Mechanisms of maintenance of species diversity

Peter Chesson. Annual Reviews in Ecology, Evolution, and Systematics 31: 343-366. Mechanisms of maintenance of species diversity

Species' relative abundances fluctuating over time under various models of coexistence. If you just want to think of it as a pretty picture, that's probably OK too. Figure 2 from Chesson (2000).

Species’ relative abundances fluctuating over time under various models of coexistence. If you just want to think of it as a pretty picture, that’s probably OK too; we won’t tell anyone. Figure 2 from Chesson (2000).

Will Pearse

Will Pearse

I’m shocked how many people consider this a classic that “must be read” and yet haven’t read it themselves because it contains “too much math”. I don’t care if you haven’t read this (I’ve not read every paper most people consider a classic), and I think focusing on what people haven’t done is pointless, but I think this is a very readable coverage of what is (otherwise) very difficult math. I do find this field very math-heavy, but with only 9 equations (~1 per 3 pages) this is a really insightful review that for novices like me is helpful.

Chesson very nicely re-phrases coexistence around the ratio of species’ competitive differences and their niche differences; if species are sufficiently different, or compete sufficiently little, they will coexist. This came up an awful lot in the community phylogenetic talks I attended at ESA last year, in part because it torpedoes the assumption made by many that closely related species are more similar to one-another and that’s all that matters for co-existence. I manically circled the idea that species’ niches have an effect (e.g., reducing resources) and a response (e.g., ability to grow given certain resources). Chesson seems desperate for us to stop thinking of a niche as something that is solely a function of a species itself: it’s a function of the context within which we see the species, and that is always shifting. Even when we say a species’ niche involves competing with other species, we’re still missing the key component that the species is using up resources, thus warping the effect-response space that all the species around it experience and themselves modify.

Which sounds a lot like I’m saying the paper is about non-linearities and changes through time; it isn’t, and Chesson very artfully points out that a lot of insight can still be had by setting up simple models in well-considered ways. It is pretty dismissive of Neutral dynamics (…although it was written in 2000!) and the take-home from the section/paragraph “nonequilibrium coexistence” could be paraphrased as “stop using this unhelpful phrase”. I was particularly struck by how Chesson viewed the importance of Neutral Theory for exploring biogeographical dynamics; more than a decade later we’re starting to do this, and we’re even having discussions of how species can evolve, under neutral dynamics, to not be neutral today. This opens a whole can of worms as to what we can usefully call a N/neutral model, but more importantly helps us unify questions at a number of different levels of ecology. Which is a good thing!

Much of what’s in this paper is probably uncontroversial to a community ecologist (right?), but I think remarkably little of it has found its way into the mainstream evolutionary biology literature. I find that interesting because I’m often surprised by how much evolutionary biologists keep track of what ecologists are doing. I can’t think of any serious modelling studies where species’ effect and response traits are seriously modelled across a phylogeny (please correct me!), and I wonder what we would find if we looked.

Lynsey McInnes

Lynsey McInnes

I can’t imagine I am the only PEGE person to have cited this paper on the mechanisms of species’ coexistence without having carefully read it? Its been cited 1694 times! So, choosing it for this week’s PEGE was a great excuse to actually sit down and use some train time to read it through and through.

Now, I have always cited this paper when writing about macroevolution and the idea of equilibrium species numbers and turnover in ‘evolutionary’ time. Oops. Chesson sidelines this view of species’ coexistence early on in his paper and instead focuses throughout on ecological/contemporary notions of species’ coexistence in a – relevantly sized, more or less closed – patch. No matter, I am convinced that most if not all of what he talks about is also relevant or at least interesting for people working at longer timescales.

The problem perhaps is that too many people have jumped on the bandwagon of these ideas being relevant to understanding the build up and maintenance of species diversity that we have become blinkered to the possibility that ecological limits might not be constraining diversity at broad (temporal and spatial) scales. It feels so easy and so neat to extrapolate Chesson’s (and others) equations and explanations of how populations of species manage to stably coexist with each other (a delicate balancing act of getting more intraspecific than interspecific competition, with divergence along at least one relevant niche axis) to how ENTIRE species diversify in the presence of one another until their niche space (in physical or ‘hyper’ space) is full. It is also really easy to obtain patterns, for example in phylogenies, that agree with the idea of diversification slowing as niches fill up.

I have a feeling we are on the cusp of entering a new phase of macroevolutionary analyses where we break ranks with trying to match one for one ecological and evolutionary phenomena. I think it is already much more common to think of what units within species evolve (populations/metapopulations depending on gene flow) and also to think of niches as much more labile (e.g., what about niche construction or extent of niche breadth). Similarly, in both macroecology and macroevolution, biotic interactions are moving from postscript to centre stage as more data becomes available to address the effects of biotic interactions on the patterns we can observe and experimental systems are emerging where these effects can be empirically manipulated.

Turns out no matter how hard I try to leave it behind, I am still a macro-scale biologist at heart and it is fun to pull out the macro-scale implications of more or less any paper that I read.


About will.pearse
Ecology / evolutionary biologist

One Response to Mechanisms of maintenance of species diversity

  1. Pingback: Book review: Community Ecology by Gary Mittelbach, and Community Ecology by Peter Morin | Dynamic Ecology

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