Western tanager. Courtesy USFWS.
This feature was first published in the 2009 Wilderness Magazine. To receive the annual magazine and quarterly newsletters from The Wilderness Society, become a member today!
Writer David S. Wilcove is a professor of ecology, evolutionary biology, and public affairs at Princeton University and the author of No Way Home: The Decline of the World’s Great Animal Migrations.
By David S. Wilcove
Sometime in early August, during one of the first chilly nights of the season in Yellowstone National Park, a western tanager will awaken, fly to the top of a lodgepole pine tree, and launch itself into the ink-black sky, thereby beginning the first leg of its annual migration to its winter quarters in western Mexico. In March, as snow piles up inside Yellowstone, herds of bison will leave the park in search of accessible forage.
Yellowstone National Park, like virtually all of our public lands, is filled with migratory animals, including birds, mammals, fish, and insects, a diverse array of creatures employing a diverse array of navigational tricks to reach destinations across the West and across the hemisphere. These animals are, for the most part, driven by opportunism. They take advantage of abundant food and other resources that are present in Yellowstone for only a portion of the year. For the western tanager, the park’s coniferous forests offer a smorgasbord of insects during the spring and summer, more than enough to raise a family. But once the cold weather sets in and the insects disappear, tanagers and other birds must find somewhere else to spend the winter. Similarly, Yellowstone’s lush meadows and grasslands can sustain thousands of bison and elk during the warm months, but heavy snow may eventually render that food inaccessible, forcing them to move to lower elevations inside and outside the park.
In addition to opportunism, one other characteristic unites Yellowstone’s diverse migrants: vulnerability. Logging and farming are destroying the montane forests of Mexico and Central America where western tanagers (and many other birds from the western U.S. and Canada) seek refuge during the winter. The fragile riparian woodlands that serve as crucial rest and refueling stops for them as they pass through the deserts of the Southwest are being degraded by overgrazing and development, while obstacles and dangers of all sorts—from skyscrapers to feral cats—have made the entire route more dangerous.
For bison, the primary enemies are a tiny bacterium and a lot of intolerance. The bacterium Brucella abortus was brought into the U.S. via imported cattle from Europe, and it spread to Yellowstone’s bison a century ago. Brucellosis (as the disease is called) has little effect on bison. Ranchers, however, detest it because it causes some of their cows to abort their fetuses and reduce their milk production. Fear of brucellosis has made Montana’s politicians and agriculture officials determined to keep Yellowstone’s bison away from Montana’s cattle. Unfortunately, a small number of ranchers continue to graze livestock on public and private lands adjacent to the park. So when bison leave the park, as often happens during harsh winters, state and federal officials first try to chase them back, using helicopters, snowmobiles, off-road vehicles, and people on horseback. Those that refuse to return are killed.
Nor are bison the only big mammals in trouble in Yellowstone. Biologist Joel Berger has estimated that over half of the elk migratory routes and three-quarters of the pronghorn routes in the Greater Yellowstone Ecosystem have been destroyed by residential development, oil and gas exploration, and the building of fences and other barriers.
Across the country and across the world, migratory animals are declining as their journeys become increasingly treacherous. This loss is not only aesthetic but ecological. Birds, for example, help to keep populations of defoliating insects in check, thereby reducing damage to our forests and croplands. Bison increase the productivity of grasslands by consuming the older, rank forage and by redistributing nutrients via their dung, all of which benefits other plants and animals, including pronghorn, prairie dogs, and grassland birds. Migratory salmon sustain grizzly bears, bald eagles, and other animals. The list goes on.
“No man is an island, entire of itself,” proclaimed the poet John Donne. The same holds true for our national parks, national forests, wildlife refuges, and other public lands. The well-being of many of the animals found on our public lands depends greatly on what happens on adjacent lands or even in distant countries. Saving these animals will require greater coordination among individuals, agencies, and nations, combined with a commitment to protecting them while they are still common. Migration is fundamentally a phenomenon of abundance. If we wait until these species are close to extinction, we will have lost both the glory and the ecological value of migration.
Western tanagers head south after a summer of feasting on insects in the Northern Rockies, some flying as far south as Mexico. Courtesy USFWS.
Evidence suggests that over half of the elk migratory routes and three-quarters of the pronghorn routes in the Greater Yellostone Ecosystem have been destroyed by residential development, oil and gas exploration, and the building of fences and other barriers. Courtesy BLM.