RESPONSES OF VEGETATION TO A CHANGING REGIME OF DISTURBANCE - EFFECTS OF FERAL PIGS IN A CALIFORNIAN COASTAL PRAIRIE

Disturbance can eliminate sensitive native species and facilitate invasions by exotics, but disturbance is also important in the maintenance of many native-dominated ecosystems. Because of this dual role, disturbance can have complex implications for biodiversity. I have investigated the effects of an introduced agent of disturbance, the feral pig Sus scrofa L., in meadows in northern California. Pigs were the principal agent of soil disturbance at this site, annually overturning an average of 7.4% of the total surface area. Grubbed areas revegetated rapidly, but grubbing had significant effects on the composition of the affected vegetation. Species richness was reduced in grubbed plots in the first year following disturbance, but rose thereafter, often exceeding the richness of undisturbed controls. Disturbance did not exclusively benefit either native or exotic species. Changes in richness primarily reflected the early colonization of disturbed plots by natives, particularly annuals, although alien annual grasses also increased in disturbed sites. Consequently, though non-natives did respond positively to disturbance, at least in the short-term they did not simply replace natives. Pigs' effects may typify the complicated events to be expected when an ecosystem's regime of disturbance is significantly altered, either by direct human intervention or as a consequence of a biological invasion.