus hudsonicus) midden site selection and conifer species compositions. Environ Manag Sust Dev 7:15–33
Fahey RT, Atkins JW, Gough CM, Hardiman BS, Nave LE, Tallant JM, Nadehoffer KJ, Vogel C, Scheuermann CM, Stuart-Haëntjens E, Haber LT, Fotis AT, Ricart R, Curtis PS (2019) Defining a spectrum of integrative trait-based vegetation canopy structural types. Ecol Lett 22:2049–2059
Fahey RT, Fotis AT, Woods KD (2015) Quantifying canopy complexity and effects on productivity and resilience in late-successional hemlock–hardwood forests. Ecol Appl 25:834–847
Flaherty S, Patenaude G, Close A, Lurz PWW (2012) The impact of forest stand structure on red squirrel habitat use. Forestry 85:437–444
Fotis AT, Curtis PS (2017) Effects of structural complexity on within-canopy light environments and leaf traits in a northern mixed deciduous forest. Tree Physiol 37:1426–1435
Fotis AT, Morin TH, Fahey RT, Hardiman BS, Bohrer G, Curtis PS (2018) Forest structure in space and time: biotic and abiotic determinants of canopy complexity and their effects on net primary productivity. Agric For Meteorol 250:181–191
Franklin JF, Dyrness CT (1973) Natural vegetation of Oregon and Washington. U.S. Department of Agriculture, Forest Service, Pacific Northwest Research Station, Portland, OR
Franklin JF, Spies TA, Van Pelt R, Carey A, Thornburgh D, Burg DR, Lindenmayer D (2002) Disturbances and the structural development of natural forest ecosystems with some implications for silviculture. Forest Ecol Manag 155:399–423
Frazer GW, Trofymow JA, Lertzman KP (2000) Canopy openness and leaf area in chronosequences of coastal temperate rainforests. Can J For Res 30:239–256
Gurnell J (1987) The natural history of squirrels. Christopher Helm, London
Hamilton RC (1993) Characteristics of old-growth forests in the intermountain region. USDA Forest Service, Ogden, UT
Hardiman BS, Bohrer G, Gough CM, Vogel CS, Curtis PS (2011) The role of canopy structural complexity in wood net primary production of a maturing northern deciduous forest. Ecology 92:1818–1827
Hardiman BS, Gough CM, Halperin A, Hofmeister KL, Nave LE, Bohrer G, Curtis PS (2013) Maintaining high rates of carbon storage in old forests: a mechanism linking canopy structure to forest function. Forest Ecol Manag 298:111–119
Hatten JR (2014) Mapping and monitoring mount Graham red squirrel habitat with Lidar and Landsat imagery. Ecol Model 289:106–123
Hendry AP (2016) Eco-evolutionary dynamics. Princeton University Press, Princeton
Irwin DE (2019) Assortative mating in hybrid zones is remarkably ineffective in promoting speciation. bioRxiv:637678
Koprowski JL (2002) Handling tree squirrels with a safe and efficient restraint. Wildlife Soc B 30:101–103
Korkmaz S, Goksuluk D, Zararsiz G (2014) MVN: an R package for assessing multivariate normality. R J 6:151–162
Larsen KW, Boutin S (1994) Movements, survival, and settlement of red squirrel (Tamiasciurus hudsonicus) offspring. Ecology 75:214–223
Lê S, Josse J, Husson F (2008) FactoMineR: an R package for multivariate analysis. J Stat Softw 25:1–18
Lindsay SL (1982) Systematic relationship of parapatric tree squirrel species (Tamiasciurus) in the Pacific Northwest. Can J Zool 60:2149–2156
Lipshutz SE (2018) Interspecific competition, hybridization, and reproductive isolation in secondary contact: missing perspectives on males and females. Curr Zool 64:75–88
Lowry DB, Rockwood RC, Willis JH (2008) Ecological reproductive isolation of coast and inland races of Mimulus guttatus. Evolution 62:2196–2214
McAdam AG, Boutin S, Sykes AK, Humphries MM (2007) Life histories of female red squirrels and their contributions to population growth and lifetime fitness. Ecoscience 14:362–369
McCune B, Grace JB (2002) Nonmetric multidimensional scaling. Analysis of ecological communities. In: McCune B, Grace JB, Urban DL (eds) Analysis of ecological communities. MjM Software Design, Gleneden Beach, OR, pp 125–142
McKinney ST, Fiedler CE (2010) Tree squirrel habitat selection and predispersal seed predation in a declining subalpine conifer. Oecologia 162:697–707
Merrick MJ, Bertelsen SR, Koprowski JL (2007) Characteristics of Mount Graham red squirrel nest sites in a mixed conifer forest. J Wildl Manag 71:1958–1963
Merrick MJ, Koprowski JL (2016) Evidence of natal habitat preference induction within one habitat type. Proc R Soc B 283:20162106
Merrick MJ, Koprowski JL, Wilcox C (2013) Into the third dimension: benefits of incorporating LiDAR data in wildlife habitat models. In: Gottfried GJ, Ffolliott PF, Gebow BS, Eskew LG, Collins LC (eds) Merging science and management in a rapidly changing world: Biodiversity and management of the Madrean Archipelago III and 7th Conference on Research and Resource Management in the Southwestern Deserts. US Department of Agriculture, Forest Service, Rocky Mountain Research Station, Fort Collins, CO, pp 389–395
Michel P, Jenkins J, Mason N, Dickinson KJM, Jamieson IG (2008) Assessing the ecological application of lasergrammetric techniques to measure fine-scale vegetation structure. Ecol Inform 3:309–320
Nelson R, Keller C, Ratnaswamy M (2005) Locating and estimating the extent of Delmarva fox squirrel habitat using an airborne LiDAR profiler. Remote Sens Environ 96:292–301
Nosil P, Sandoval CP (2008) Ecological niche dimensionality and the evolutionary diversification of stick insects. PLoS One 3:e1907
Nosil P, Vines TH, Funk DJ (2005) Reproductive isolation caused by natural selection against immigrants from divergent habitats. Evolution 59:705–719
Parker GG, Harding DJ, Berger ML (2004) A portable LIDAR system for rapid determination of forest canopy structure. J Appl Ecol 41:755–767
Peterson BK, Weber JN, Kay EH, Fisher HS, Hoekstra HE (2012) Double digest RADseq: an inexpensive method for de novo SNP discovery and genotyping in model and non-model species. PLoS ONE 7:e37135
Pritchard JK, Stephens M, Donnelly P (2000) Inference of population structure using multilocus genotype data. Genetics 155:945–959
R Development Core Team (2018) R: a language and environment for statistical computing. R Foundation for Statistical Computing, Vienna http://www.R-project.org
Ramsey J, Bradshaw HD, Schemske DW (2003) Components of reproductive isolation between the monkeyflowers Mimulus lewisii and M. cardinalis (Phrymaceae). Evolution 57:1520–1534
Rothwell R (1979) Nest sites of red squirrels (Tamiasciurus hudsonicus) in the Laramie Range of southeastern Wyoming. J Mammal 60:404–405
Rusch DA, Reeder WG (1978) Population ecology of Alberta red squirrels. Ecology 59:400–420
Siepielski AM, Benkman CW (2008) A seed predator drives the evolution of a seed dispersal mutualism. Proc R Soc Lond B 275:1917–1925
Sikes RS, Animal Care and Use Committee of the American Society of Mammalogists (2016) 2016 Guidelines of the American Society of Mammalogists for the use of wild mammals in research and education. J Mammal 97:663–688
Simonson WD, Allen HD, Coomes DA (2014) Applications of airborne lidar for the assessment of animal species diversity. Methods Ecol Evol 5:719–729
Smith CC (1968) The adaptive nature of social organization in the genus of three squirrels Tamiasciurus. Ecol Monogr 38:31–64
Smith CC (1970) The coevolution of pine squirrels (Tamiasciurus) and conifers. Ecol Monogr 40:349–371
Smith CC (1981) The indivisible niche of Tamiasciurus: an example of nonpartitioning of resources. Ecol Monogr 51:343–363
Smith AA, Mannan RW (1994) Distinguishing characteristics of Mount Graham red squirrel midden sites. J Wildl Manag 58:437–445
Sobel JM, Chen GF, Watt LR, Schemske DW (2010) The biology of speciation. Evolution 64:295–315
Song B, Chen J, Silbernagel J (2004) Three-dimensional canopy structure of an old-growth Douglas-fir forest. For Sci 50:376–386
Vähä JP, Primmer CR (2006) Efficiency of model-based Bayesian methods for detecting hybrid individuals under different hybridization scenarios and with different numbers of loci. Mol Ecol 15:63–72
Van Pelt R, Nadkarni NM (2004) Horizontal and vertical distribution of canopy structural elements of Pseudotsuga menziesii forests in the Pacific Northwest. For Sci 50:326–341
Vogeler JC, Cohen WB (2016) A review of the role of active remote sensing and data fusion for characterizing forest in wildlife habitat models. Rev Teledetec 45:1–14
Wood DJ, Drake S, Rushton SP, Rautenkranz D, Lurz PW, Koprowski JL (2007) Fine-scale analysis of Mount Graham red squirrel habitat following disturbance. J Wildl Manag 71:2357–2364
Worton BJ (1989) Kernel methods for estimating the utilization distribution in home-range studies. Ecology 70:164–168
Yang X, Schaaf C, Strahler A et al (2013) Study of bat flight behavior by combining thermal image analysis with a LiDAR forest reconstruction. Can J Remote Sens 39:S112–S125
Zimble DA, Evans DL, Carlson GC, Parker RC, Grado SC, Gerard PD (2003) Characterizing vertical forest structure using small-footprint airborne LiDAR. Remote Sens Environ 87:171–182
Zugmeyer CA, Koprowski JL (2009) Habitat selection is unaltered after severe insect infestation: concerns for forest-dependent species. J Mammal 90:175–182
Acknowledgments
We are grateful to S. Doyle, A. Esposto, G. Heyer, N. Israel, D. Kirchmeier, and S. Malinich for field assistance. We thank two anonymous reviewers for useful suggestions on the manuscript. The genomic sequencing was carried by the DNA Technologies and Expression Analysis Cores at the UC Davis Genome Center, supported by NIH Shared Instrumentation Grant 1S10OD010786-01.
Data accessibility
Analyses reported in this article can be reproduced using the data provided by Fotis et al. (2019).
Funding
This research was supported by the startup fund from The Ohio State University for Andreas Chavez.
Author information
Authors and Affiliations
Contributions
ATF and ASC contributed to the conceptualization and study design. ATF performed the LiDAR data collection and ecological analyses and SP conducted the genomic data collection. ASC conducted the home range and admixture analyses. ATF and ASC contributed to the writing and revising of the manuscript.
Corresponding author
Ethics declarations
Ethical approval
All animal handling techniques were approved by The Ohio State University’s IACUC (protocol #2017A00000031) and adhered to the American Society of Mammalogists guidelines for the use of wild mammals in research (Sikes & The Animal Care and Use Committee of the American Society of Mammalogists 2016).
Conflict of interest
The authors declare that they have no conflict of interest.
Additional information
Communicated by A. I Schulte-Hostedde
Publisher’s note
Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
Rights and permissions
About this article
Cite this article
Fotis, A.T., Patel, S. & Chavez, A.S. Habitat-based isolating barriers are not strong in the speciation of ecologically divergent squirrels (Tamiasciurus douglasii and T. hudsonicus). Behav Ecol Sociobiol 74, 32 (2020). https://doi.org/10.1007/s00265-020-2814-5
Received:
Revised:
Accepted:
Published:
DOI: https://doi.org/10.1007/s00265-020-2814-5