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

Disturbance can eliminate sensitive native species and facilitate inva sions by exotics, but disturbance is also important in the maintenance of many native-dominated ecosystems. Because of this dual role, distu rbance can have complex implications for biodiversity. I have investig ated the effects of an introduced agent of disturbance, the feral pig Sus scrofa L., in meadows in northern California. Pigs were the princi pal agent of soil disturbance at this site, annually overturning an av erage of 7.4% of the total surface area. Grubbed areas revegetated rap idly, but grubbing had significant effects on the composition of the a ffected vegetation. Species richness was reduced in grubbed plots in t he first year following disturbance, but rose thereafter, often exceed ing the richness of undisturbed controls. Disturbance did not exclusiv ely benefit either native or exotic species. Changes in richness prima rily reflected the early colonization of disturbed plots by natives, p articularly annuals, although alien annual grasses also increased in d isturbed sites. Consequently, though non-natives did respond positivel y to disturbance, at least in the short-term they did not simply repla ce natives. Pigs' effects may typify the complicated events to be expe cted when an ecosystem's regime of disturbance is significantly altere d, either by direct human intervention or as a consequence of a biolog ical invasion.