Climate, grazing and invasive plants: Interacting drivers of change in arid grasslands

The consequences of a growing human population increasingly challenge the resilience of native ecosystems. Changes in climate, land-use practices and invasive species can reduce biological diversity, degrade the functioning of ecosystems, and threaten the natural resources upon which humans depend. Drylands, comprising one-third of Earth's terrestrial ecosystems, are productive systems that can rapidly shift into barren lands, often without clear identification of causal factors. I conducted research into common drivers of community change in arid grasslands and place this scientific information in a useful framework for improving management in dryland systems.; In the American Southwest, livestock grazing interacts with climatic variability and invasive species in ways that can force a radical reorganization of ecological communities. In an experimental framework, I studied plant community responses along a gradient of grazing intensity (cattle removal, moderate grazing, and high-impact grazing). In a high-elevation, arid grassland I document strong directional effects of high-impact grazing that led to an increase in exotic species and a 50% decline in native cover. An eight-fold increase in the frequency of the invasive annual grass, Bromus tectorum, in the high-impact treatment followed a severe drought that peaked in the sixth year of the study. Multiple lines of evidence suggest strong grazing-by-climate interactions. In an analysis of resource availability, I found strong support for the supposition that high-impact grazing increases litter cover and soil nitrogen, thereby promoting biological invasion by exotic annual plants. Despite the negative effects of high-impact grazing, the opposite strategy of removing cattle offered little improvement in plant composition and had a negative effect on plant species richness.; The grassland community studied here shows an unexpected level of tolerance to intermediate levels of grazing even during extended periods of drought. The moderate grazing treatment maintained greater native species richness and lower exotic species richness than either cattle removal or high-impact grazing. Furthermore, defoliation of the plant community led to a 30% stimulation of aboveground plant production. The dominant species of this grassland rarely showed negative responses to defoliation, and some of the perennial grasses consistently responded with increased production. These results suggest that sustainable grazing, where economic production does not lessen long-term ecological resilience, may be possible in this dryland system.