Experts Will Need to Be Sly to Solve Mystery of the Vanishing Island Foxes

Channel Islands National Park biologist Tim Coonan has a mystery to solve.

Clues are scarce, and time is of the essence.

At stake is the island fox--a subspecies found only on the Channel Islands.

Four years ago, more than 450 of the little foxes bounded through the coyote brush on the wind-swept plateaus of San Miguel.

Today their numbers have plummeted to 40.

No one knows why--the decline perplexes rangers and biologists alike.

Golden eagles have picked off foxes on some of the other Channel Islands. But there appear to be no golden eagles on San Miguel.

Depletion of habitat due to development--or lack of food--can cause such a decline, but the mouse population the foxes feed on has not changed.

Stranger yet, although the number of foxes on San Miguel has been dropping for several years, populations of the animal had remained stable on two other islands in the park. But this year they fell sharply too.

“That was a shock,” Coonan said. “This year we are coming to grips with the fact that they are disappearing on Santa Cruz and Santa Rosa as well. . . . It’s a real conundrum.”

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But now, armed with a $50,000 environmental grant, the National Park Service will conduct blood tests and perform necropsies on the tiny island foxes to try to find out what is killing them.

Often swathed in clouds and fog, San Miguel is the most isolated and least accessible of all the islands in the park.

Coonan recently flew here to check traps. He walked over the dry grasses. There, in a tiny 2 1/2-foot-by-8-inch trap, was a pup--lured into the metal box by cat food.

The biologist reached in and lifted out the scared creature, smaller than a Yorkshire terrier. The fox’s dark eyes darted about nervously.

Coonan grasped the animal by the chin and scratched it behind its cinnamon buff ears.

Biologists believe that the island fox, related to the mainland gray fox, floated out to the islands on pieces of wood more than 20,000 years ago. Over the millenniums the foxes on the different islands have gradually become distinct--both from the mainland gray fox and from each other.

Like many island mammals, they are smaller than their mainland cousins, in this case, about 18% smaller. But they are the megafauna of the island ecosystems--larger than any other species except the spotted skunk.

Artifacts from Chumash graves reveal that on some islands the foxes were domesticated. At death they were buried with their masters.

The little foxes on San Miguel have survived overgrazing by cattle, drought and massive soil erosion.

But now they are threatened anew.

In the 1980s, the population plunged to a level so low that the state listed the animals as endangered. In 1993, the park service began a basic monitoring program for all terrestrial island creatures, including foxes.

Researchers inserted tags the size of grains of rice between the foxes’ shoulder blades with needles. The tags can be read like supermarket bar codes to keep track of individual foxes.

By 1994, the fox population on San Miguel was higher than it had been in years--450. But it plunged to 300 the next year and has continued to drop. There were 100 foxes in 1996 and only 70 last year.

This year the number reached its lowest point yet: 40.

“[In] the early part of the decline, we kept waiting for them to bounce back,” Coonan said. But they didn’t. And officials began to become alarmed.

“It’s a crisis time,” Coonan said. “This year’s numbers are even more disturbing. . . . It’s unlikely that a population with that few animals could make it if struck by disease.”

Preliminary testing of the foxes’ blood and feces has revealed some cases of canine ideno virus--which can cause hepatitis--and heartworm. The virus hits pups hardest and could explain why they are not surviving year to year; heartworm cuts short the foxes’ lives.

Biologists do not know how the diseases could have reached the island, 55 miles off the Ventura coast.

They doubt that mosquitoes, which carry some viruses, could have flown across the channel. But in wetter years, like 1995, there were more mosquitoes on the island.

Biologists also wonder if sailboat owners mooring at the island’s sandy beaches are letting their dogs roam free in the isolated environment--introducing new diseases such as heartworm, to which the fragile fox population has built no immunity.

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But Coonan emphasized that evidence of the two diseases does not mean they are killing the foxes. Foxes on islands farther south, outside the park, also have heartworm, but they are doing fine. Still, the San Miguel subspecies could have lower resistance.

Although rangers have not spotted any golden eagles over San Miguel, they cannot rule the aerial predators out either. Fox carcasses on Santa Cruz and Santa Rosa show evidence of bird attacks.

The foxes “probably haven’t seen an eagle before, so they are very vulnerable,” Coonan said. “They haven’t looked to the skies to see what’s up there. They haven’t had to.”

At this point, park biologists believe that disease is the culprit. But more research is needed.

“We don’t have the smoking gun yet,” Coonan said. “That’s why I would really like to look at some of the carcasses.”

The environmental grant from Canon USA will help with such tests. This year, Canon doled out the $50,000 grants to 17 national parks.