The effects of livestock grazing and climate variation on vegetation and grasshopper communities in the northern Chihuahuan Desert

Grasshoppers are important herbivores of North American semi-arid grasslands and shrublands, and vegetation and climate are key factors controlling their species compositions and population dynamics. Domestic livestock grazing is a historic and a current landscape-scale ecological perturbation that has caused reductions of perennial grasses and increases in woody shrubs and weedy annual herbs in desert grassland communities. Climate variation also affects vegetation and grasshopper production, and the combined effects of livestock grazing and climate variation on vegetation and grasshoppers have not been adequately studied in the American Southwest. I measured vegetation and grasshoppers for five years at a series of five semi-arid sites in the northern Chihuahuan Desert to evaluate the interactive effects of short-term livestock grazing and climate variation on plant and grasshopper community structure and species abundances. The study sites ranged from shrub dominated to grass dominated landscapes, with livestock fence lines separating land that was grazed at 30% annual forage utilization, and lands on the other sides of the fences excluded from grazing for at least 20 years. I assigned grasshopper species to life-form guilds based on their ecomorphologies and their microhabitat substrate uses that I observed. A wet spring/dry summer El Niño event occurred at the beginning of the study, and a dry spring/wet summer La Niña event occurred at the end of the study. Livestock grazing changed plant and grasshopper species compositions and abundances significantly during those wet years, further favoring annual forbs, annual grasses and non-graminicole grasshoppers on grazed lands during wet years, while favoring perennial grasses and graminicoles on non-grazed lands also during wet years. The biotic communities at all sites probably supported more perennial grasses and more graminicoles prior to European settlement and livestock grazing that began over a century before this study.