Impacts of Fera! Livestock on Island Watersheds1

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. LIVESTOCK GRAZING CAN have dramatic effects on watershed systems (Mwendera and Saleem 1997). Grazing reduces plant density, ground cover, and litter (Naeth and Chanasyk 1996); when vegetation cover declines, soil bulk density increases and organic matter content and aggregate stability decrease, resulting in decreased rate of water infiltration and increased sediment production (Mwendera and Saleem 1997). Simulations showed that runoff and sediment yield from rainfall increased with increased grazing intensity and reduced ground cover (Johnston 1962). As a result, grazing in many sensitive watersheds is restricted (Holechek et al. 1989). Watersheds on islands may be particularly vulnerable to the effects of grazing, especially by feral livestock. Because island ecosystems typically lack large herbivores, endemic plants lose their defenses against herbivory; consequently, the introduction of livestock can be especially devastating to island vegetation (Bowen and Van Vuren 1997). Further, grazing by feral livestock is often unrestricted on islands, and densities can reach high levels (Coblentz 1978, Van Vuren and Coblentz 1987, Parkes 1993). 1 Manuscript accepted 20 October 2000. 2 Department of Wildlife, Fish, and Conservation Biology, University of California, Davis, California 95616 (E-mail: [email protected]). 3 John Muir Institute of the Environment, University of California, Davis, California 95616. Pacific Science (2001), vol. 55, no. 3:285-289 © 2001 by University of Hawai'i Press All rights reserved Feral sheep and. goats exist on numerous islands worldwide (Rudge 1984). Their impacts on vegetation and soils through grazing and trampling have been documented (Coblentz 1978, Van Vuren and Coblentz 1987), but resultant effects on island watersheds have never been studied. Our purpose in this study was to determine the effects of overgrazing by feral sheep on stream flows in watersheds on Santa Cruz Island, California. We hypothesized that overgrazing would alter stream flows, presumably by increasing surface runoff following major winter storms.