Foraging Preference by Wild Deer on Toyon Populations from Santa Catalina Island versus the Santa Monica Mountains

Post-fire regeneration of Toyon (Heteromeles arbutifolia), a chaparral shrub on Santa Catalina Island is impeded by mule deer (Odocoileus hemionus), a non-native species introduced during the 1930’s. Mainland and island populations of Toyon were examined for potential differences in defense mechanisms that act as natural deterrents to over-browsing by ungulate herbivores, with the hypothesis that island populations, void of natural ungulate herbivores, may lack such defense mechanisms. Several characteristics of leaf morphology, including number and length of spines, leaf area and depth, and tensile strength of leaves, were compared between island and mainland populations. Leaves of Toyon from Santa Catalina had larger leaves with fewer and smaller spines and less tensile strength than adult and resprout plants seen on the mainland. In addition, island populations had significantly lower concentrations of cynogenic glulcoside than seen in mainland populations. Preliminary feeding trials indicate a preferential browse by mainland, free-ranging mule deer on saplings from Santa Catalina Island