Since 1999, when fewer than 100 island fox were thought to inhabit Channel Islands National Park, a recovery program has brought the species to nearly full recovery on the park's islands.
The decline of the Channel Islands' populations of the species, which is listed as federally endangered, was attributed to predation by golden eagles, a predator that hadn't nested on the islands until the 1990s, according to the National Park Service.
They were able to colonize the islands because of several factors. First, bald eagles were absent from the Channel Islands, having disappeared by the mid-20th century due to both human persecution and the presence of DDT in the environment. Territorial bald eagles may have deterred goldens from establishing. Second, golden eagles arriving on the islands found food sources that were not available prior to the ranching era: feral pigs on Santa Cruz Island, and mule deer on Santa Rosa. Golden eagles nested on both Santa Cruz and Santa Rosa Islands form the mid-1990s to as recently as 2006.
To help recover the island foxes, efforts to live-trap and relocate golden eagles to the mainland began in the late 1990s. Since 1999, 44 golden eagles have been shipped ashore. With the eagles largely out of the way, the fox population rebounded, in part with the help of a captive breeding program. Today more than 1,700 of the small foxes -- which weigh only about 5-6 pounds as adults -- roam the islands.
According to Channel Island biologists, the fox population on San Miguel Island has grown to about 320 individuals from just 15 that were captured in 1999 for a captive-breeding program. Today, survivorship of island foxes on the island is 94 percent, an above-normal level, they say.
Santa Cruz Island, the largest and most diverse Channel Island, lays claim to more than 1,000 foxes, with an annual survival rate of about 96 percent, according to the biologists. For context, this fox population has increased from a low of less than 70 foxes in 2000, they note.
However, golden eagles on Santa Rosa Island are impeding a population boom among the foxes there. Park biologists say the fox population currently stands at just under 400 animals. Of note, this past spring 11 radio-collared foxes died from eagle predation, the park noted. Golden eagle feathers were found at two fox mortality sites and several sightings of golden eagles occurred in the same time period, the biologists say. Consequently, annual fox survival dipped from near 90 percent at the beginning of 2009 to 61 percent presently.
Subsequently, in May two island foxes succumbed to eagle predation on the west end of Santa Cruz Island following sightings of golden eagles in the area. These are the first fox kills in association with golden eagle sightings in over a year on this island.
Nevertheless, officials are pleased with the overall growing fox population in the park.
“The rapid recovery of island foxes may be one of the most successful recoveries of an endangered species to date,” said Dr. Lotus Vermeer, Director of The Nature Conservancy’s Santa Cruz Island Preserve.
“We measure fox recovery by looking at the overall populations and are excited by the progress we are seeing,” said Russell Galipeau, superintendent of Channel Islands National Park. “The continued predation by golden eagles is unfortunate and is being monitored closely.”
While golden eagle predation seems to be largely under control, it now seems that the park's bald eagles are developing a taste for fox. In late May in the bald eagle nest at Pelican Harbor on Santa Cruz Island bald eagle webcam viewers observed one of the adults bringing an island fox carcass into the nest, park officials say. It is not known whether the adult bald eagles preyed upon this fox or whether they recovered a dead fox carcass.
Bald eagles are known to primarily feed on fish, seabirds, and marine mammal carcasses and it is not their natural tendency to prey on terrestrial mammals, the park says.
This week a group of approximately 60 biologists and managers from various agencies and organizations is holding its annual meeting in Ventura, California, to discuss island fox recovery. They will present the current status of island fox populations on each of the six Channel Islands that supports this endemic species and will identify measures for continued island fox monitoring, research, and protection.