A functional approach reveals community responses to disturbances

D. Mouillot et al., 2012. Trends in Ecology and Evolution 28(3): 167-177. DOI:10.1016/j.tree.2012.10.004. A functional approach reveals community responses to disturbances

Below, we give our first impressions of this article. Please comment below, or tweet Will or Lynsey (maybe use #pegejc). Think of this as a journal club discussion group!

Will Pearse

Will Pearse

I like the work of David Mouillot and Sébastien Villéger, and in my opinion they shouldn’t need to write a review paper explaining the suite of  approaches they’ve developed, because we should all be using them already. However, we’re not, and so they have, but we have the consolation prize of a really nice summary of some really nice approaches.

I’m what used to be called a community phylogeneticist – I use phylogenies to understand community ecological data. We are often (and rightly) accused of using phylogenetic data when trait data would probably be better for the question at hand – the phylogenetic middleman problem – and this paper describes the kinds of analyses we should do instead. However, we also come under fire because when we assume that phylogenetic overdispersion reflects competition, because it’s hard to map these observed patterns onto mechanisms (yes, I’m citing that paper again). The problem is that this exact critique can be applied to functional trait studies – if I find a community of species that are extremely dissimilar in terms of their functional traits, does that mean similar species have engaged in strong excluding competition, does it mean facilitation of dissimilar individuals is taking place, or does it mean something else? These issues are probably easier to tackle with trait data, but I still get worried when I start mapping trait patterns onto ecosystem processes.

Another thing that always confuses me in functional trait ecology is choice of traits. I don’t mean the boring (but important) “how many, and which, traits do we need to pick” issue, I mean what do we do when there’s variation in how important the traits we’ve collected are? These multidimensional trait spaces, to my mind, implicitly assume that all traits are equally informative. What if xylem diameter really doesn’t matter as much as specific leaf area? Can we weight the dimensions of these functional trait spaces to take this into account? How could we even detect that particular trait axes were more important? I think this is particularly important given we measure traits, in part, on the basis of what’s easy to collect – they’re what we hope are proxies for things we think are important in a system. Presumably someone has thought about this and I’ve just missed that literature – I’d be grateful for any links to papers.

There was very little about intraspecific variation in this paper, but then again it’s simple to incorporate into any of the measures: instead of abundances of each point in trait space, have lots of points (all of them close together, likely) for each measurement of a species. There was also little about choice of null models; this is something that does concern me slightly, because I’m unsure how to tell if we can compare measurements from a five-dimensional space to those from a ten-dimensional space safely. This probably reflects my dodgy maths! Moreover, in many other areas of ecology people get very concerned about the particular null model being used to test data – I just permute species identities for trait studies, and I can’t remember seeing anyone else doing anything else. Is that OK? Are there better ways of disentangling species composition and abundance?

Lynsey McInnes

Lynsey McInnes

Another strange pick from me that fell right into the community phylogeneticist niche that Will occupies. What a gift for him! Anyways, I was drawn to this paper because I like traits and functionality and am dubious (in a largely uninformed way) about most ways of defining communities. This paper kind of covers most of these topics…

I enjoyed this paper, but found it a bit difficult to follow. I think this is mostly due to me joining the discussion without having really thought that much about these ideas and issues beforehand. I liked the links to the niche/neutral debate, especially that the authors refrained from being really shitty about neutral theory and the contribution it can bring to thinking about ecological patterns and processes.

Some thoughts that came to mind…

Data availability/processing. The methods put forward in this paper look data/effort hungry. Which traits to use? How to score them objectively? Collating abundance data? Worrying about missing rare species/traits (although maybe this doesn’t matter)? Worrying about trait x trait interactions? Minefield! But in terms of providing early warning signals for community functioning probably worthwhile?

My next worry is how common is it to have this detailed information on shifting abundances following disturbance in order to benefit from these early warning signals of impending local extinction? Perhaps it is fairly common (or at least feasible) in communities where we try hard to obtain this information. Also (and there probably exists a whole research field on this), maybe we want to be focussed on some emergent properties of communities that can be quantified without monitoring abundances of all species involved, i.e., if functional traits are what is important rather than species diversity or identity then don’t we want some way of measuring community functionality that is not dependent on being able to monitor species’ abundances…

All this emphasis on traits over species diversity made me wonder how these techniques and approaches fit within the whole debate on species identification via traits vs. DNA barcodes? If a species is largely functionally redundant, do we care about its species’ status? I’m being artificially simplistic, of course, and I fully believe there is a place for these functional approaches and DNA barcoding approaches depending on both the aim of the exercise (discovering diversity vs. predicting responses to disturbance being an obvious distinction) and the nature/location of the community.

I’ve always been on the edge of the field of community phylogenetics but never had the guts to dive right in. But it does seem that a big criticism of CP (beyond the maze of dodgy metrics) is the focus on species as the be all and end all of most measures, rather than figuring out how trait differences (big and small) influence the different community structures that we observe (filtered, overdispersed, etc.). CP would probably benefit from incorporating some of this focus on traits, no?

The focus here is clearly on quite rapid timescales but with the increasing recognition that ecological and evolutionary timescales have a big chunk of overlap, I found it interesting that the authors didn’t devote much time to speculating on any possible evolutionary responses of species that decline in abundance. Yep, the scope is probably minimal and the trait values they would ‘need’ in the truncated functional space are likely already ‘taken’, but even so…

And finally, back to what is a community? It would be fun if these kinds of methods could somehow be adapted to identify clusters of function that somehow correspond meaningfully to the abstract idea of a community that most of us already hold in our heads…


About will.pearse
Ecology / evolutionary biologist

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