New Findings on Island Fox Diet

Providing the best opportunity for island foxes to survive in the wild means understanding how they use resources and interact with other species.

Researchers, headed by Brian Cypher, examined the diet of island foxes across all six Channel Islands. Their findings have just been published in Global Ecology and Conservation, 2(2014) 255–66. Their article: “Multi-population comparison of resource exploitation by island foxes: Implications for conservation,” looks at the dietary differences between the six island subspecies, compares seasonal dietary shifts and dependence on introduced species, and recommends considerations for future conservation efforts. (Original paper)

The scientists gathered island fox scat across the six islands during four seasons in 2009. The scat samples were analyzed at California State University-Stanislaus’ Endangered Species Recovery Program office in Bakersfield, CA.

San Miguel Island deer mouse, courtesy of C. Schwemm

The researchers discovered that sixteen species were most frequently found in island fox scat or droppings:

• Animal Prey - island deer mice, birds, lizards, beetles and beetle larvae, Jerusalem crickets, silk-spinning sand crickets, and grasshoppers
• Plant Fruits - from toyon, manzanita, prickly pear cactus, and summer holly
• Introduced Species - earwigs, European snails, fruits of ice plant and Australian saltbush

toyon fruit, courtesy of K. Dearborn animalbytes.net

Like many members of the canine family, island foxes are generalists when it comes to searching for food. They hunt small prey, search out seasonal fruit, but will also eat carrion. To survive on the Channel Islands, island foxes have evolved to exploit all available food resources. The graph below is a simplified comparison of the frequency with which different items appeared in the diet of the six island fox subspecies over a year. (for complete tables and percentages see the original paper, below)

Arthropods in the Diet
An interesting finding is the importance of numerous insect species. Insect parts constituted a “significant proportion of each scat.” The dark purple band in the graphic above depicts the frequency of insects found in scat on each island. On all six islands, beetles were a primary food item year-round (most identifiable were darkling beetles and ten-lined June beetles). Insect prey offers calorie-rich food for little energy expenditure, making them an efficient food source. In addition there was evidence island foxes occasionally ate cockroaches, dragonflies, butterfly caterpillars, and wasps.

European earwigs are now found across North America, including the Channel Islands. This introduced species has become an important food resource for island foxes. Earwigs were a primary food source on San Nicolas, San Miguel, Santa Rosa, and Santa Cruz, and were an important food source on San Clemente in the spring and on Santa Catalina during the fall and winter.

Crustaceans–beach hoppers and various crabs–were occasionally found in scat, demonstrating that some island foxes search for food along shorelines. Introduced European garden snails are now found on several of the Channel Islands and island foxes have included them in their foraging. On the two islands, San Clemente and San Nicolas, garden snails (red band on graphic) have become a primary food resource for island foxes.

Vertebrate Prey 

island fox with 3 deer mice in jaws, courtesy NPS

Many people think of foxes as primarily mammal and bird predators. While the island fox does hunt, these larger prey items appeared less frequently in the diet than you might think. The blue band represents the frequency of deer mouse remains and the green band represents the frequency of lizards (on San Clemente and San Miguel) or birds (on Santa Rosa). At a glance you can see that lizards and birds are not primary food sources on three islands and, on all islands, deer mice are less frequently found in the diet than insects.

Some additional prey animals were found occasionally in fox scat: Catalina California ground squirrels (only found on Catalina), scavenged seal or sea lion, spotted skunk (competitors on Santa Rosa and Santa Cruz), a few black rats, a house mouse, and a bat. While island foxes have been seen catching snakes, no evidence of snake remains was found in 2009.

Native Fruiting Plants

Santa Cruz Island fox in tree searching for fruit

The larger the Channel Island, the greater its plant diversity. The two largest islands, Santa Cruz and Santa Catalina, offer island foxes abundant plant resources. On these two islands, native fruit is a major part of the diet. The orange band on the graphic above shows that, on Santa Cruz, the primary island-fox diet consists of insects and fruit. 

On Santa Rosa, where large introduced browsing animals (deer and elk) continued to degrade native plants until 2011, deer mice were more frequently found in the diet than fruit. As the plant resources on Santa Rosa Island recover, researchers will be watching to see if fruit becomes a greater part of the island fox diet.

While fruit is an important part of the diet for most island foxes, there can be a great deal of difference in the species consumed between islands. The chart below shows the plant species found in the scat with a more than 10% frequency. 

On Santa Catalina and Santa Cruz the number of native plant species is high and no introduced non-native plant species are used frequently by island foxes. The islands that have suffered the greatest alteration of their native plants, San Clemente, San Nicolas, and San Miguel, are currently sustaining island foxes with introduced plant species. Notice on San Miguel, no native plant fruit was found to be consumed by island foxes.

Conservation Implications 

Removing introduced animal species from the islands has eliminated some carrion opportunities for island foxes. However, carrion of large introduced animals is not believed to have ever been a primary food source for these omnivorous canines.

Removing introduced non-native plant species is a major goal across the Channel Islands, but this study reveals an important caution. Island foxes are consuming introduced ice plant and the garden snails it harbors. Before invasive ice plant and Australian saltbush can be removed, native fruiting plants will need to be reestablished so that island foxes have enough resources to survive.

Future Study
This study provides the first comprehensive look at island fox diet across the Channel Islands and through the seasons. However, it represents one year: 2009. Native plant resources fluctuate with annual weather. Climate change and drought are impacting native plants and fruit production. Have impacts to plant resources in 2013 and 2014 challenged survival for island foxes and their smaller prey, which are also dependent on plant foods? Have island foxes altered their diet in the face of drought? (Parasite threat from food source on San Miguel) Can conservation measures to restore the natural ecosystem be managed so that resources are not compromised for island foxes? And will island foxes expand their diet as their ecosystem is restored? Continued study of island fox diet is vital to informed conservation decision making.

Download the original Research Paper “Multi-population comparison of resource exploitation by island foxes: Implications for conservation”