Migrations aid the environment in four ways. First, grazing herbivores contribute to the health of grasses by grazing and by fertilizing the soil during migration. Second, when migrating birds or fish leave an area, insects, flowering plants, plankton, small fish, and invertebrates receive time to reestablish their numbers. Third, migrating animals often restore balance to ecosystems they enter by controlling prey numbers or providing food to predators. Fourth, migration contributes to genetic diversity within a species by allowing members of groups to mingle, split off from the herd, or join the herd.
Migration patterns consist of the times, distances, and routes taken, called migration corridors. Migration corridors comprise strips of land, air, or water where a migrating group travels unimpeded, and they are influenced by three things: resources for food and shelter, habitat destruction along the route, and population density—both animal and human. Migrating animals need places along their routes to rest and eat before continuing their trek. (Some species migrate nonstop, however, over remarkably long distances.) Migration corridors provide space for the migration’s numbers, safety from predators, and food.
Many migration corridors have been destroyed or blocked, and biologists have noted a decline in migratory species in the ocean, in air, and on land. Pipelines, roads, housing developments, and walls and fences all act as barriers to migrations. (Roads are a critically hazardous obstacle to California’s endangered tiger salamander.) When the United States–Mexico border wall was proposed, Christine Haas of the Arizona Audubon Society said, “If the wall goes up, it will be a complete and absolute barrier for terrestrial wildlife.” Haas’s prediction has played out at the border fencing in the Sonoran Desert that now blocks migrations of jaguar, pronghorn, and owls. Biologist Emil McCain told Audubon in 2007, “It’s [the border fence] probably the finest example of habitat fragmentation you can think of.” Other human-caused actions have exerted subtle effects on migrations, such as nighttime light emitted by cities that upsets bird migrations, which take place at night. This disturbance is called light pollution or photopollution.
Caribou migrations are among the largest migrations in the Western Hemisphere. These caribou in Alaska’s Togiak National Wildlife Refuge migrate in herds, behavior characteristic of animals that must eat while they watch for predators during their migration. Habitat fragmentation and blocked migration corridors present very dangerous threats to migrating species. (Aaron Collins, U.S. Fish and Wildlife Service)
The instincts that guide animals along their migrations have evolved over the millennia, yet Princeton University ecology professor David Wilcox stresses that migration in these times represents an extraordinary “act of faith.” Migrating animals do not know what might have become of their wintering grounds, spring nesting sites, or stopovers that have provided food and water for centuries. If migration corridors continue to erode, animals will have one less option for survival.
