Chapter 7: Soil Formation on Santa Cruz Island: Influences of Livestock on a Developing Landscape

Bridget Tracy & Aaron King

Soils on Santa Cruz Island (SCI) have developed for millennia on steep slopes in the presence of intense periods of wind and rain. These factors contributed to soil development on the island and built up a landscape that housed pine and oak woodland, shrubs and grassland. The natural processes of weathering and slope failures occurred throughout the island’s history and created a dynamic backdrop for the evolution of its ecosystem. In the 1800s, ranchers introduced sheep and other non-native grazers to SCI. As the populations of these grazers exploded over the next century, they wreaked havoc on the landscape. Large feral herds denuded hillslopes and permanently removed much of the native vegetation, creating an unstable landscape populated with non-native annual grasses. Increased incidence of shallow soil slips as well as gullying and deep-seated landslides resulted. The destructive influence of these grazers was undeniable and by 1978 the Nature Conservancy bought 90% of the island with the goal of restoring a more natural landscape. The National Park Service purchased the remaining land and in 2000 all the sheep were removed from SCI. Studies have shown that sheep removal has reduced slope failure and increased vegetative cover. These studies provide hope that with time SCI can return to its historic regime of soil and landscape formation.