Outside of Alaska, virtually all large, migrating mammals in the United States live in the greater Yellowstone National Park ecosystem. Migration for many of these animals has been significantly impeded. Bisons, for example, rarely venture beyond the park's borders owing to the risk of being shot by ranchers. More than half the elk migration routes and three quarters of the pronghorn antelope routes have been lost to land and energy development. The migration routes of pronghorn antelopes in the northern part of the park have been entirely blocked, leading to high fawn mortality and declining populations. Pronghorns migrating to the south must pass through a narrow bottleneck called Trapper's Point, where movement is hampered by major oil and gas fields and housing developments.
Ecologists believe oil drilling, which has been proposed in Alaska's 19-million-acre Arctic National Wildlife Refuge (ANWR), could be equally disruptive to the migration of 125,000 caribou in the Porcupine River herd. After spending the winter in the Canadian Yukon, the herd travels 400 miles to the coastal plain of ANWR in the early summer where females give birth to as many as 50,000 calves. The animals feed on tundra grass and flowers to fatten up before the journey back to their wintering grounds. The ANWR site is ideal for caribou because of the dearth of predators and steady winds that keep mosquitoes from pestering the young. Oil exploitation, according to a United States Geological Survey report, would restrict available calving grounds, leading to reduced calf weight and reduced survival.
Of course, concerns about migration are not limited to the United States. The migration of 250,000 wildebeest across the Kalahari Desert in Botswana ended 25 years ago when the government built cattle fences that blocked their passage. "From wildebeest in Africa to antelope in Wyoming, long-distance migrations are among the world's most stunning yet imperiled biological processes," comments Joel Berger, a biologist with the Wildlife Conservation Society. Berger and other conservationists are trying to protect migration not simply because it is a "stunning" phenomenon. Migratory animals must move to survive; when their migration routes are cut off, these animals may go extinct.
Efforts to save whooping cranes, the most endangered of the world's 15 crane subspecies, offer a rare instance of animals learning about migration by watching what human do. Starting in 2001, humans flying ultralight aircraft have led whooping crane chicks on their first migratory flight south from a wildlife refuge in Wisconsin to a newly-created wintering habitat in a Florida preserve--a 1,200-mile journey.