Sunday, July 20, 2008

Do Mammal Species Avoid Living with Their Relatives?

As the reconstructed tree of life grows, phylogeneticists increasingly find themselves revisiting classic questions in ecology and evolutionary biology. Among the most interesting of these questions involve evolution's contribution to the composition of ecological communities. Two studies published a few months ago use the recently-completed mammalian supertree to ask whether communities tend to consist of particularly distantly-related species. This pattern of "phylogenetic overdispersion" has long been considered a symptom of strong competitive interactions among closely-related -- and presumably ecologically similar -- species.

Cardillo et al. (2008) address overdispersion by asking whether a global dataset of island assemblages tend to consist of phylogenetically overdispersed samples from their putative source populations. Although a few communities exhibited overdispersion, the overwhelming majority of their samples consisted of random samples (see figure). A subsequent study by Cooper et al. (2008) [subscription required] took a closer look at monkey, squirrel, and possum communities over large geographic regions. They found evidence for overdispersion in five of eight analyses of pooled samples, but never in individual communities.

The absence of general conclusions from these studies mirrors earlier results from studies of plant communities, where both overdispersion and its converse (i.e., phylogenetic clustering) have been recorded (see papers by Silvertown, Caveder-Bares, etc). Together these observations suggest that -- at least for the time being -- the phylogenetic contribution to community composition must be evaluated on a case by case basis. Perhaps more importantly, the inability of broad scale analyses to distinguish among the many possible alternative explanations for the diverse patterns they observe should be seen as a call for detailed, biological-informed studies of specific examples.

Cardillo, M., J. L. Gittleman, and A. Purvis. 2008. Global patterns in the phylogenetic structure of island mammal assemblages. Proc. Royal Soc. B 275:1549-1556.

Cooper, N., J. Rodriguez, and A. Purvis. 2008. A common tendency for phylogenetic overdispersion in mammalian assemblages. Proc. Royal Soc. B 275:2031-2037.


Luke J. Harmon said...

I think the key issue here is that it's not the trees that matter, just the traits. Phylogenetic similarity is just not a very good predictor of trait (or ecological) similarity, especially at deep levels.

Glor said...

Good point. Assumptions about "niche conservatism" and "phylogenetic signal" are key to making and testing phylogenetic predictions about community structure, but the conclusions about those patterns also tend to be clade or scale specific. This is something the mammal studies consider, but aren't able to address in detail due to the breadth of their analyses. Of course, geographic and taxonomic scales of analyses have already been considered as potential explanations for the lack of generality in the plant studies as well.