Impacts of Feral Livestock on Island Watersheds

Abstract

We assessed the effects of overgrazing by feral sheep on watersheds on Santa Cruz Island, California. Overgrazing had a marked effect on stream flow; flows were much greater in overgrazed than in lightly grazed watersheds early in the rainy season, but the difference vanished later in the season. This pattern can be explained by reduced infiltration and increased surface runoff of rainfall in overgrazed areas. Thus, feral livestock may affect island species not only directly by grazing and trampling, but also indirectly by altering hydrologic processes and therefore species that are dependent on these processes.