Mountain ranges harbour exceptionally high biodiversity, which is now under threat from rapid environmental change. However, despite decades of effort, the limited availability of data and analytical tools has prevented a robust and truly global characterization of elevational biodiversity gradients and their evolutionary origins. This has hampered a general understanding of the processes involved in the assembly and maintenance of montane communities. Here we show that a worldwide mid-elevation peak in bird richness is driven by wide-ranging species and disappears when we use a subsampling procedure that ensures even species representation in space and facilitates evolutionary interpretation. Instead, richness corrected for range size declines linearly with increasing elevation. We find that the more depauperate assemblages at higher elevations are characterized by higher rates of diversification across all mountain regions, rejecting the idea that lower recent diversification rates are the general cause of less diverse biota. Across all elevations, assemblages on mountains with high rates of past temperature change exhibit more rapid diversification, highlighting the importance of climatic fluctuations in driving the evolutionary dynamics of mountain biodiversity. While different geomorphological and climatic attributes of mountain regions have been pivotal in determining the remarkable richness gradients observed today, our results underscore the role of ongoing and often very recent diversification processes in maintaining the unique and highly adapted biodiversity of higher elevations.
A key question in predicting responses to anthropogenic climate change is: how quickly can species adapt to different climatic conditions? Here, we take a phylogenetic approach to this question. We use 17 time-calibrated phylogenies representing the major tetrapod clades (amphibians, birds, crocodilians, mammals, squamates, turtles) and climatic data from distributions of > 500 extant species. We estimate rates of change based on differences in climatic variables between sister species and estimated times of their splitting. We compare these rates to predicted rates of climate change from 2000 to 2100. Our results are striking: matching projected changes for 2100 would require rates of niche evolution that are > 10,000 times faster than rates typically observed among species, for most variables and clades. Despite many caveats, our results suggest that adaptation to projected changes in the next 100 years would require rates that are largely unprecedented based on observed rates among vertebrate species.
Aim Climatic niche breadth (the range of climatic conditions that a species experiences over space and time) is a fundamental topic in ecology, biogeography and evolution. But what determines the climatic niche width of species? In 1967, Janzen suggested that climatic niche widths for temperature were determined by levels of seasonal fluctuation in temperature at each locality, such that niche breadths are narrow in tropical species and broad in temperate species. However, it is unclear whether climatic niche breadths of species are determined more by seasonal variability within sites as opposed to climatic variation between sites across the species' range. We address this question here.Location Global. MethodsWe analysed three vertebrate clades (plethodontid salamanders, hylid frogs and phrynosomatid lizards) for which we had phylogenetic information and climatic data from localities throughout each species' geographic range, collectively including 409 species. We tested how climatic niche breadths of localities (i.e. temporal variation) are related to overall species climatic niche breadths (i.e. temporal and spatial variation) using phylogenetic comparative methods, focusing both on temperature extremes and precipitation. ResultsAcross the three clades, we find that niche breadths for single localities generally span most of the species' climatic niche breadth, and are strongly correlated with overall species niche breadths. However, species with wider climatic niches also tend to show greater climatic divergence between localities. Main conclusionsThe extent to which the climatic niche breadths of species are determined by variation within localities versus spatial variation between localities has been largely unexplored. Our results suggest that within-locality seasonal variation explains most variation in climatic niche breadths among species. However, between-locality variation and local adaptation may also play some role. These results require more general testing, but have several important implications.
Mountain systems are exceptionally species rich, yet the associated elevational gradients in functional and phylogenetic diversity and their consistency across latitude remain little understood. Here, we document how avian functional and phylogenetic diversity and structure vary along all major elevational gradients worldwide and uncover strong latitudinal differences. Assemblages in warm tropical lowlands and cold temperate highlands are marked by high functional overdispersion and distinctiveness, whereas tropical highlands and temperate lowlands appear strongly functionally clustered and redundant. We additionally find strong geographic variation in the interplay of phylogenetic and functional structure, with strongest deviations between the two in temperate highlands. This latitudinal and elevational variation in assemblage functional structure is underpinned by nuanced shifts in the position, shape and composition of multivariate trait space. We find that, independent of latitude, high‐elevation assemblages emerge as exceptionally susceptible to functional change.
Spatial variation in biodiversity is the result of complex interactions between evolutionary history and ecological factors. Methods in historical biogeography combine phylogenetic information with current species locations to infer the evolutionary history of a clade through space and time. A major limitation of most methods for historical biogeographic inference is the requirement of single locations for terminal lineages, reducing contemporary species geographical ranges to a point in two-dimensional space. In reality, geographic ranges usually show complex geographic patterns, irregular shapes, or discontinuities. In this article, we describe a method for phylogeographic analysis using polygonal species geographic ranges of arbitrary complexity. By integrating the geographic diversification process across species ranges, we provide a method to infer the geographic location of ancestors in a Bayesian framework. By modeling migration conditioned on a phylogenetic tree, this approach permits reconstructing the geographic location of ancestors through time. We apply this new method to the diversification of two neotropical bird genera, Trumpeters (Psophia) and Cinclodes ovenbirds. We demonstrate the usefulness of our method (called rase) in phylogeographic reconstruction of species ancestral locations and contrast our results with previous methods that compel researchers to reduce the distribution of species to one point in space. We discuss model extensions to enable a more general, spatially explicit framework for historical biogeographic analysis.
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