Variable Effects of Feral Pig Disturbances on Native and Exotic Plants in a California Grassland

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.