Multi-population comparison of resource exploitation by island foxes: Implications for conservation

Imperiled island foxes are inherently resource-limited by their insular ecology. We examined food use on all 6 islands where they occur to assess resource exploitation patterns. Over 40 different food items were identified with item use varying among islands. Sixteen items occurred with ≥10% frequency in annual fox diets: deer mice, birds, lizards, beetles, beetle larvae, Jerusalem crickets, silk-spinning sand crickets, grasshoppers, earwigs, snails, and fruits of toyon, manzanita, prickly pear cactus, ice plant, Australian saltbush, and summer holly. Foxes used a diversity of food items with variations among islands attributable to island-specific availabilities. Deer mice in particular appeared to be preferred. Foxes also exhibited extensive use of non-native items, such as ice plant fruits, European snails, and earwigs, and foxes may even be dependent on these items on some islands. To increase food security and promote population stability, we recommend (1) continuing and enhancing habitat restoration efforts on all islands, (2) increasing the abundance of native items in association with any removals of non-native species used by foxes, and (3) monitoring annual trends in abundance of key food items as well as periodic monitoring of item use by foxes to determine functional responses to changes in item availability. Keywords: Channel islands, Endangered species, Food-item selection, Foraging ecology, Island fox, Urocyon littoralis