Is regional species diversity bounded or unbounded?

Howard V. Cornell, 2013. Biological Reviews 88(1): 140-165. DOI:10.1111/j.1469-185X.2012.00245.x. Is regional species diversity bounded or unbounded?

This post is PEGE’s contribution to the first PEGE/EvoBio journal clubs crossover. Add your comments to the bottom of this post and then come and join us with the guys over at EB-JC (evobiojournalclub.wordpress.com) next Monday (May 13th, 4:30p ET) to discuss things further over video chat.

EB-JC works a bit differently from us so we are keen to join forces and see what happens.

Is speciation rate bounded, unbounded, or... sort of both?

Is speciation rate bounded, unbounded, or… sort of both? (from Cornell 2013)


Will Pearse

Will Pearse

This paper is a brave attempt to reconcile the debate over what limits diversity through evolutionary time. Cornell’s ‘damped increase hypothesis’ is a compromise between a constant (unbounded) rate of speciation and one where clades have a (bounded) carrying capacity determined by ecological interactions. He acknowledges that diversity tends to increase through time, but this increase can be tempered by ecology; those looking for fireworks should look elsewhere (go read ‘that’ Wiens paper), because this is an attempt to reconcile and move forward.

It’s easy(ish) to derive models of evolutionary diversification, the problem is finding reliable data to validate them. You can’t sequence what isn’t here, so molecular phylogenies can’t incorporate extinct species, while the fossil record has biased sampling and makes it difficult to distinguish among species and higher taxonomic groups. Moreover, distinguishing between a clade becoming more diverse because of increased speciation or decreased extinction would be hard even with perfect data. Despite a number of really cool methods (I like these), I sometimes worry we may never be able to sort this mess out.

Much of this paper hinges on whether niche space ever gets filled. If species can fill up niche space, then it’s reasonable to expect that competitive effects would limit diversification, and Cornell’s first conclusion is that we need more experimental tests of whether these kinds of competitive effects exist. That’s not to say we need more research on competition – we have decades of that in ecology – but we need more explicit tests in extant clades that biogeographers are examining. We can’t turn back the clock, but we can validate the assumptions of our models in the present where we’re not data-limited. Now that is eco-evolutionary modelling!

Cornell spends a good deal of time discussing the importance of migration on clade dynamics, and species moving into an area and occupying niches is a rather thorny problem. However, I’m just not sure we’re ever going to be able to handle this; after decades of work, we’re only just beginning to understand how we can model species’ distributions in the present day, and attempting to do that using only biased fossil distributions for all of evolutionary time sounds like an incredibly tall order. Perhaps one thing we could do is look for correlated extinctions/speciations in the fossil record. Imagine a new species has just evolved that occupies a new kind of niche; if we assume it spreads essentially instantaneously in evolutionary time, any slow-down in other clades’ diversification rates should be immediately detectable.


Lynsey McInnes

Lynsey McInnes

This paper has been in my to read pile since I spotted it sometime last year. It probably remained there because it’s long, dense and comprehensive and takes more than a commuter train ride to get through. So when Rafael Maia over at Evo Bio journal club challenged Will and I to find a super cool paper on the geography of speciation, we decided to go with this one as it more or less covers ALL super cool papers on the geography of speciation ever published. So, thanks Howard Cornell for summarized this research field for us and providing us with plenty of food for thought for our first EB/PEGE journal club crossover.

For those of you who know me/have read my posts here at the site, you can guess that I chose this one and that it appeals to my interest in understanding the spatial nature of diversification. First, a note about the style of the paper. I really enjoyed reading it, and managed the whole thing in a single sitting and I think that is down to Cornell’s extremely clear writing style and his total command of the subject he is writing about. Wow. I was also impressed with his ability to navigate through a field that has become quite contentious in recent years; he managed to extract relevant points from a suite of Rabosky & Wiens papers without explicitly acknowledging the animosity apparent between these two camps. In fact, he elegantly concludes that regional diversity is, in effect, both bounded and unbounded and puts forward the damped increase hypothesis to cover this inclusive idea. Hoorah.

Enough rambling, what has Cornell left us to talk about?

First, I really liked his repeated emphasis on taxonomic scale. By this, he meant the conclusions we reach on the diversification of a clade will dependent on the breadth of diversity found within the clade (i.e., a subclade of mammals, a clade restricted to a single region, a clade found in a region with ecologically-similar species from different clades, a clade that has newly-colonised an area, and vice versa). I think this is an underappreciated point within macroevolution and one that deserves more explicit treatment in the future. If we are using a phylogeny to understand diversification of a group, we want to use a well-defined monophyletic clade, maybe we even want to compare two or more than two monophyletic clades. This is fine and admirable, but when doing so, we have to remember that the clade does not exist in isolation (ecologically-(dis)similar species probably occupy the same area, the clade probably included additional taxa that have now gone extinct, the size and nature of the area it currently occupies have probably changed substantially since the origin of the clade, and so and so on). If we really, really want to understand how clades diversify, we have to try really, really hard to account for at least some of the above. At the very least we have to circumscribe what we are trying to understand (How an area became populated? How a clade diversified? How extrinsic/intrinsic traits shape diversity/diversification?)

Years ago, I remember Joaquin Hortal – community ecologist extraordinaire – scoff in the face of the idea of ecological limits to diversity. His reasoning was that he knew of no system that was totally full, there was always scope to squeeze in new species if only the new species could be transported to the area in question. I kinda believed him, but somehow decided that he was talking at some kind of narrower scale than me, that at the continental scale or global scale of entire clade (what is an entire clade?!?!), limits could be reached. I think Cornell’s paper is the first I have read that both implicitly recognises and reconciles these multiple scales that I had in mind. I do wonder if we will ever work out a way to identify the optimal spatial and taxonomic scale to look at how diversity is generated or even if such an optimum exists?

To conclude a bit of a rambley post, I really appreciated this paper. It summarized the weight of evidence for two hypotheses that had somehow recently become quite polarized in the literature and emerged with a happier medium incorporating the best bits of both. Cornell also sets out his vision for making progress in this field and this involves taking a more thoughtful approach to improving our understanding of regional diversity patterns: collect more and better data, incorporate multiple types of data in any analysis, analyse it properly, and perhaps most importantly think about the SCALE of your analysis before you set out and as you interpret your results. Onwards and upwards!

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About will.pearse
Ecology / evolutionary biologist

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