Convergent structure of multitrophic communities over three continents

Segar et al. Ecology Letters 16(12): 1436-1445. Convergent structure of multitrophic communities over three continents

Figs and fig wasps. Taken from the excellent figweb site (c) Simon van Noort (Iziko Museums)

Figs and fig wasps. Taken from the figweb site. (c) Simon van Noort (Iziko Museums)

Will Pearse

Will Pearse

Put simply, this paper is excellent. The authors have amassed an impressive dataset, performed a thoughtful and sophisticated analysis, and then explained the whole thing so clearly that it almost sounds easy. I look forward to trying to play around with some of these ideas in other systems!

It seems like there really has been convergence here: distantly related species are doing the same thing as each other in different places. So how the hell did this happen? While many evolutionary biologists I speak to seem to have a pretty good idea what they think convergence is, I think we’re still lacking a formal mechanistic model that can be tested. Yes, we can isolate parts of a phylogeny that looks convergent, but I don’t think we have a model of trait evolution we can use to model this and I’m not sure what it would even look like (what is the opposite of a Brownian walk?). Perhaps convergence happens when there’s insufficient dispersal for pre-adapted species to move in and occupy a particular niche. Perhaps convergence can only happen when there’s sufficient flexibility in a particular trait, thus labile behavioural traits should show more convergence and things like the Baldwin Effect will become important. Maybe there’s something special about fig wasps, and their emergence and mating on the surface of figs (they do that, right?) that makes them more susceptible to all this. Maybe it’s none of these things.

Perhaps the most important limiting factor would be the evolution of the figs themselves; I wonder if the most important methodological advance would be simultaneous evolution of fig and wasp traits, and simultaneous diversification/extinction of both taxa. Obviously work has been done on this already, but I’m talking about a more explicit derivation where, instead of individuals in a population interacting, there are individuals from two separate populations (figs and wasps) interacting according to some fixed set of rules. Thus a particular trait shift in one population would have to be matched by a complimentary shift in the other. I sense the maths would get quite intractable quite quickly (well, it would for me…), but simulation shouldn’t be impossible.

Lynsey McInnes

Lynsey McInnes

To maintain full disclosure, I am about to start collaborating with senior author, James Cook, so it is in my interests here to be constructive and probably err on the side of positivity. That said, I enjoyed this paper a lot! The fig wasp system is inherently cool and I thought the analyses here were exceedingly ambitious.The authors set out to test the relationships among fig wasp communities across three continents. According to measures of phylogenetic and ecological distance, do they follow the ‘inheritance’ (long term co-diversification, same ecological and phylogenetic diversities), ‘convergence’ (same ecological diversities got through different phylogenetic routes) or ‘constraint’ (ecological roles divergent because of constraints on colonisation and/or niche shifts by resident species (meaning phylogenetic diversity also different among communities) hypotheses. They find most support for fig wasp communities being similarly structured through ecological convergence.There are two sides of the fence on which one could sit with regard to this paper. On the one hand, the authors have built up a perhaps overly-complicated methodology in order to demonstrate ecological convergence when one has the feeling they already knew this result would emerge. These are fig wasp experts after all. For instance, the authors could have put the wasps into their guilds without any analyses at all. Similarly, I still don’t fully understand the ins and outs of the PVR and how that setup is able to decompose the variance into ecological, phylogenetic and joint components. I also worry about the low sample sizes and the power of a 35 species family spanning a ton of other wasp species (these qualms might be unfounded, I imagine Will would know).

BUT…my interest lies in rolling out such a methodology more broadly, perhaps to sets of communities with which one has little expertise. Then, for example, an objective way to delimit guilds is vital. And a step by step framework for analysis (the authors’ figure one) is a great tool. My mind is already ticking over to the time when one could stack various cross-continental analyses of community structure across groups into a big metaanalysis. Is convergence the norm? My feeling is such a meta-analysis is a long way off though.

One can imagine also developing the methodology within the fig wasp system (imagine having the data to do this for each of the 750 fig/fig wasp systems) or, as the authors suggest, looking at the structure in different parts of a single fig tree species’ range. I wonder if there are environmental correlates of the different signals?

I also liked a lot that the authors quantified both richness and relative abundance. I liked a lot that they had explanations for the reasons behind the signal of convergence (weird fig traits, niche shifts). I also liked that the authors distinguish constraint vs. convergence and wonder whether convergence ever follows constraint (and whether you could tell?).

I wonder if you could ever roll out these studies in some kind of experimental mesocosm? It would be cool to see the genetic underpinning of the various routes to similarity in community structure and how many replicates would get stuck in some setup due to constraint vs. reach the same ‘end’ due to convergence? You could add in the effect of various historical events (climate change! meteorites!), the possibilities are endless.

One final idea, it would also be interesting to look at multitrophic communities much closer together in space and see how movement across communities affects the patterns observed. Although the authors do suggest that their setup would work best for bounded communities. Hm.

So, yes, a very cool project. Thanks James and co 😉


About will.pearse
Ecology / evolutionary biologist

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