Effects of global change on savanna grassland ecosystems

Humans are altering the environment locally and globally through climate and land use change. Global temperature is increasing, precipitation patterns are becoming more variable, disturbance regimes are being altered, and ecosystems are being simplified as multi-species communities are replaced by monocultures of crops or livestock. Grasslands, which cover approximately a third of the terrestrial lands, are ecologically and economically significant, thus their responses to environmental change will have dramatic consequences for global patterns of productivity, biodiversity, and food production. This dissertation research investigates how the three main drivers of mesic grassland ecosystems – precipitation, grazing, and fire - interact to affect community composition, structure, and dynamics. I utilized an existing cross-continental study to determine the degree to which mechanisms controlling diversity and dynamics in North American (NA) savanna grasslands apply to Southern African (SA) systems, and vice versa. I conducted my research in two savanna grassland ecosystems: Kruger National Park (Kruger), in northeastern South Africa, and Konza Prairie Biological Station (Konza), in northeastern Kansas, USA. Overall, this dissertation shows that on small scales different grasslands exhibit similar responses to grazing and fire, and to grazing, fire, and drought. However, the generality seen in small-scale responses may not transfer to larger landscape scale processes because patch dynamics within these landscapes are strongly affected by grazing and fire in NA but not in SA. This research also showed that grazing more frequently affected community properties like diversity, cover, and ANPP, while both drought and changes in rainfall variability rarely affected those properties. Instead grasslands responded to changes in rainfall by altering stem densities, and only then in the presence of grazing. As the majority of grasslands globally are grazed, current climate change experiments in grasslands may be underestimating the effects of altered precipitation patterns on the population dynamics of species within these ecosystems.The National Science Foundation, The United States Department of Agriculture, The University of New Mexico Biology Department, The University of New Mexico Office of Graduate StudiesBiologyDoctoralUniversity of New Mexico. Biology Dept.Collins, ScottDahm, CliffordLitvak, MarcyKnapp, Ala