Maintaining Migration Corridors

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Migrations take place in the air, on land, and in the ocean. Of the 900 or so bird species living in North America, about 75 percent do some sort of migration; worldwide more than 1,000 species migrate. Several sandpiper species travel more than 8,000 miles (12,875 km) from the Arctic to the southern tip of South America, but the champion may be the Arctic tern, which flies more than 11,000 miles (17,703 km) from the Arctic Circle to the Antarctic Circle each breeding season, almost half Earth’s circumference. Other migrations extend a mere 500 miles; some songbirds migrate no more than a few miles or less each season. Migrating birds and any other species that must cover distance to find food, breed, or raise young contend with three main threats to life: exhaustion, lack of food, and predators.
Ocean migrations cover equally impressive distances. Gray whale migrations are the longest distance among mammals. The whales migrate between the Chukchi Sea off northern Alaska and Baja California in Mexico, about 12,000 miles (19,315 km).

Animal migrations have followed the same paths for centuries. Wildlife biologists do not know all the factors that influence the timing and the routes programmed into the genes of each migrating species, or how generations know to follow the exact same routes. In many cases, however, migrations take place for very practical reasons. African wildebeests, for
example, migrate in herds of more than one million animals every year in a constant search for fresh grazing land after their enormous numbers have depleted the local vegetation. Their migration follows a clockwise route more than 1,800 miles (2,900 km) over the Serengeti in Tanzania to Kenya’s Mara River. As the wildebeests travel, predator lions, cheetahs, and hyenas sustain their young by taking the herd’s unhealthy members.

Migration corridors like those of the wildebeest and the gray whale consist of a strip of land or sea through which the wildlife moves. Each migration corridor must contain a minimum amount of territory to provide sufficient food and protection from weather and predators. Because urban areas have encroached on the lands that create corridors, many migration routes now receive legal protection. In the United States, the Migratory Bird Treaty Act of 1918 bans the killing of migratory birds when they are flying or during their stopovers. The law protects grasslands and plains, coasts, river corridors, and forested areas that the birds
use along the migration route, called a flyway. The Endangered Species Act (1973) and the Marine Mammal Protection Act (1972) also outlaw the hunting of migrating animals. International cooperation provides a necessary help toward protecting migration corridors. For example, the International Ramsar Convention (Convention on Wetlands of International
Importance Especially as Waterfowl Habitats) of 1972 sets migratory bird treaties among the United States, Canada, and Japan. The United States also cooperates with Mexico for bird and mammal migration protection under the same law.

In the United States, government agencies and wildlife organizations have established joint programs for maintaining corridors. As an example, the Yellowstone to Yukon Conservation Initiative includes the work of more than 800 organizations in Canada and the United States to maintain a migration and habitat corridor extending from the Peel River in Canada’s northern Yukon Territory to the Wind River Valley in Wyoming. This corridor supports the migration patterns and breeding of grizzly bears, black bears, elk, wolves, mountain lions, plus many bird and fish species. The Yellowstone to Yukon Conservation Initiative explains the rationale for developing the corridor: “The Yellowstone to Yukon region is one of the last places left in the lower forty-eight [states] where the full historical suite of carnivores and ungulates—including grizzly bears and caribou—can still be found.” It goes on to say, “Addressing conservation on the large-landscape scale is revolutionary and timely. It’s no longer enough to preserve isolated forests, valleys, and wilderness areas. Connection
of habitats is key to the long-term health of ecosystems and the biological diversity that supports both wildlife and human communities.” There may be no better definition of ecology.

Urban habitat corridors serve similar purposes as they do in the natural environment. Migration corridors through urban areas help wildlife with

 

The migration corridor that extends from the Canadian Rocky Mountains to Yellowstone National Park is called an ecoregion because it acts as an immense ecosystem spanning different climates and elevations and thousands of square miles. This migration corridor has been important in the reestablishment of grizzly bear and gray wolf populations.

the following: connecting fragmented habitats; escaping from floods and fires; escaping from predators; seeking new food sources; providing options for shelter, dens, and nests; providing rest stops for migrating birds; and avoiding construction, traffic, noise, and harassment from humans. corridors in cities. Traffic from urban sprawl has taken a toll on local biodiversity. In 2007 three rare Florida panthers died on roadways in a single week, because traffic does not discriminate between rare or abundant species, and these accidents kill mammals, birds, amphibians, reptiles, and even marine mammals that climb onto land and enter roadways.

Urban habitat corridors are usually of two kinds: riparian corridors along streams and rivers, and hard-surface corridors, hedgerows, tree lines, landscaping, or fencing. Sometimes people unintentionally build migration corridors, as they do when they plow roads in winter in Yellowstone National Park. Plowed roads allow greater access for snowcraft tours, snowmobiles, and snowshoeing, but the roads also enable the park’s bison
to roam farther in search of scarce food. The roads have helped weak animals because they no longer must expend energy pushing through snow 

 

Habitats threatened by fragmentation can be helped by preserving corridors between
them. Habitat corridors allow isolated populations to find food and interbreed. The
best habitat corridors are protected by buffer zones that contain little or no human
interference.

to find a few bites of grass. Once these animals wander outside Yellowstone, ranchers have shot them in fear the animals will transmit disease to their cattle. Migration corridors, therefore, must be built or maintained with a sound plan as to their effect on both humans and wildlife.

Fencing and other barriers prevent wildlife road losses, but at the same time these barriers halt migrations. Wide divided highways such as interstate highways also cause habitat fragmentation. Amy Masching, a conservation biologist at the Denver Zoological Foundation, explained to Audubon magazine in 2007, “Highways prevent species from accessing their historic migration and breeding areas, which reduces their genetic diversity.” For these reasons civil engineers have built landscaped bridges and tunnels in the United States, Canada, the Netherlands, and Australia. Structures have also been developed for large and small mammals, marsupials, birds, amphibians, and reptiles.