Abstract
Whether the gradients of global diversity conform to equilibrium or non-equilibrium dynamics remains an unresolved question in ecology and evolution. Here, we evaluate four prominent hypotheses which invoke either equilibrium (more individuals, niche diversity) or non-equilibrium dynamics (diversification rate, evolutionary time) to explain species richness and functional diversity of mammals worldwide.
Global. We combine structural equation modelling with simulations to examine whether species richness and functional diversity are in equilibrium with environmental conditions (climate, productivity) or whether they vary with non-equilibrium factors (diversification rates, evolutionary time).
We use the newest and most inclusive phylogenetic, distributional and trait data for mammals. We find that species richness and functional diversity are decoupled across multiple regions of the world.
While species richness correlates closely with environmental conditions, functional diversity depends mostly on non-equilibrium factors (evolutionary time to overcome niche conservatism). Moreover, functional diversity plateaus with species richness, such that species-rich regions (especially the Neotropics) host many species that are apparently functionally redundant.
We conclude that species richness depends on environmental factors while functional diversity depends on the evolutionary history of the region. Our work further challenges the classic notion that highly productive regions host more species because they offer a great diversity of ecological niches.
Instead, they suggest that productive regions offer more resources, which allow more individuals, populations and species to coexist within a region, even when the species are apparently functionally redundant (the more individuals hypothesis).