Effects of livestock grazing on native and exotic vegetation in vernal pools

California grasslands provide a dramatic example of large-scale conversion from native- to exotic-dominated landscapes. Although livestock grazing has been implicated in this transformation, land managers are now exploring grazing as a tool for increasing native plant diversity in these invaded landscapes. Vernal pools are embedded in some of these grasslands but have remained relatively rich in natives. However, most vernal pools have been destroyed, and those that remain are now threatened by invasion. We used experimental and comparative approaches to evaluate effects of sheep and cattle grazing on vernal pools in Sonoma County, California. Using an exclosure experiment, we examined the influence of spring sheep grazing on 7 pools. Grazing impacts were concentrated in pool bottoms and resulted in small changes in abundance of three exotic plant groups:  decreased annual forbs, and increased perennial forbs and perennial grasses. In a comparative study, we evaluated 45 pools on 4 sites that had been either grazed year-round by cattle or ungrazed for at least 10 years. Grazing-related differences were greatest at pool edges, and grazed pools had greater cover of most native plant groups and greater native species richness. The contrasting results of these two studies reflect the complexity of using livestock to increase native species. Livestock species, grazing regime, and initial community conditions will all influence outcomes of grazing. Nevertheless, for vernal pools with robust native plant populations embedded in exotic-dominated grasslands, carefully-planned livestock grazing may be a useful tool for maintaining and even increasing native plant richness and cover.