Island Fox - Taxonomy and Evolution

Taxonomy and Evolution

The island fox shares the Urocyon genus with the mainland gray fox, the species from which it is descended. Its small size is a result of insular dwarfism, a form of allopatric speciation. Because the island fox is geographically isolated, it has no immunity to parasites and diseases brought in from the mainland and is especially vulnerable to those the domestic dog may carry. In addition, predation by the golden eagle and human activities devastated fox numbers on several of the Channel Islands in the 1990s. Four island fox subspecies were federally protected as an endangered species in 2004, and efforts to rebuild fox populations and restore the ecosystems of the Channel Islands are being undertaken. Radio collars are being attached to foxes in an effort to track and locate the young foxes. To date these efforts have been largely successful.

There are six subspecies of the island fox, each of which is native to a specific Channel Island, and which evolved there independently of the others. The subspecies are:

• Urocyon littoralis littoralis of San Miguel Island,
• Urocyon littoralis santarosae of Santa Rosa Island,
• Urocyon littoralis santacruzae of Santa Cruz Island,
• Urocyon littoralis dickeyi of San Nicolas Island,
• Urocyon littoralis catalinae of Santa Catalina and,
• Urocyon littoralis clementae of San Clemente Island.

Foxes from each island are capable of interbreeding, but have genetic and phenotypic distinctions that make them unique; for example, the subspecies have differing numbers of tail vertebrae.

The small size of the island fox is an adaptation to the limited resources available in the island environment. The foxes are believed to have "rafted" to the northern islands between 10,400 and 16,000 years ago. Initially, fox populations were located on the three northern islands, which were likely easier to access during the last ice age—when lowered sea levels united four of the northernmost islands into a single mega-island (Santa Rosae) and the distance between the islands and the mainland was reduced—it is likely that Native Americans brought the foxes to the southern islands of the archipelago, perhaps as pets or hunting dogs.

Based on the limited fossil record and genetic distance from its gray fox ancestors, the northern island fox subspecies are probably the older subspecies, while the San Clemente island fox has been only resident on its island for about 3,400–4,300 years, and the San Nicolas island fox established itself as an independent group about 2,200 years ago. The Santa Catalina island fox is potentially the most recently evolved subspecies, having been on its island for about 800–3,800 years. The fox did not persist on Anacapa Island because it has no reliable source of fresh water; Santa Barbara Island is too small to support the food demands of the fox.