Abundance of feral sheep on SCI in the late nineteenth and early twentieth centuries was at least 50,000 (Van Vuren 1981). Ongoing attempts were made to control or contain the herd with roundups, hunting, and fencing, but these efforts had limited success. The sheep population remained largely unchecked until 1981, and anecdotal information indicates there were substantial fluctuations in their abundance (Van Vuren 1981). There are no data on pig abundance on SCI prior to the 1990s, but ranchers' memoirs provide clear evidence there were years when their population was very high. Rooting was extensive, especially in vineyards and agricultural areas, as well as oak woodlands and grasslands.
The Nature Conservancy (TNC) purchased an interest and a conservation easement in the western 90% of SCI in 1978, and soon began to consider eradication programs for the sheep and pigs (Schuyler 1993). As the initial step in the sheep eradication program they conducted a study of the sheep population ecology (Van Vuren 1981), as well as studies on the effects of overgrazing on the flora (Hochberg et al. 1980), fauna (Laughrin 1982), and soils (Brumbaugh 1980) of the island. TNC also conducted a brief survey of distribution and habitat use by the pigs (Baber 1982). All of the studies documented the enormous effects the sheep were having on the island, including loss of rare plants and vegetation communities, accelerated soil erosion, and reduced populations of some of the native animals (Van Vuren and Coblentz 1987). The strong conclusion from all of the studies was that TNC needed to implement a sheep eradication program as soon as possible. Although TNC felt it was desirable to attempt to eradicate both pigs and sheep simultaneously, the co-owner of the western 90% of the island would consent only to a program for the sheep. TNC began systematic hunting late in 1981 when there were an estimated 20,000 sheep on SCI (Schuyler 1993). By early 1989, 37,171 sheep had been shot off of the western 90% of the island (Schuyler 1993). Between 1500-5000 remained on the eastern 10% of the island through 2001, but systematic hunting and fencing prevented them from re-colonizing the western part of the island. The National Park Service acquired full ownership of the east end of SCI in 1997, and over the next 4 years rounded up and moved the remaining sheep to the mainland.
When the co-owner of the western part of SCI died unexpectedly in December 1987, full ownership of that part of the island passed to TNC. Although they were not prepared to begin a pig eradication program, TNC did decide to remove cattle from the island. Cattle ranching had been the primary land use on the island since the 1930s, with approximately 1800-1900 head occurring over 40-50% of the island during the 1980s. From 1988 to 1989 TNC rounded up all but six of the herd (they were kept for historical purposes) and shipped them to the mainland.
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