A new dynamic null model for phylogenetic community structure

Pigot & Eitenne 2015 A new dynamic null model for phylogenetic community structure. Ecology Letters 18(2) 153-163

Figure 2 from Pigot & Etienne. Plots of likelihood of community membership under low (top) and high (bottom) rates of local extinction. Or, if you prefer, a series of variously coloured and not-coloured phylogenies.

Figure 2 from Pigot & Etienne. Plots of likelihood of community membership under low (top) and high (bottom) rates of local extinction.


Will Pearse

As if by magic, the ‘new’ approaches I hoped would appear last post have done so. It’s almost as if I know the posting schedule ahead of time!… Alex Pigot and Rampal Etienne have produced an analytical framework within which we can distingush between speciation, extinction, and colonisation in structuring an assemblage’s phylogenetic structure. Beyond that, they have developed a method that uses phylogeny to its true potential: not a proxy but unique data that helps us estimate evolutionary (speciation and extinction) and ecological (migration) processes of interest.

This is an important contribution because the problem of dispersal is one that has vexed many for some time, yet (with notable exceptions) received relatively little attention. Dispersal from a wider source pool provides an important link between ecological and evolutionary time-scales that we need to model. A species ‘appearing’ can either have an evolutionary (speciation) or ecological (dispersal) origin, and that their DAMOCLES model can at least start getting at that distinction is important. It is of little surprise to anyone who has followed these sorts of studies that phylogenetically overdispersed communities can result from something other than competition, but linking its origin to evolutionary and dispersal events outside a community is interesting.

There are, of course, additional complexities that could be built on top of this model, and I’m not going to bore you by rambling on about traits because I think you all know where we want that to be going. However, I think it’s important to explicitly consider the meta-community (or source pool, if you prefer) from which these species are being drawn. Focusing on one assemblage is useful, but the reality is that speciation and extinction dynamics are happening at biogeographic scales, and we desperately need to link community-scale models such as these with those. Considering multiple assemblages undergoing these kinds of dynamics could be a good place to start. I wonder if the limiting factor may be finding something analytically tractable; while simulating individual communities linked within a wider system is reasonably feasible, doing so analytically (as the authors seem to have started doing) is more difficult.


Lynsey McInnes

Lynsey Bunnefeld

Alex Pigot has a way with null models. He’s already shown that the arc of species’ range size over the course of a species’ lifetime is not necessarily the result of deterministic processes and now he (and Rampal Etienne) have shown that common patterns of community assembly need not be the result of negative biotic interactions if an appropriate null model is used. Wow.

This paper is a great example of a couple of key points that I am most definitely guilty of ignoring. 1. taking time to think whether your null model is biologically as well as statistically null is important. 2. important insights can be made even before your model includes every last contributing factor (see Will’s post above). 3. data examples are important to illustrate your method. Nice.

I’m now going to largely disregard all those things I just said were important and wonder how you might extend this model and wonder what pesky real world effects might topple the null expectation.

I wonder how biotic interactions with non-clade members affect community assembly, i.e., competitors, predators, prey, hosts, etc. I wonder what a null model for this might look like? Should hosts/parasites (for example) evolve in tight coevolution, or not? I wonder what repeat processes of community assembly look like? Always the same, or not (I’m sure this has been treated in microcosms and by Gould). How would phylogenies help with these questions? In the same vein, I wonder how what happens to the new sister species that does not enter the community of his ancestor? I guess I am pondering the effects of space. I have no doubt the authors have too, and would not be surprised if they already have the answers up their sleeve. The authors deal elegantly with variation in a quantitative trait (or traits) meant, I think, the characterise niche. I wonder what happens when you throw in variation in other traits, probably dispersal ability (haha, with all the trauma that goes with defining and measuring that!).

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About will.pearse
Ecology / evolutionary biologist

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