Evolution of dispersal strategies and dispersal syndromes in fragmented landscapes

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Cote et al. Evolution of dispersal strategies and dispersal syndromes in fragmented landscapes. Ecography, in press. (Image from http://sarinasunbeam.deviantart.com/art/Seed-Dispersal-Infocomic-606992414)


Lynsey McInnes

Lynsey Bunnefeld

PEGE journal club has morphed into a hybrid in-person/online journal club hosted by the University of Stirling. One half of the PEGE admin has moved to Stirling as a lecturer (me) and is hoping to harness the insights of the department as a whole when discussing matters in the PEGE realm.

We are still straightening out details and may migrate to a new website soon, but in the meantime, a rotating series of bloggers from the Biological & Environmental Sciences department at the University of Stirling will write up a short blog summarising a paper and our discussion every two weeks. As before, we’d be really happy to hear your thoughts on the paper and our interpretation in the comments below. In case you are wondering, Will Pearse is now an assistant prof at Utah State University and we’re even still friends! 

This week, I (Lynsey) chose the paper and committed to writing up our discussion. What follows is my own interpretation of events, apologies if I have misrepresented anything we discussed.

 

I chose a paper by Cote and colleagues from a recent special issue on fragmentation published in Ecography. I was excited about this paper as it promised to integrate three areas of interest of mine: space (fragmentation), intraspecific trait variation (evolving strategies) and species categorisation (dispersal syndromes). However, these grand promises proved problematic. To summarise our discussion: we came out sceptical of the framework proposed by the authors to integrate these three angles; we deemed it infeasible at best and foolish at worse.

Dispersal is a fiendishly difficult phenomenon to get your head around. Do we mean dispersal capacity or propensity? Is a mean or a kernel adequate to categorise the dispersal ‘ability’ of all members of a species? How much intraspecific variation in dispersal ability exists? Is this variance constant? How does it evolve? The authors acknowledge all of these issues and propose to address them head on. They put forward the idea of dispersal syndromes with covarying traits that either enable, enhance or match – the authors thus do not consider dispersal ability as a trait, but rather an emergent feature that comes about as a result of a bunch of possible traits. So far, so interesting.

Where the paper crumbles (for me) is that they go on to overlay the complexity of categorising dispersal syndromes on top of a fragmented landscape. I’m no expert on the process of fragmentation, but I do know it’s a fiendishly complicated topic too. The authors list four ways in which fragmentation modifies a landscape: it reduces habitat quality, increases number of habitat patches, reduces patch size and increases isolation among patches. Each of these four effects are likely to interact with dispersal capacity AND propensity in non-linear ways. And that’s without even considering these effects as selective pressures promoting the evolution of increased or reduced dispersal.

And so we got stuck. We didn’t feel that we have a good grasp (even for a single snapshot of time) of how to adequately characterise dispersal (although we all agreed it was an interesting problem) and so we were hesitant as to the utility of a framework of predicting how dispersal ability (or the traits that covary with it) are likely to interact with or evolve in response to a fragmenting landscape. A pragmatic solution we came up with was to think about holding some variables constant and looking at the evolution of dispersal strategies in those contexts (for example, varying only one of fragmentation’s four effects, not all four).

To conclude, the authors’ aims were admirable, but we were unsure whether we are really in a position to populate their proposed framework at the moment and, even if we were, we were unsure what generalities could emerge: because dispersal ability is a complex phenomenon we were not convinced a framework could be developed that robustly predicts how it might respond and evolve in species found on fragmented landscapes. Are there not too many unknowns and idiosyncracies of species * landscape? Saying that, we would be happy to be proven wrong!

Next week, John Wilson has chosen a recent paper from Ecology by Menzel et al for us to discuss: Mycorrhizal status helps explain invasion success of alien plant species. Join us!

 

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