Ecological implications of introduced mule deer to island endemic shrubs on Santa Catalina Island, California

Islands commonly harbor unique species that are susceptible to damage by introduced organisms. Historically, no ungulates existed on Santa Catalina Island, but several were introduced including mule deer (Odocoileus hemionus) in 1930.  In this study we evaluated the effects of mule deer browsing on regeneration of island scrub oak (Quercus pacifica). The study included a combination of randomly positioned seedling belt transects, evaluation of diets of mule deer, and sixteen experimental seedling plots that were planted with 100 tree seedlings each in early winter 2005 and then monitored through November 2006. We also examined browse impacts to two species of endemic shrubs; Dendromecon harfordii and Ceanothus arboreus.  Here, we purposely altered two exclosures that were established in 1999, such that several of each species were exposed to browsing (matched fenced and non fenced trees were then monitored for browsing). Mule deer on Catalina Island consume forbs and grasses during the wet season and woody plants including many endemic shrubs during the dry season. Relatively large seedlings are rare across the entire island, potentially related to exposure to nonnative ungulates including bison (Bos bison) and mule deer.  Survival of oak seedlings was negatively linked to trampling by bison and deer, and dense growth of non-native grasses around seedlings.  D. harfordii and C. arboreus trees newly exposed to mule deer were immediately targeted for twig consumption; browsing impacts were dramatic and very few current annual growth twigs remained on exposed trees after 15 months.