Seasonally dry tropical forests of the Gulf of Mexico: A degraded landscape undergoing homogenization or a promising, resilient reservoir?
Introduction
Forested landscapes around the world are being transformed at alarming rates by different human activities, such as the over-exploitation of forest, the expansion of the agricultural frontier, and urban sprawl, among others (Melo et al., 2013; Banda et al., 2016; Maxwell et al., 2016). As a result, the extent of forest cover has been drastically reduced, impacting biodiversity, impairing its ability to provide ecosystem services (e.g. carbon capture, soil formation) and threatening vital ecological functions, including natural forest regeneration (Schulz et al., 2016, 2018). Pervasive consequences of deforestation and forest fragmentation include the reduction in the effective population size of native species and the local extirpation of sensitive species, while at the same time favoring population increase for the handful of species that are highly tolerant to disturbance (i.e. oligarch or winner species; McKinney and Lockwood, 1999). The latter also leads to a higher degree of similarity in species composition between different forest patches (i.e. loss of β-diversity), a process known as biotic homogenization (Solar et al., 2015).
In highly degraded landscapes, biotic homogenization can bring about changes in vegetation physiognomy (e.g. dominance by shrubs) and in the functional attributes of plant communities (e.g. an increase in the proportion of fast-growing heliophytic species) in just a few tree generations. In these landscapes, remnants of primary or old-growth forest rich in slow-growing tree species are often replaced by secondary forest with impoverished assemblages, thus reducing regional diversity (i.e. gamma diversity; Socolar et al., 2016), and ecosystem resilience (Janzen, 1988; Norden et al., 2009). In the case of seasonally dry tropical forest (SDTF, sensu Pennington et al., 2009; Dirzo, 2011) that has been disturbed, wind-dispersed species may colonize disturbed sites or old-fields much faster and more abundantly than the species that are dependent on frugivorous vertebrates for seed dispersal, and thus could delay or deflect secondary succession (Janzen, 1988). Additionally, in highly fragmented landscapes the opposite process of biotic differentiation may also occur when the agricultural matrix completely isolates forested patches and halts the exchange of individuals (i.e. seeds) between separate patches, thus increasing differences in species composition between patches (Arroyo-Rodríguez et al., 2013; Solar et al., 2015). The resulting increase in β-diversity in landscapes subjected to biotic differentiation may contribute to the maintenance of regional or gamma diversity, even when alpha diversity may be decreasing from the loss of certain species within some patches (see “the dominance of β-diversity hypothesis”; Tscharntke et al., 2012).
Multi-scale studies of the patterns of diversity distribution are essential to understanding the local ecological processes and regional dynamics (e.g. land use changes) that drive the biotic homogenization or differentiation of a landscape. For plants, topographic (e.g. contrasting slopes) and edaphic (e.g. differences in sand content) properties at the local scale are strong determinants of spatial variation in community composition (Méndez-Toribio et al., 2017; Martínez-Camilo et al., 2018), while at the regional scale historic and biogeographic processes such as the presence of barriers (e.g. mountain uplift), which induce spatial gradients or geographic isolation, are usually more important (Almazán-Núñez et al., 2012; Martínez-Camilo et al., 2018). The relative importance of these local and regional factors varies greatly between landscapes and ecosystem types, thus the results of a given site or scale of study may not be applicable to other regions or scales.
The study of diversity distribution patterns is particularly important for seasonally dry ecosystems in the Neotropics, which are found from Mexico to Argentina. These ecosystems are regarded as being among the most threatened in the world, and are also recognized for their rich biodiversity and high proportion of endemic species, and so the regions in which these ecosystems are still present are priority areas for conservation (Dirzo, 2011; Banda et al., 2016). A distinctive feature of SDTF is the presence of several woody species whose life strategies and functional attributes allow them to withstand relatively long periods of drought (5–7 months) and warm temperatures, representing a crucial genetic pool on a warming planet. On the coastal plain of the Gulf of Mexico in the central part of the state of Veracruz, Mexico, there is a seasonally dry tropical region of great interest for its rich plant diversity (Castillo-Campos and Travieso-Bello, 2006; Williams-Linera and Alvarez-Aquino, 2016). However, knowledge of the extent and distribution of current types of land use and vegetation in this region is limited and any understanding of the spatial changes in community composition and its diversity distribution (β-diversity) even more so. Since this region is currently characterized by heavily modified landscapes, it is crucial to gather scientific information on the current state of the remaining native biodiversity and on its spatial distribution in order to develop feasible conservation strategies and secure the provision of ecosystem services by these forests.
The main objective of this study is to assess the spatial variation in plant diversity for the highly fragmented seasonally dry tropical region of the Gulf of Mexico at three different scales: among different forest types, within each forest type (comparing forest patches of the same type), and within each forest patch or fragment. This multi-scale approach was used with the aim of answering the following questions: i) At which spatial scale is floristic differentiation highest?; ii) What is the relative contribution of each vegetation type and sampled forested patch to regional diversity?; and iii) To what extent does this highly disturbed region experience biotic homogenization? Additionally, to gain a better understanding of the drivers of β-diversity we assessed spatial variables (e.g. elevation, distance to the nearest settlement) in the forested patches. To the best of our knowledge this is the first study to describe the variation in β-diversity patterns using a multi-scale approach in a seasonally dry tropical region in the Neotropics that also takes into account the conservation status of the remaining forest patches and their contribution to regional diversity.
Section snippets
Study area
The seasonally dry tropical region of the coastal plain of the Gulf of Mexico is located in the central part of the state of Veracruz, between 19°16' – 19°48′ N and 96°19' – 96°48′ W. Mean annual temperature is 22–26 °C, and mean annual precipitation is 1,200 to 1,500 mm. Precipitation is markedly seasonal, characterized by a five-to eight-month-long dry season (<100 mm per month) and a rainy season (>180 mm/month) from June to September (Travieso-Bello and Campos, 2006). Prior to human
Results
Deforestation within the 1,100 km2 of the study area is pervasive: less than 25% of the area had forest cover in 2014, while 74% was non-forest cover. Just one of all the forest fragments was larger than 500 ha, at 1,905 ha, which includes 700 ha of a private protected area. This, the largest fragment in the area, accounted for 10% of total forest cover. Three quarters (75%) of the forest cover was scattered among more than 500 fragments smaller than 10 ha (NMS in prep.).
Discussion
Neotropical landscapes are heavily threatened by expanding human activities and the seasonally dry tropical region of the coastal plain of the Gulf of Mexico in central Veracruz is no exception. However, in spite of the extensive deforestation and extremely high degree of forest fragmentation in our study area, there is still a remarkably rich assemblage of tree and shrub species growing in the sparse and relatively small forest patches that are immersed within the agricultural matrix, and
Funding information
This study was funded by The Rufford Foundation (Rufford small grant #206761, to NMS); the Instituto de Ecología, A. C. (project INECOL, 20030-10281, to JLD); and the Consejo Nacional de Ciencia y Tecnología (CONACYT, Scholarship #263474, to NMS).
Author contributions
N.M-S, J. L., and F. E. conceived the ideas and hypotheses, and designed the study. N. M-S collected data. N.M-S, J. L., and F. E. conducted statistical analyses and interpretation of the results. N. M-S coordinated the writing of the manuscript. All the authors substantially contributed to the manuscript and agreed with the final version for submission.
Acknowledgments
We are grateful to Victor Vazquez, Noe Cardona, Miguel Ramirez, Evodio Martínez, Tomás Higueredo, and Enrique Romero for their help during the field work and to those in charge of the Amelco UMA; also to René Altamirano, Noé Cardona, and the Higueredo Family for granting us permission to work on their properties. Also, we thank Claudia Gallardo for her help with species identification. Bianca Delfosse revised the English.
References (52)
- et al.
Biotic homogenization: a few winners replacing many losers in the next mass extinction
Trends Ecol. Evol.
(1999) - et al.
On the hope for biodiversity-friendly tropical landscapes
Trends Ecol. Evol.
(2013) - et al.
Grazing deteriorates the soil carbon stocks of Caatinga forest ecosystems in Brazil
For. Ecol. Manag.
(2016) - et al.
How should beta-diversity inform biodiversity conservation?
Trends Ecol. Evol.
(2016) - et al.
Changes in composition, diversity and structure of woody plants in successional stages of tropical dry forest in southwest Mexico
Rev. Mex. Biodivers.
(2012) - et al.
Beta-diversity in seasonally dry tropical forests (SDTF) in the Caatinga Biogeographic Domain, Brazil, and its implications for conservation
Biodivers. Conserv.
(2014) - et al.
Value of small patches in the conservation of plant-species diversity in highly fragmented rainforest
Conserv. Biol.
(2009) - et al.
Plant β-diversity in fragmented rain forests: testing floristic homogenization and differentiation hypotheses
J. Ecol.
(2013) - et al.
Patterns of β-diversity in a Mexican tropical dry forest
J. Veg. Sci.
(2002) - et al.
Plant diversity patterns in neotropical dry forests and their conservation implications
Science
(2016)
Primary and secondary vegetation patches as contributors to floristic diversity in a tropical deciduous forest landscape
Biodivers. Conserv.
A new statistical approach for assessing similarity of species composition with incidence and abundance data: a new statistical approach for assessing similarity
Ecol. Lett.
Rarefaction and extrapolation with Hill numbers: a framework for sampling and estimation in species diversity studies
Ecol. Monogr.
Coverage‐based rarefaction and extrapolation: standardizing samples by completeness rather than size
Ecology
Exploraciones botánicas en Veracruz y estados circunvecinos I. Pisos altitudinales de vegetación en el centro de Veracruz y zonas limítrofes con Puebla
Ciencia y El Hombre
Second Growth: the Promise of Tropical Forest Regeneration in an Age of Deforestation
Rates of change in tree communities of secondary Neotropical forests following major disturbances
Philos. Trans. R. Soc. B.
Resilience of tropical dry forests - a meta-analysis of changes in species diversity and composition during secondary succession
Oikos
Co‐inertia analysis: an alternative method for studying species–environment relationships
Freshw. Biol.
Package “Ade4”
iNEXT: iNterpolation and EXTrapolation for Species Diversity
Instituto nacional de Estadística y geografía
Management of habitat fragments in a tropical dry forest: growth
Ann. Mo. Bot. Gard.
Entropy and diversity
Oikos
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