Week 5 – The Abundance and Distribution of Species

Week 5 Reading:

Macroecology: Chapter 4

Roy, K. et al. 2009. A macroevolutionary perspective on species range limits. Proc. R. Soc. B 276: 1485-1493.

There are many inherent problems with documenting a species’ range. Primarily, it is hard to visualize on a large scale. However, Dr. Brown points out some general patterns across taxa and space.

Dr. Brown lays out three separate models to explain a species’ distribution: a continuous model in which abundance is greatest in the center of the range, a distribution in which one edge of the range borders a severely unfavorable habitat and abruptly ends, and a distribution in which there are multiple peaks in density due to heterogeneity. It is striking that spatial distributions of taxa across space are so similar. For instance, the example given on pg. 55 (Fig. 4.5) shows that the abundance of a species is orders of magnitude greater in one area of its range and rare towards some edge – and this is repeatable across taxa and remains stable throughout time!

Another pattern Dr. Brown points out is the observation by Charles Darwin that there are few abundant species and many rare species. Again, this pattern is seen within all taxonomic groups. It is noteworthy to point out that the widely distributed species are more numerous in a particular habitat than the species that are well adapted to that habitat. The mechanism proposed by Dr. Brown is that species that become specialist have a trade-off: be highly competitive in one habitat at the loss of tolerating a wider range of abiotic factors. It is curious that this relationship only holds for assemblages of closely related species or those that occupy similar ecological guilds; I would have expected similar species to diverge rather than be conserved.

Roy et al. (2009) proposes that the reason for such conservatism is due to evolutionary history setting range limits for species. In this study, the researchers find an asymmetry between what factors set the northern and southern range limits. Using a macroevolutionary approach, the researchers showed that range limits are most influenced by species richness (number of species within a genus) than by age of a lineage. That is, as the number of species increases within a genus, there is a higher chance of diversifying to new habitats and climatic regimes. They claim that this is due to bivalves restricting their range during times of glaciations (effect of evolutionary history affecting modern species), and then expanding, which resulted in speciation within genera as a means to expand geographic range rather than old clades occupying one continuous range from the tropics to the poles.

Are these results intuitive? How does this paleobiological study compare to modern observations discussed in Macroecology?