This research, in collaboration with Jon Chase at Washington University and Jim Grace at the USGS, investigates the nature of the strong correlation between climate and species richness. Several mechanisms proposed to explain this relationship predict that access or exposre to energy promotes species richness. However, if some kinds of organisms locally out-compete others for available energy, the strength of the energy richness relationship may vary between these groups. In forest communities, trees out-compete herbaceous plants for limiting light resources. Thus, we would expect the energy-richness relationship to vary by growth form. By compiling data about climate, plant growth form, and forest structure and diversity from across the United States, we revealed a previously unidentified macroecological pattern. Namely, terrestrial herbaceous plants exhibit a unique response to regional energy gradients. This result is relevant to several climate-based mechanisms proposed to explain latitudinal gradients in biodiversity.

This research, in collaboration with Anne Greenberg, an undergraduate environmental studies major at Washington University, addresses predictions of an alternative mechanism for explaining the energy richness correlation. The Evolutionary Rates Hypothesis predicts that environmental energy limits rates of molecular evolution, population divergence and ultimately speciation. Therefore diversification should occur more slowly in comparatively low-energy environments, such as the forest understory, or temperate regions. To the test these predictions we are conducting a comparative phylogeography of two widespread plant genera, Mitchella (Rubiaceae) and Liquidambar (Hamamelidaceae). The former is an understory vine and the latter is a canopy tree. Both have similar distributions, with closely related species in temperate regions of Eastern Asia and Eastern North America, with the latter species including certain populations that extend into tropical Mesoamerica. Because of the similarity of their distributions, these genera are taken to be representatives of a widespread ancient forest that became fragmented by geological and paleoclimatic changes since the Tertiary. If levels of exposure to environmental energy have not varied greatly between lineages or regions since that time, the difference in population subdivision between temperate and tropical regions should be greater for the canopy tree, Liquidambar, than for the understory herb, Mitchella. This unique prediction serves to distinguish the effect of the evolutionary rates hypothesis on this system from alterative mechanisms for the latitudinal gradient in biodiversity.

This research was funded by the Environmental Studies Department of Washington University