Biotic and Abiotic Forcing During the Transition to Modern Grassland Ecosystems: Evolutionary and Ecological Responses of Small Mammal Communities Over the Last 5 Million Years

Abstract

Understanding the origin of modern communities is a fundamental goal of ecology, but reconstructing communities with durations of 103–106 years requires data from the fossil record. Early Pliocene to latest Pleistocene faunas and sediments in the Meade Basin and modern soils and rodents from the same area are used to examine the role of environmental change in the emergence of the modern community. Paleoenvironmental proxies measured on modern surface soils and paleosols are described, and faunal dynamics of fossil rodents are discussed. Mean annual precipitation (MAP) was estimated from elemental concentrations and magnetic properties, and warm-season temperature and δ18O of soil water was estimated using carbonate isotope paleothermometry on pedogenic nodules. MAP and temperature estimates from paleosols exhibit no short-term variability, no long-term trends, and generally bracket modern values. Estimated soil water δ18O values increased through time, suggesting aridification played a role in the evolution of the regional grassland ecosystem. Carbon isotope analyses of biomarkers are used to examine the abundance of C4 grasses, which suggest more C4 biomass and more variability in C4 biomass than carbonate proxies. Rodent species richness remained constant due to balanced rates of extinction and immigration, both of which show episodic spikes consistent with a balance between forcing mechanisms that result in equilibrium on long time scales. Overall, these results suggest that different mechanisms of faunal change may be acting at different time scales, although the stratigraphic resolution of paleoenvironmental proxies needs to be increased, and body size and dietary distributions of rodents need to be determined before which processes of change are most important can be decided.