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.

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).

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.

Study Area

Santa Cruz Island, located 40 km south of Santa Barbara, is 39 km long and 3-11 km wide (249 km²). An east-west system of interior valleys, dominated by the large Central Valley, bisects the island longitudinally along a geological fault. The major drainage of Santa Cruz Island has its headwaters in Griffith Canyon, on the south slope of Picacho Diablo, the highest peak on the island (750 m), and drains east through the Central Valley. Topography is rugged and slopes may exceed 30°. Climate is an oceanic, Mediterranean type characterized by hot, dry summers and mild, wet winters. Most of the 52 cm average annual precipitation falls from November through April. Vegetation on the island is diverse, with many endemic species (Junak et al. 1995). Grassland, chaparral, oak woodland, and coastal sage scrub communities cover 89% of the island (Minnich 1980).

Sheep ranching began on Santa Cruz Island during the 1850s, but ranching was abandoned and the sheep became feral (Van Vuren and Coblentz 1989). At the time of our study the island supported at least 20,000 sheep (Van Vuren and Coblentz 1989), but density of sheep varied spatially because of past control efforts. A fence extended in a generally east-west direction across the south slope of Picacho Diablo; because of sheep control south of the fence, tributaries entering Griffith Canyon from the south and southwest supported low densities of feral sheep (0.2/ha), but those entering from the north supported densities about 10 times greater (2.1/ha), which is extraordinarily high by any measure (Van Vuren and Coblentz 1989).

Figure 1.

Map of the study area on Santa Cruz Island, showing the six watersheds in Griffith Canyon and the fence that divided areas of high sheep density (north of the fence) and low sheep density (south and southwest of the fence).

Materials and Methods

For our study we selected six tributaries of Griffith Canyon that were similar in physical characteristics (e.g...