In a coastal grassland in northern California, Cushman et al. conducted a four-year exclosure experiment to assess the effects of soil disturbances by feral pigs (Sus scrofa) on plant community composition and soil nitrogen availability. The data support the generalization made by other studies; the richness of both native and exotic plant taxa increased in pig-disturbed areas, and exotic richness increased with increasing amounts of disturbance. These plant species are short lived and are especially adept at colonizing openings because they produce abundant amounts of seed that persist in the soil of disturbed areas, also because they disperse into disturbed areas as seed rain from surrounding patches. Soil disturbances by pigs increased the richness of exotic plant species by 29% and native taxa by 24%. Although native perennial grasses were unaffected, disturbances reduced the biomass of exotic perennial grasses by 52% in tall patches and had no effect in short patches. Pig disturbances led to a 69% decrease in biomass of exotic annual grasses in tall patches but caused a 62% increase in short patches. Native, nongrass monocots exhibited the opposite biomass pattern as those seen for exotic annual grasses, with disturbance causing an 80% increase in tall patches and a 56% decrease in short patches. Native forbs were unaffected by disturbance. There was no evidence to support the idea that pig disturbances affected nitrogen mineralization rates or soil moisture availability. Despite the variability documented in this study, they conclude that the overall effect of feral pigs on grasslands will be to promote further invasion of exotic grasses and forbs.
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