Long-Term Impacts of Exotic Grazer Removal on Native Shrub Recovery, Santa Cruz Island, California

Abstract

A combination of overgrazing and exotic species introduction has led to the degradation of habitats worldwide. It is often unclear whether removal of exotic ungulates will lead to the natural reestablishment of native plant communities without further management inputs. I describe here my return to sites on Santa Cruz Island, California, 12 years after initial sampling in order to gain a long-term view on native shrub reestablishment into exotic grasslands after exotic grazer removal. Santa Cruz Island was grazed by feral sheep and cattle for over a century; these exotic grazers were removed in the late 1980s and feral pigs were removed in 2005–2006. I resampled 5 sites on south-facing slopes in the Central Valley of the island to quantify native shrub cover, density, and size. Previous data suggested that one species, Eriogonum arborescens, would be able to naturally recruit in exotic grass–dominated areas. Native shrubs have shown a modest increase in cover over time, although more striking was a sharp increase in the amount of dead shrub cover and density. Recruitment events during high rainfall years probably led to the slight increase in Eriogonum cover between sampling periods. Recent drought periods, however, have probably increased mortality and otherwise slowed shrub reestablishment in these arid sites.