Feral pigs have created ecological havoc in many parts of California, uprooting native plants and turning meadows into mudholes. But nowhere have their effects been as dramatic as on the Channel Islands, where they have caused a complete restructuring of the food web, threatening the native island fox with extinction. A team of biologists has now documented the remarkable extent to which the introduced pigs have disrupted the island ecosystem. They are reporting their findings in the scientific journal Proceedings of the National Academy of Sciences (the article will be published online on December 18 and will appear in print in the January 8 issue of the journal).
The story involves not only exotic pigs and native foxes, but also golden eagles that have recently colonized the islands and a native spotted skunk. Golden eagles were initially drawn to the islands by the abundance of tasty piglets on Santa Cruz, the largest of the Channel Islands. But they began preying on the island fox as well, causing the fox population to plummet not only on Santa Cruz but also on the nearby islands of Santa Rosa and San Miguel. The cat-sized island fox, once the dominant predator in the ecosystem, is rapidly disappearing. Meanwhile, skunk populations are booming due to reduced competition from foxes.
"The presence of exotic pigs has totally restructured the food web on Santa Cruz Island," said Josh Donlan, science director for the nonprofit Island Conservation and Ecology Group based at the University of California, Santa Cruz, and a coauthor of the paper.
At first glance, it might appear that feral pigs are outcompeting the foxes, but the two species are not direct competitors, said lead author Gary Roemer, now at New Mexico State University.
"This phenomenon is called apparent competition--the two prey species share a predator that has an asymmetric impact, causing one species to decline," Roemer said. "This is the first case that documents apparent competition from an exotic species causing a decline in a native species, and it may be an important mechanism in the global decline of biodiversity."
Roemer, Donlan, and coauthor Franck Courchamp of the National Center for Scientific Research in France combined field data with a mathematical model of the competitive and predatory relationships of the four species to elucidate the changes in food-web dynamics on the islands.
The feral pigs enabled golden eagles to colonize the islands by providing an abundant food source, said Roemer, who began studying the foxes on Santa Cruz Island in 1993 as a graduate student at UCLA. He first found evidence of eagles preying on foxes in 1994, but it wasn't until the following year that he saw the fox population start to decline. "As the study progressed, it became clear that eagle predation was taking a toll on the foxes," he said.
Island foxes are more vulnerable to golden eagle predation than pigs or skunks due to basic differences in the natural histories of the three prey species, Donlan said. Feral pigs reproduce rapidly and can produce large litters throughout the year, and piglets that survive eventually grow too big to be prey for golden eagles. Skunks are nocturnal, so are only occasionally preyed on by eagles, which hunt during the day. The foxes, however, are active both at night and during the day, have just one small litter in the spring, and both pups and adults are preyed on by golden eagles.
"The foxes are a nice eagle meal for their entire lives, and they are often active during the day, so they are constantly preyed on by the eagles," Donlan said.
Pigs were introduced to Santa Cruz Island during the 1850s, along with sheep; yet golden eagles didn't become a problem until the 1990s. A resident population of bald eagles on the islands may have kept golden eagles at bay, but by 1960 the bald eagles had died out due to hunting and DDT pollution, said Brian Latta, a biologist with the Santa Cruz Predatory Bird Research Group based at UCSC.
"Bald eagles are highly territorial, and they would nest on the perimeter of the islands because their diet is mostly marine-based," said Latta, who has been capturing golden eagles on Santa Cruz Island and relocating them to the mainland in cooperation with the National Park Service and other agencies. "We can only speculate as to why golden eagles started eating foxes in the early 1990s, but it seems there were enough living year-round on the island by then to start having an impact on the fox population," he said.
Channel Islands National Park encompasses five of the eight Channel Islands, including the three where golden eagle predation has caused fox populations to plummet. Foxes also occur on three southern islands where eagles are not a problem, but on one of them, Santa Catalina, the foxes were decimated by distemper virus introduced in 1998, Roemer said.
The National Park Service launched a major effort in 1999 to save the island fox, which has been proposed for listing as an endangered species. In addition to Latta's golden eagle relocation project, a pig eradication project is planned to start next year on Santa Cruz Island, in cooperation with the Nature Conservancy, which owns 76 percent of the island. Captive breeding of foxes is under way on San Miguel (where only one fox is left in the wild) and on Santa Rosa (where no wild foxes remain). Fewer than 100 foxes remain on Santa Cruz, where captive breeding is also being considered.
Although eradication of the feral pigs on Santa Cruz Island won't be easy, Roemer and Donlan said chances are good for a recovery of the fox populations. "I think it can happen, but it will take a lot of work and a lot of resources," Donlan said.
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