Ecological character displacement: glass half full or half empty?

Yoel E. Stuart and Jonathan B. Losos. Trends in Ecology and Evolution 28(7): 402-408. DOI:10.1016/j.tree.2013.02.014. Ecological character displacement: glass half full or half empty?

A glass for the eternal optimist – for sale from ThinkGeek

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Will Pearse

I think I’m not the only one with a slight science-crush on Jonathan Losos, and it’s papers like this that do it. Short, sharp, and to the point. The authors argue that tests of ecological character displacement haven’t been as strict as they should have been, and judge case studies according to the criteria the field itself set.

Let’s briefly cover obvious potential gotchas. These six criteria are well-known (>450 citations, and I’d heard of them), but they’re probably not the only criteria and it might be unfair to judge a field by its adherence to one paper’s suggestion. That said, while you might be able to think of some more (please chime in!), I think they’re all pretty fair and I’d be surprised to receive hate-mail about how dreadful the criteria are.

I think it might be worth reflecting on why we’ve been publishing ever-more-exciting sounding examples of character displacement, instead of actually examining whether the examples we have are definitely character displacement. Cynically, I think we all prefer (and fund) nice shiny new example that look great on the cover of Nature, not the boring follow-ups that fill in the (necessary) details. What’s worse, I think we’re all guilty (to some extent) of confirmation bias, and maybe we don’t want to look too carefully at systems that have earned us front-covers of journals in case we find something we don’t want to see.

But back to the biology. There’s a reason figure 3 shows that the least-confirmed criterion is demonstrated competition in nature: it requires ecological data and ecological fieldwork, both things that evolutionary biologists would probably rather not be doing. The last few decades have seen some amazing increases in statistical firepower in evolutionary biology, in part because we have only so much data and we must soak up every ounce of signal we can. However, ecological data isn’t limited in the same way, and I (and others) seem to think that ecological experiments might be an excellent way to improve our understanding of evolution.

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Lynsey McInnes

It’s hard to argue with the conclusions of this paper. Thoughtful, thorough and interesting, it’s a plea to be a bit less lax when purporting to find evidence for instances or the prevalence of ecological character displacement (ECD). ECD -such a satisfying idea, yet difficult to conclusively demonstrate. Schluter and Mcphail’s six criteria provide a comprehensive ticklist to complete, and appear exceedingly difficult to meet (without a shitload of effort).

But what is the appeal of ECD? It’s an exciting phenomenon, bridging ecology and evolution and providing an interesting explanation for divergence. More interesting, say, than adaptation to different abiotic environments or just some other non-adaptive mechanism of divergence.

And yet maybe ECD has been elevated to too high a status. Maybe it is just one interesting mechanism of adaptive divergence, alongside apparent competition or haphazard adaptation to available niches, or some other mechanism and it has been credited with undue (and certainly undemonstrated) importance?

Anoher thing I noticed: studies that meet all six criteria are from well studied systems, sticklebacks, finches, anoles, etc. If other studies had similar amounts of time devoted to them would the other criteria have been met? I didn’t check whether not meeting them equated to them not having been tested for or them actually failing to be met?

The authors highlight the idea that climate change and invasive species are now providing great conditions to witness evolution in real time and thus to test for instances of ECD, as novel communities are brought together providing opportunities or competition for resources and character displacement. Indeed this seems like an opportunity too good to miss, but will nonetheless require careful delineation of what responses are expected and high levels of study to dismiss alternative mechanisms.

I also wonder how ECD fits in with the current trend to look for niche conservatism and/or niche evolution in every clade of organisms. If a clade shows niche conservatism along some environmental axis, do they often also show ECD along some complementary axis? Perhaps we will be understand diversification if researchers in the different camps talked more to one another and there was a better integration of the potential effects of various abiotic and biotic factors.