Are moderate dispersal abilities best for species richness?

Submitted by editor on 14 January 2020.Get the paper!

Beetles come in all shapes and sizes. [Source: Insects Unlocked project at the University of Texas at Austin].

 

"An inordinate fondness for species with intermediate dispersal abilities" (Ashby et al. 2019)

JBS Haldane, one of the most influential evolutionary biologists of the 20th century, is widely known to have once quipped that “the Creator, if he exists, has an inordinate fondness for beetles”. While the quote in its most succinct form may be apocryphal, Haldane also wrote about this idea, describing what has become a longstanding conundrum in evolutionary biology: why are some groups of species, such as beetles, so much more diverse than others?

It has long been thought that how well or how poorly organisms are able to disperse to new environments is a key driver of speciation, and hence species diversity. Verbal models propose that species richness peaks at intermediate dispersal rates, because too low dispersal constrains the colonization rate of organisms, hence reducing opportunities for speciation, and because too high dispersal leads to so much gene flow between populations that distinct species cannot form. However, when searching through the literature, we were amazed to discover that this intuitively plausible pattern has surprisingly little theoretical backing. We therefore set out to simulate speciation in a metacommunity, to understand the interplay between dispersal and species diversity.

By combining dispersal-speciation theory with recent advances in niche modelling of different resource types (Ashby et al. 2017 Nature Ecology & Evolution), we showed that species richness is indeed typically maximised for intermediate dispersal, and that this occurs across a wide range of parameters and modelling assumptions. This pattern is strongest when the resources that constitute the “niche space” are non-substitutable (e.g. food source and nest sites), as in the animation below.

Species self-organise to occupy non-overlapping niches, leading to highest diversity at intermediate dispersal due to a balance between the colonisation rate and gene flow.

This is because species self-organise into non-overlapping, arbitrary associations between non-substitutable resources, allowing different associations to evolve in each patch. But for this to happen, dispersal must be sufficiently high to ensure multiple patches are colonised, while not being so high as to prevent divergence between patches. Thus, our model both supports the dispersal-speciation hypothesis, while also revealing a new mechanism for generating high species diversity due to competition for non-substitutable resources.

 

Written by: Ben Ashby

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