Pigs threaten foxes

The case for removing feral pigs (Sus scrofa), has less opposition in the Channel Islands National Park and other islands off the coast of southern California. This is a case where one invasive exotic mammal species, the feral pig, has changed the ecological relationships among several native predators. On four islands, feral pigs introduced into the ecosystem are indirectly leading to the extinction of four subspecies of island fox (Urocyon littoralis), a tiny animal smaller in size than a house cat. The smallest member of the family Canidae in North America, island foxes show little fear of humans and were probably once kept as pets by Native Americans.

There are no island foxes left in the wild on San Miguel and Santa Rosa Islands, where captive breeding programs are aiming to save the island fox subspecies from extinction. Fewer than 200 island foxes are left on Santa Catalina Island, in part because of canine distemper virus vectored by domestic dogs. The island fox subspecies on Santa Cruz Island has seen its population decline from 1,300 to fewer than 100. The U. S. Fish and Wildlife Service proposed the four rare island fox subspecies for protection under the Endangered Species Act, and eliminating feral pigs from the Channel Islands is part of the plan to save the island foxes.

Feral pigs and other introduced grazing mammals like rabbits, sheep, goats, cattle, deer, elk, and horses contribute to degradation of the island habitat. The presence of feral pigs also introduces an indirect ecosystem food chain effect contributing to the demise of the island fox. Feral pigs serve as prey, allowing golden eagle (Aquila chrysaetos) populations to flourish. Historically, golden eagles are a mainland species that has neither bred nor overwintered on the islands. The islands have historically been nesting grounds for the bald eagle (Haliaeetus leucocephalus), which preys mostly on marine mammals and fish.

Basically, the introduction of feral pigs allowed the golden eagle, a mainland species, to become established on the islands. Unlike bald eagles, golden eagles prey on island foxes. One study found that with 90% golden eagle predation, island fox numbers declined to zero. Released from competition with island foxes, island spotted skunk (Spilogale gracilis amphiala) populations also increase as an indirect consequence of feral pigs supporting golden eagle populations. In other words, feral pigs feed the golden eagles which eat the foxes which frees up space for the skunks.

Golden eagle removal and relocation is part of the plan to save the island fox and restore the ecosystem. However, satellite telemetry studies indicate that golden eagles relocated to the mainland will try to return to the islands as long as feral pigs are available as a food source. Reintroduction of the bald eagle is also being considered to help restore the ecosystem. The bald eagle is very territorial, and may deter the golden eagle from nesting. However, removing feral pigs as a food source is the more important factor in preventing reestablishment of golden eagle populations, restoring the ecosystem, and saving the island fox.

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