Community‐level consequences of cattle grazing for an invaded grassland: variable responses of native and exotic vegetation

Abstract

AbstractQuestionsDoes grazing by cattle mediate the composition of a coastal grassland community and do native and exotic taxa from different plant groups, based on life‐history and life‐form characteristics, vary in their response to grazing? To what degree does grazing impact the growth and fecundity ofBromus diandrus, the most dominant exotic grass invader in this system?LocationCoastal grassland/prairie nearCarmel, centralCalifornia,USA.MethodsUsing a 3‐yr exclosure experiment, we evaluated the effects of light winter/spring grazing by cattle (average of 0.75 cow‐calf pairs·ha−1) on species richness and percentage cover of native and exotic plants from different life‐history and life‐form groups.ResultsWe found that cattle grazing reduced total vegetative cover by 25% and increased bare ground by 40%. Grazing also caused a 70% reduction in above‐ground plant biomass and a 40% reduction in the cover of exotic annual grasses, which were by far the most dominant group in this system. Grazing also reduced the height and spikelet production of the exotic annual grassB. diandrus. In contrast, grazing increased the cover of exotic annual forbs by 15%. Contrary to our expectations, grazing did not affect the cover of native forbs and grasses, and we hypothesize that these taxa possess life‐history characteristics and/or depleted seed banks that prevented them from responding to grazing during our experiment. Grazing also did not affect the species richness of any native or exotic plant group.ConclusionsOur 3‐yr exclosure experiment has shown that grazing by cattle caused major changes in a coastal grassland community, with the responses of native and exotic taxa having different life‐history and life‐form characteristics varying markedly. Understanding the responses of these divergent groups is not only important for basic research evaluating the effects of mammalian herbivores on plant communities, but is also critical for developing science‐based approaches for sustainably managing invaded grasslands.