Effects of Wild Pigs on Seedling Survival in California Oak Woodlands

Wild pigs were established around coastal Spanish settlements in California in the 1800s and expanded over the last century by hunting introductions, domestic releases, and natural dispersal. The current distribution of wild pigs is closely associated with oak woodlands where foraging and rooting by the species may impinge on many native species. Rooting disturbance by wild pigs may be especially problematic for regeneration of oak woodlands by physical damage to tree seedlings. In 1998 we initiated a long-term study of the ecological effects of wild pigs in California oak woodlands using multiple exclosures paired with nearby control plots that were established in two state parks in the north and central coast region of California, Austin Creek State Recreation Area and Henry Coe State Park. Mesh size and height of exclosure fences were designed to repel wild pigs but allow access by other organisms. We were especially interested in rooting effects on oak seedling number and size. The focus period for the 8-year study ended in May 2005, during which we monitored seedlings in (1) large 50 X 50 m exclosure and control habitat plot pairs centered in oak woodland, and (2) smaller 3 X 3 m exclosure and control canopy plot pairs established beneath the canopies of individual trees producing large crops of acorns in fall 1998 and 1999. In May 2005 the differential for seedling number in exclosure compared to control plots exceeded 400 percent at Austin Creek SRA. An outbreak of tent caterpillars limited seedling production at Henry Coe SP prior to 2003. Nevertheless, mean seedling height was significantly taller for all exclosure plots at both research sites. Rooting was reduced at Henry Coe SP after a control program was initiated in 2002, indicating that such approaches can improve the prospects for regeneration of oak woodlands in California.