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Forecasting the impacts of climate change on biological diversity requires better ways to incorporate competitive interactions into predictions of species' range dynamics and persistence. This problem has been studied extensively in a different context by theoreticians evaluating the coexistence of species in spatially heterogeneous environments. Here, we show how spatial coexistence theory can be adapted to provide a mathematical framework for understanding species persistence in competitive communities under climate change. We first show how the spatial low-density growth rate provides the relevant metric of species persistence along a climate gradient. We then analyse a model of multiple migrating competitors to show how mechanisms contributing to low-density growth rates quantify the effect of different competitive processes on persistence, and how these processes change in strength with species' asynchronous migration under climate change. Finally, we outline the empirical utility of the framework, showing how the theory can scale up from local measurements of species performance and competitive interactions to range-scale metrics of persistence. Treating species' range dynamics as a geographical-scale coexistence problem presents its own set of challenges, but building from a well-established body of theory may greatly improve the predictability of species persistence in competitive communities under climate change., (© 2018 John Wiley & Sons Ltd/CNRS.)

The tropical forests of Borneo and Amazonia may each contain more tree species diversity in half a square kilometre than do all the temperate forests of Europe, North America, and Asia combined. Biologists have long been fascinated by this disparity, using it to investigate potential drivers of biodiversity. Latitudinal variation in many of these drivers is expected to create geographic differences in ecological and evolutionary processes, and evidence increasingly shows that tropical ecosystems have higher rates of diversification, clade origination, and clade dispersal. However, there is currently no evidence to link gradients in ecological processes within communities at a local scale directly to the geographic gradient in biodiversity. Here, we show geographic variation in the storage effect, an ecological mechanism that reduces the potential for competitive exclusion more strongly in the tropics than it does in temperate and boreal zones, decreasing the ratio of interspecific-to-intraspecific competition by 0.25% for each degree of latitude that an ecosystem is located closer to the Equator. Additionally, we find evidence that latitudinal variation in climate underpins these differences; longer growing seasons in the tropics reduce constraints on the seasonal timing of reproduction, permitting lower recruitment synchrony between species and thereby enhancing niche partitioning through the storage effect. Our results demonstrate that the strength of the storage effect, and therefore its impact on diversity within communities, varies latitudinally in association with climate. This finding highlights the importance of biotic interactions in shaping geographic diversity patterns, and emphasizes the need to understand the mechanisms underpinning ecological processes in greater detail than has previously been appreciated.

Understanding how species diversity is maintained is a foundational problem in ecology and an essential requirement for the discipline to be effective as an applied science. Ecologists' understanding of this problem has rapidly matured, but this has exposed profound uncertainty about the spatial scales required to maintain species diversity. Here we define and develop this frontier by proposing the coexistence-area relationship-a real relationship in nature that can be used to understand the determinants of the scale-dependence of diversity maintenance. The coexistence-area relationship motivates new empirical techniques for addressing important, unresolved problems about the influence of demographic stochasticity, environmental heterogeneity and dispersal on scale-dependent patterns of diversity. In so doing, this framework substantially reframes current approaches to spatial community ecology. Quantifying the spatial scales of species coexistence will permit the next important advance in our understanding of the maintenance of diversity in nature, and should improve the contribution of community ecology to biodiversity conservation.