Fishing, farming, and pesticides have decimated the bald eagle population of California's Channel Islands. But current efforts to reintroduce the birds could likewise threaten some of the islands' other endangered species, including foxes, according to a new study.
Bald eagles once ruled the Channel Islands, an archipelago of eight islands just off the coast of southern California. For thousands of years, they feasted on fish, seabirds, and waterfowl alike. But things began to deteriorate in the late 1700s. First the Spanish colonized the islands and began commercially harvesting fish and sea otters, competing for the eagles’ prey. Then in the 1850s, ranchers introduced sheep, cows, goats, and pigs, which overgrazed the islands and disrupted coastal eagle breeding grounds. Finally, in the mid-1960s, Channel Island farmers began spraying their crops with the pesticide DDT, which thins the eggshells of birds, wiping out what was left of the islands' bald eagle population.
Since the early 1980s, conservationists have been attempting to reintroduce the eagles. At first, results were mixed because DDT still lingered in the ecosystem. But recent successful hatchings on three islands in the chain, Santa Cruz, Santa Catalina, and Santa Rosa, suggest that the ecosystem may finally be clear enough of contaminants to support a breeding population.
That's good news for bald eagles but potentially bad news for island foxes, says Seth Newsome, a conservation biologist at the University of Wyoming in Laramie. In the new study, he and his colleagues reconstructed historical bald eagles' eating habits based on isotope analyses of bones and feathers collected from ancient remains, century-old abandoned nests on the islands, and museum collections. In bones and feathers, higher concentrations of the isotopes delta-13C and delta-15N—stable varieties of carbon and nitrogen, respectively—indicate that the animal ate more marine prey, whereas lower levels point to more terrestrial prey.
The data reveal that when marine resources of food, such as fish, weren't available, eagles readily adapted to terrestrial alternatives. In the past, that meant eating carrion from sheep and pigs, but ranching no longer takes place on the island. Enter the endangered island foxes, who've seen their numbers decline due to a combination of habitat destruction and, since the 1990s, predation by golden eagles. "The types of resources the [bald] eagles historically consumed just aren't there in the same numbers," Newsome says. "The eagles are very good at adapting and being opportunistic," so island foxes might become a target.
The eagles could even put themselves in danger by feeding on new sources of food. For example, Newsome says, they could be poisoned if they feed on carrion from sea lions and seals, which accumulate high levels of environmental toxins.
These problems are a consequence of conservationists focusing on hatching the eagles but not paying enough attention to what they'll be eating, says Newsome, whose team reports its findings online today in the Proceedings of the National Academy of Sciences. He proposes lobbying for stronger protections for fish populations around the island, which have declined due to commercial fishing. A rehabilitated fish stock would take a lot of pressure off the foxes, he says.
The paper demonstrates that eagles can adapt their eating habits very rapidly when the situation calls for it, says Peter Sharpe, a wildlife biologist at the Institute for Wildlife Studies on Santa Catalina Island. But he says there’s no reason to think the bald eagles will run out of marine food sources anytime soon. “We’ve been watching nests on Catalina for more than 20 years, and we’ve never seen a fox in one,” he says.
Lotus Vermeer, director of The Nature Conservancy’s Santa Cruz Island Preserve, says that eagle population planners have considered the island foxes' safety since they first began conservation efforts. She agrees with Sharpe that bald eagles are unlikely to start preying on island foxes. "First and foremost, bald eagles are marine predators," she says. And, she says, the Northern Channel Islands, which include Santa Cruz Island and Santa Rosa Island, are home to enough seabirds to support a growing bald eagle population.
Still, Newsome says, his study—and history—show that bald eagles are highly capable of changing their diet if their preferred prey declines.