| The Magazine of the CALIFORNIA ACADEMY OF SCIENCES |
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Habitats A Murder of Crows
Californians have been marking West Nile Virus's steady progress across North America since it flew to New York from Israel in 1999. Since then it has killed about 560 people, tens of thousands of birds, and an unknown number of other animals nationwide. Long before it arrived here last summer, probably in the bloodstream of a mosquito trapped on an airplane, scientists knew that West Nile Virus would flourish in California's climate. They knew, too, that its favorite bird hosts and mosquito vectors were waiting here in abundant numbers. And they expected it all to begin with a signature Hitchcockian omen: a dead crow. When the first infected crow was found in southern California early in the summer of 2004, scientists and public health officials braced for the toll on the state's human population: thousands would be infected, hundreds would have flu-like symptoms, and a handful would die before the year was out. The risk to humans is real, but not devastating. Most people exposed to the virus never know it. A small percentage suffer flu-like symptoms or muscle paralysis, and fewer than one percent of infected humans die from encephalitis or meningitis, the disease's most serious ramifications. Most deadly cases involve the elderly or the immunologically suppressed. What scientists still do not know is how hard the virus is going to hit California's wildlife and ecosystems. But there are good reasons to believe that it may hit them very hard indeed, says Walter Boyce of the University of California, Davis Wildlife Health Center. Boyce is the lead author of a July report, "Potential Impacts of West Nile Virus on Wildlife in California," published by the Center this summer. Particularly vulnerable, he says, will be the state's rare and endangered bird species, whose population numbers may be too low to rebound from an epidemic. Unfortunately, there isn't much we can do for them except to try to minimize West Nile's main way of getting from bird to bird: the mosquito. When we romanticize bygone wild California-the skies darkened with birds and the Sacramento River brimming with salmon-we leave out the mosquitoes. They were once so thick around San Francisco Bay that Native Americans retreated to the hills in the summer months. Many of us would welcome a return of the wolf and the herds of elk they preyed upon, but I have yet to meet a restoration ecologist or even an Earth First!er who advocates bringing back the sky-darkening swarms of mosquitoes. The hungry grizzlies were teddy bears compared to death from ten thousand bites. Mosquitoes take the prize for the world's most deadly animal. Every year they infect more than 300 million people with malaria, dengue fever, and other diseases. The World Health Organization estimates that more than a million people die annually from malaria alone. Many other creatures are plagued by the deadly insects, too. Invasive mosquitoes are especially lethal to immunologically naive organisms. Mosquito-borne malaria, for instance, wiped out nearly half of Hawaii's native birds when it arrived on the islands around the beginning of the twentieth century. In California, mosquitoes were sometimes deadly to humans, too, and their control was one major motive for the orgy of wetland draining and filling that went on a hundred years ago. The mosquito's egg and larval stages require standing water, and marsh is their idea of reproductive heaven. In addition to filling wetlands, early twentieth-century mosquito controllers coated standing marsh water with diesel fuel and other poisons to spoil the pools' larvae. As usual, the benefits gained in human health and comfort came at an environmental cost to wildlife. In those days, however, few were paying much attention to that kind of collateral damage. Today at least we note and regret it. It is not epidemiologically significant-but still an interesting fact-that the same summer West Nile took its first human victim in California, the state launched the biggest wetland fix this side of the Florida Everglades. The South Bay Salt Pond Restoration Project, which aims to make 100,000 acres of functioning wetland out of salt pond, will surely raise large-scale mosquito-abatement issues from the dead. The methods of mosquito control used today, which we will certainly see applied in force in response to West Nile's arrival, are less overtly destructive to wildlife than, say, spraying diesel fuel was. Still, they are controversial. It's generally agreed that the most benign form of mosquito poisons are the larvacides, which target the early, water-borne stages of the mosquito's life cycle. A gallon of water containing mosquito larvae can be easily treated, but once the insects in that water become airborne adults, you'd have to spray an entire acre to kill them. The pesticides targeting adults-adulticides-are less specific to mosquitoes and much more likely to kill other invertebrates. They can have health consequences for humans and other mammals, too, according to Shawnee Hoover, special projects director for Beyond Pesticides, a toxics watchdog group in Washington, DC. So in addition to the primary impacts of the virus itself, Hoover and Boyce agree, California wildlife will have to contend with the significant but little-understood impacts of increased pesticide use. Furthermore, bats, dragonflies, birds, and numerous species of fish make a living eating mosquitoes. If mosquito populations are reduced to a minimum, those animals will either have to turn to other food sources, or starve. While few would argue for protecting the rights of mosquitoes, killing too many of them may harm some species we do want to protect. While humans can take effective steps to protect themselves from this virus (drain standing water, wear long pants and sleeves in the evening, wear insect repellent, and install screens on open doors and windows), animals cannot. Other than suppressing mosquito populations in the infected areas humans can reach, says Boyce, California doesn't really have a plan to protect wildlife against the disease. Public health officials already complain that funds are too scarce to slow West Nile's assault on human beings. Even fewer funds are available for tracking the disease's impact on wild populations. But before we can address West Nile in nature, says Boyce, we will have to understand both the virus and our wildlife much better than we do now. Studies so far have painted a grim but far from hopeless picture of the epidemic. West Nile virus can infect at least 200 species of North American birds including pigeons, house sparrows, chickens, cardinals, mockingbirds, mallards, parakeets, peacocks, macaws, flamingos, bald eagles, and whooping cranes. Some birds, notably the corvids-crows, jays, magpies-are especially susceptible. In one lab test, crow mortality hit 100 percent. Two field studies of the American crow estimated local losses in New York and Oklahoma in 2001 at 30 percent and 50 percent, respectively. On the other hand, some bird species are as resistant as people to serious forms of the disease. At least 20 mammal species have also been shown to carry and get sick from the virus, including horses (by midsummer 2004, 22 had died in California), cats, dogs, chipmunks, striped skunks, bats, and alpacas. Even alligators are susceptible to the disease, which they probably contract not from mosquito bites but from eating infected birds. But the population-level impacts on amphibians, reptiles, and mammals are largely unknown, says Boyce. In the UC Davis study, researchers used computer databases to identify West Nile virus hotspots, areas where "amplifier species" such as corvids, which carry high concentrations of the virus, overlap with rare or endangered species. The areas of greatest concern in the state are the Central Valley, coastal regions, western Sierra Nevada, the Salton Sea, and the lower Colorado River basin. The study's maps identify 27 endangered birds at risk in California, including the brown pelican, Swainson's hawk, peregrine falcon, bald eagle, and sandhill crane. It also identifies 21 vulnerable mammals-among them the kit fox, wolverine, and bighorn sheep; ten reptiles such as the rubber boa, desert tortoise, and blunt-nosed leopard lizard; and ten amphibians including the Shasta salamander, black toad, and the long-toed salamander. All of these animals could prove targets for the virus, says Boyce. Species not considered endangered but which have small, concentrated ranges may also be hard hit. The yellow-billed magpie, for instance, is relatively abundant in the Central Valley and it is neither state nor federally listed, but its distribution is restricted to areas with a very high risk for transmission, as is the island scrub jay, another corvid, found only on Santa Cruz Island off southern California. If either bird's mortality climbs near those of crows, it could become endangered or even go extinct. Although crows may be the most obvious victims of West Nile virus, Boyce doesn't think the microbe poses a long- Gordy Slack is a freelance writer and contributing editor to California Wild. You can contact him at g.slack@gmail.com. |
