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Abstract
J. B. S. Haldane is widely quoted to have quipped that the Creator, if one exists, has an inordinate fondness for beetles. Although Coleoptera may not be the most speciose order once Hymenopteran diversity is fully accounted for, as a whole the very clear differences in species diversity among taxa require an explanation. Here we use stochastic simulations to show that dispersal has eco‐evolutionary effects that predict taxa to become particularly species‐rich when dispersal is neither too low nor too high. Our model combines recent advances in understanding coexistence in niche space with previously verbally expressed ideas, where too low dispersal imposes biogeographic constraints that prevent a lineage from finding new areas to colonize (reducing opportunities for speciation), while too high dispersal impedes population divergence, leading to few but widely distributed species. We show that this logic holds for species richness and is robust to a variety of model assumptions, but peak diversification rate is instead predicted to increase with dispersal. Our work unifies findings of increasing and decreasing effects of dispersal rate on speciation, and explains why taxa with moderate dispersal abilities have the best prospects for high global species richness.
Highlights
Dispersal can influence biodiversity through both ecological and evolutionary processes
In line with the verbal model, we find that species richness is typically maximised at intermediate dispersal (Fig. 3A; parameter values specified in figure captions), a finding that is extraordinarily robust across different modes of resource competition, a wide range of model parameter values (Supplementary material Appendix 1 Fig. A1–A7), and over longer time scales (Supplementary material Appendix 1 Fig. A8)
The pattern for species richness is qualitatively similar for substitutable and non-substitutable resources, but we find that the extent to which potential niches are filled at the global level depends both on dispersal and resource type
Summary
Dispersal can influence biodiversity through both ecological and evolutionary processes. Dispersal can enhance colonization, which increases local diversity if all else is equal (MacArthur and Wilson 1967). All else is typically not equal: increased movement, by increasing contact with similar species, can limit diversity through competitive exclusion (Macarthur and Levins 1967). The outcome can be scale-dependent: theory suggests that regional diversity should decrease with increasing dispersal (Hubbell 2001, Mouquet and Loreau 2003), while local diversity is often maximized at intermediate dispersal (unimodal pattern). Experimental studies that manipulate dispersal have typically found a unimodal relationship between dispersal and diversity maintenance, such that local (Cadotte 2006) or regional
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