Biogeography and traits

A map of climates in the Cape Floristic region and how they relate to shoot design for 88 species of Leucadendron.

Research on species traits generally follows one of three avenues: traits are related to other traits, environmental variables where the plants were measured, or phylogenetic relationships.

Although we've made good strides over the past decade in understanding trait relationships, relationships with environmental gradients, and understanding the distribution of traits over phylogenetic space, we have progressed little in understanding how traits affect the distribution and abundance of species. A fourth road is just as important, but less frequently traveled: biogeographic distributions of species.

As far as I can tell, there is really only one paper to attempt this. Thuiller et al. (2004) measured traits on 88 species of Leucadendron (proteas), and then derived the biogeographic distribution of each species. With these data, they generated mean climate parameters for their occurrence.

The simple pairings led to some novel insights. First, species in drier, more Mediterranean regions had smaller leaves and smaller stems, but also smaller cones. This might not be a surprise to many, but sometimes we need ways to quantify the obvious. More interesting, species in more arid regions had narrower niche breadths and were more likely to have seeds dispersed by wind, rather than by ants, which were more prevalent in the more subtropical, continental climates.

In all, I'd have to add this paper to my list of needle-in-haystack papers, but it's actually been cited relatively well (42 cites since 2004), more than my arbitrary cap of 5 citations a year. That said, the specific approach hasn't penetrated trait research. As a consequence, research on trait distributions have little mechanistic underpinnings. The Thuiller paper is a good model for future research that hasn't caught on yet.

Thuiller, W., S. Lavorel, G. Midgley, S. Lavergne, and T. Rebelo. 2004. Relating plant traits and species distributions along bioclimatic gradients for 88 Leucadendron taxa. Ecology 85:1688-1699.