MAMMAL DIVERSITY IN RELATION TO LANDSCAPE HISTORY

One of the fundamental biogeographic patterns in modern ecosystems is the increase in species diversity in topographically complex regions, compared to adjacent lowlands or plains. This diversity gradient is striking across continents today for mammals, birds, and vascular plants. Rodents, which represent over half of modern mammal diversity, manifest this gradient well. In principle, ecological or evolutionary processes that have different consequences in different landscapes could cause this gradient. Steep environmental gradients in topographically complex regions result in more microclimates and habitats than in areas of low relief, and thereby can accommodate more species. Alternatively, speciation rates may be higher in topographically complex regions because of fragmentation of populations across climatic and topographic barriers. The relative contributions of these and other geological, ecological, and evolutionary processes to this diversity gradient invite multidisciplinary analysis that encompasses tectonic and climatic history, paleontology, phylogenetics, phylogeography, and community assembly.

The fossil record of western North America offers a strong contrast in landscape histories and a rich fossil record of rodents. Over the last 25 Ma, rodent diversification histories differed between the tectonically active intermontane west and the Great Plains. The modern gradient of higher diversity in topographically complex regions was strong during the Miocene Climatic Optimum from 17 to 14 Ma, an interval of widespread tectonic activity. Before and after this interval, diversity fluctuated in both regions without a consistent difference. Within smaller tectonic provinces, significant diversification rates were associated with episodes of climatic or tectonic activity. The taxonomic composition and ecological character of rodent assemblages changed across the Neogene, with both pulsed and gradual replacement of archaic rodent groups by members of the dominant modern families. Phylogeographic analyses place the divergence of several basal rodent clades within the Miocene Climatic Optimum. These different approaches imply a strong role for landscape history in the diversification of lineages and the assembly of faunas over geologic time.