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| Aim Whether the gradients of global diversity conform to equilibrium or nonequilibrium
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.
Location Global.
Methods 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.
Results 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 nonequilibrium
factors (evolutionary time to overcome niche conservatism).
Moreover, functional diversity plateaus with species richness, such that speciesrich
regions (especially the Neotropics) host many species that are apparently
functionally redundant.
Main conclusions 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). Together these findings demonstrate how
ecological (the total amount of resources) and evolutionary factors (time to
overcome niche conservatism) might have interacted to generate the striking
diversity of mammals and their life histories. | |
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