Complex response of vegetation to grazing suggests need for coordinated, landscape-level approaches to grazing management

Management of domesticated ungulates on grasslands has the potential to affect ecosystem function at landscape to global scales. In the southwestern United States, introduction of livestock in the 1800s corresponded with grassland degradation and dramatic shifts in vegetation, including the rapid spread of invasive plant species. In contemporary grasslands, however, evidence increasingly suggests that responsible grazing may enhance plant diversity in the region, though positive effects on diversity may or may not offer corresponding benefits to ecosystem function. Here, we examined the effects of grazing on land cover and functional composition of a semiarid grassland over a 20-year period. We found that high intensity grazing increased exposed soil and shifted community composition toward a greater proportion of annual and exotic species. This was particularly apparent following a severe drought event that initiated a significant loss of perennial plant cover, especially forbs, and was followed by a nearly 4-fold expansion of exotic species. Plots that were grazed at moderate levels consistently exhibited the lowest proportion of exotic species and were similar in functional group composition to exclosure plots. However, moderate grazing did increase soil exposure relative to exclosure plots. These findings suggest that moderate grazing could provide benefits to grassland ecosystem diversity and correlated ecosystem services like invasive species control and pollination services, while simultaneously increasing erosion, reducing water infiltration and altering nutrient cycling, due to increased soil exposure and disturbance. The potential for grazing to exert antagonistic effects on ecosystem services, depending on site conditions and grazing intensity, suggests that livestock management decisions should be tailored to individual management and conservation goals that address the inherent spatiotemporal variability of arid grasslands.