Abstract. Biological invasions are a global phenomenon that can accelerate disturbance regimes and facilitate colonization by other nonnative species. In a coastal grassland in northern California, we 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. Our results indicate that pig disturbances had substantial effects on the community, although many responses varied with plant functional group, geographic origin (native vs. exotic), and grassland type. (''Short patches'' were dominated by annual grasses and forbs, whereas ''tall patches'' were dominated by perennial bunchgrasses.) 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, whereas the biomass of exotic forbs increased by 79% with disturbance in tall patches and showed no response in short patches. In contrast to these vegetation results, we found no evidence that pig disturbances affected nitrogen mineralization rates or soil moisture availability. Thus, we hypothesize that the observed vegetation changes were due to space clearing by pigs that provided greater opportunities for colonization and reduced intensity of competition, rather than changes in soil characteristics. In summary, although responses were variable, disturbances by feral pigs generally promoted the continued invasion of this coastal grassland by exotic plant taxa.

Invasive species that increase prevailing disturbance regimes can profoundly alter the composition and structure of ecosystems they invade. Using both comparative and manipulative approaches, we investigated how native and exotic vegetation and soil characteristics at a coastal grassland site in northern California changed through time following disturbances by feral pigs (Sus scrofa). We quantified these successional changes by comparing pig disturbances of varying ages (2, 14, 26+, and 60+ months) during the spring and early summer of 2001. Our results indicate that species richness of native plants increased slowly but steadily through time following disturbances, whereas richness of exotic species rebounded much more rapidly. Percent cover of native perennial grasses also increased steadily through time after pig disturbance, whereas the cover of exotic perennial grasses, annual grasses and forbs initially increased rapidly after disturbance and then remained the same or subsided slightly with time. The cover of native forbs and bulbs either increased weakly through time following disturbance or did not change substantially. Pools of ammonium and nitrate in the soil did not change greatly through time following pig disturbance. Net mineralization rates for ammonium and nitrate also varied little with age since disturbance, although we did find that nitrate mineralization was greater at intermediate ages in one study. Neither organic matter content or particle size varied significantly with disturbance age. In summary, we have shown that native and exotic plants from different functional groups vary greatly in how they recovered from pig disturbances. Exotic taxa were generally able to rapidly colonize and persist in pig disturbances, whereas native taxa usually exhibited a slow but steady rebounding following pig disturbance. Given our results, and those of others from nearby sites, we suggest that the health of coastal grasslands may be enhanced substantially by eliminating or greatly reducing the size of feral pig populations.