Abstract
The association between species richness and ecosystem energy availability is one of the major geographic trends in biodiversity. It is often explained in terms of energetic constraints, such that coexistence among competing species is limited in low productivity environments. However, it has proven challenging to reject alternative views, including the null hypothesis that species richness has simply had more time to accumulate in productive regions, and thus the role of energetic constraints in limiting coexistence remains largely unknown. We use the phylogenetic relationships and geographic ranges of sister species (pairs of lineages who are each other’s closest extant relatives) to examine the association between energy availability and coexistence across an entire vertebrate class (Aves). We show that the incidence of coexistence among sister species increases with overall species richness and is elevated in more productive ecosystems, even when accounting for differences in the evolutionary time available for coexistence to occur. Our results indicate that energy availability promotes species coexistence in closely related lineages, providing a key step toward a more mechanistic understanding of the productivity–richness relationship underlying global gradients in biodiversity.
Highlights
The relationship between species richness and energy availability—often described in terms of ecosystem productivity—is widespread yet poorly understood [1,2,3,4,5]
Empirical support for this hypothesis has been lacking, raising the possibility that richness is higher in productive ecosystems for largely historical reasons, including the greater geological age and area of tropical biomes, which increases the period of time available for diversity to accumulate
If most clades originated in the humid tropics, species richness may have had a longer time to accumulate in regions with high energy availability, while strongly conserved physiological constraints have prevented the expansion of species or clades into colder or drier environments with lower productivity [11,14,25,32,33,34]
Summary
The relationship between species richness and energy availability—often described in terms of ecosystem productivity—is widespread yet poorly understood [1,2,3,4,5]. Higher energy fluxes and the corresponding expansion in the breadth and availability of useable resources are expected to reduce the incidence of stochastic population extinction by sustaining a larger total number of individual organisms (the “more individuals” hypothesis) [18,21] and to increase the potential for local niche partitioning required for stable coexistence [22,23,24]. If most clades originated in the humid tropics, species richness may have had a longer time to accumulate in regions with high energy availability, while strongly conserved physiological constraints have prevented the expansion of species or clades into colder or drier environments with lower productivity (the “niche conservatism” hypothesis) [11,14,25,32,33,34]. Rather than reflecting energetic constraints on community assembly, more species may coexist in productive ecosystems for largely historical reasons [2,35]
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