The bald eagle paradox
When the recovery of one species endangers another
All it takes to wreak havoc in a colony of common murres is one bald eagle. Gulls spot the eagle from a distance and sound the alarm, and the murres nesting on the cliff bob their heads nervously. As the big raptor swoops down, the stocky black-and-white seabirds flee, leaving their eggs and chicks behind. Gulls and crows then quickly move in and gobble as many as they can. "It is heart-stopping," says Roy Lowe, U.S. Fish and Wildlife project leader for the Oregon Coast National Wildlife Refuge Complex. Over the last 15 years, he's witnessed an ever-growing number of bald eagle raids on seabird colonies.
In the 1960s, the bald eagle nearly became extinct; widespread DDT use had thinned its eggshells. Once the pesticide was banned in 1972 and conservation efforts increased, populations rose dramatically, and the bird was removed from the federal endangered species list in 2007. But human activity had altered the raptor's habitat in many ways -- development encroached, available food sources shifted, the populations of some species rose while others declined. The bald eagle was brought back in hopes of helping restore ecosystem balance, but wildlife managers lacked information about what that balance originally looked like. Now, along the Pacific Northwest coast, reports abound about eagles creating havoc among cormorants, murres and other seabirds. Biologists are at a loss about what, if anything, to do about the charismatic troublemaker they worked so hard to bring back.
In Washington, the population of nesting bald eagles has grown annually by about 9 percent in the past 25 years; in 2005, there were 840 nesting pairs. In Oregon, the population has increased by 7 percent each year between 1978 and 2007. In 2007, 553 pairs were counted. "We still don't think that eagles are as abundant as they were before they declined," says Frank Isaacs, who has conducted eagle surveys and is a senior research assistant at Oregon State University. But it's all guesswork, he says, because the bird's historical population levels are unknown.
Scientists also know little about what the world was like for prey species before the eagles were extirpated. The Oregon coast now supports over a million nesting seabirds, including common murres, storm petrels, western gulls, cormorants, and tufted puffins. While none are federally endangered, Washington regards common murres and tufted puffins as species of concern (along with bald eagles); the puffins are also considered "sensitive" in Oregon. After the bald eagle's decline, these seabirds nested for generations without being harassed, says Isaacs: "I feel sorry for the seabirds." These days, on the northern part of the coast, bald eagle predation has caused the collapse of entire murre colonies, once 300,000 birds strong. "Some of our rocks that were significant colony sites are now permanently abandoned," Lowe says.
In Washington, the population of glaucous-winged gulls exploded in the '70s and '80s, says Jim Hayward, a research professor of biology at Andrews University in Michigan, largely because of the food available at overflowing landfills. Hayward noted that starting in the '90s, increasing numbers of bald eagles began harassing gull colonies, such as those located on Protection Island in Washington's Strait of Juan de Fuca. "It is just chaos out there," he says. In a paper to be published this year in the Journal of Raptor Research, Hayward and his colleagues indicate that eagle predation on the island probably contributed to a 44 percent decrease in the number of gull nests between 1993 and 2008.
These interactions between bald eagles and gulls or murres, as dramatic as they appear, may simply represent a shift back towards balance. Researchers speculate that seabirds like murres were sometimes prey for eagles in the past, and could not breed in large colonies with such a predator in the picture. The eagles might also help bring gull numbers back down to historic levels. But for less robust species, such as cormorants and great blue herons, the raptor's indiscriminate appetite could prove a problem.
Historically, bald eagles fed opportunistically on a wide variety of prey, including salmon and mammals, but decades of decline in wild salmon populations may have forced them to dine on other birds more frequently. "Predator-prey relationships are not a new threat," says Steve Mashuda, an attorney with the nonprofit environmental law firm Earthjustice. "It is only a threat in the context of an imbalance that is largely human-caused." Many seabird populations are already impaired because of human activities, including oil spills, fishery by-catch, and loss of habitat to development.
Tampering with natural systems lays the foundation for yet more tampering -- active manipulation of predator-prey relationships in an attempt to "fix" the mess. This approach is not new to wildlife managers. Peregrine falcons, like bald eagles, have made a spectacular recovery, and came off the endangered species list in 1999. Their return, though, created some unforeseen consequences. In California in the 1990s, peregrine predation began to impact California least terns, also federally endangered. Tern populations were already declining, mainly because of habitat loss, and thus were less able to withstand predation. The state initiated a non-lethal predator control program that's still in effect. Individual peregrines that disrupt least tern colonies are captured, then released into areas away from the fragile terns. Very few relocated peregrines ever return, says Brian Latta, executive director of the nonprofit The Bird Group. To Latta, the decision to step in is clear when the future of an endangered species is at stake. "It is a mitigation effort to make up for what we have done," says Latta. "If we want these terns to persist, we have to manage them since we have greatly reduced their numbers and the amount of available habitat."
A relocation program may seem like a small price to pay for healthier tern populations. But when recovered predators have to be killed to save an endangered species, the trade-offs are more complicated. California sea lions are yet another protected predator that recovered from low levels in the 1970s to around 238,000 animals now. In recent years, increasing numbers of sea lions have come to Bonneville Dam in the Columbia River to eat spring chinook salmon on their way upstream to spawn. The sea lions take between 0.4 and 4.2 percent of the run, which is listed as threatened, largely because of mortality caused by dams.
Wildlife managers attempted to scare away the sea lions, but the pinnipeds hung on to their lunch spot. In 2008, the National Marine Fisheries Service allowed Washington, Oregon and Idaho to kill a limited number of sea lions. "We have had enough experience watching marine mammals utilize human-made areas and get incredibly effective at it," says Garth Griffin, a biologist with the National Marine Fisheries Service. "If left unchecked, it is a problem that is just absolutely unforgivable in terms of wildlife management. We are obliged to manage this species in favor of the one that is the most endangered."
Even more troubling is increasing salmon predation by Steller sea lions at the site. Steller sea lions are listed as threatened and cannot be legally killed. "If the Steller sea lions step in and we can't touch them, what can we do?" says Robert Stansell, fish biologist for the U.S. Army Corps of Engineers.
The sea lions are the wrong target, says Mashuda. "If you spend a lot of time worrying about sea lion predation, you end up spending less time worrying about predation by the dams," he says. "The ESA says that we ought not to be picking and choosing between two endangered animals. Instead, we should be focusing on why both those animals are endangered and what is needed to recover them." That means addressing the root causes, such as over-exploitation of natural resources, pollution, or habitat loss. Gary Roemer, an associate professor with New Mexico State University's Department of Fish, Wildlife and Conservation Ecology, has studied conflicts between endangered species. "Unless we start dealing with the larger underlying problems that result in single-species endangerment," he says, "we will see more of these conflicts arise."
Meanwhile, along the Washington and Oregon coasts, wildlife managers are wary of taking any active measures to manage the bald eagles. "We will allow the natural processes to continue," Lowe says. Wildlife managers will focus on further protecting the seabirds from human-caused mortalities, through education and monitoring, and on building structures that give nesting birds a refuge from eagles.
Other scientists agree that too little is known about bald eagle predation to take meaningful action. Julia Parrish, a professor of fisheries at the University of Washington, is leading a long-term study of common murre and bald eagle interactions. In 2007 and 2008, the researchers found that on Tatoosh Island, off the Olympic Peninsula, eagles were such a nuisance that the murres abandoned their nests without producing any young. But by 2009, the murres had adapted. They hid their nests beneath brush, and successfully reproduced. "It may be that predators and prey have to be in association with each other for many years in order to allow for a stable set of behavioural interactions to evolve," Parrish says. "For me, it is like watching a soap opera. I know who is going to show up, I know all the characters, but I never know what is going to happen from one year to the next."
Isabelle Groc is a freelance writer and photographer based in Vancouver, British Columbia.