Using livestock to manage plant composition: A meta-analysis of grazing in California Mediterranean grasslands

The use of livestock to manage vegetation composition has become a common element of conservation planning in many regions of the world. Similar to many arid and semi-arid grasslands throughout the world, California grasslands have a history of invasion by non-native grasses and forbs. Attempts to restore native plant populations using managed grazing are common, despite the lack of an overarching quantitative basis for assessing livestock effects on different plant groups. Given the wide range of soils, climate and topography over which grasslands are found, it is important to understand the context-dependency of grazing effects across a region. We performed a meta-analysis of livestock grazing within California grasslands to investigate the response of different plant groups to grazing relative to precipitation, grassland type, soil and grazing regime. We found that exotic forbs showed a dramatic, uniform increase in cover with grazing, but no increase in richness. By contrast, native forbs increased in richness yet their cover response was weak and variable depending on grazing regime and precipitation. Exotic grass cover was unaffected while richness was enhanced by grazing. Native grass cover generally increased with grazing, although the high variation among studies was not predicted by the explanatory variables we evaluated. These results lend support to the use of grazing to enhance native forb richness and native grass cover in some settings although this must be weighed against increases in the cover of exotic forbs.