On the move

Tens of thousands of species migrate, and the journeys they take are as different as the creatures themselves. Arctic terns migrate from their nesting grounds in the Bering Sea to the Antarctic Ocean, a circumpolar voyage of 35,000 kilometers that may be without equal in the animal kingdom. At the other extreme, spotted salamanders in Maine awake from their winter hibernation in abandoned shrew burrows and trek 135 meters or so across the forest floor to their breeding ponds. Some species of birds and sea turtles appear to follow an invisible roadmap created by the earth's magnetic field. Other animals rely on landmarks such as mountain ranges and coastlines, the alignment of the stars in the night sky, or olfactory cues to determine where they're going. And for plenty of species, we simply don't know how they find their way. What we do know is that migration has become an increasingly dangerous journey for many species, resulting in greatly diminished populations. Radar images of flocks of migratory songbirds crossing the Gulf of Mexico in the springtime show a 50 percent drop in the number of flocks between the mid-1960s and the late 1980s. By studying historical records, biologist Joel Berger has calculated that the number of migratory routes used by elk and pronghorn in the Yellowstone region has declined by nearly 60 percent and 80 percent, respectively. The number of salmon swimming up the Columbia River each year to spawn has dropped by more than 90 percent since the time of Lewis and Clark, and most of the fish that make the journey today are produced in hatcheries, not in the wild. The primary threats to animal migrations are habitat destruction, physical obstacles, overexploitation, and climate change. Habitat destruction, often described as the primary threat to biodiversity in general, is an especially serious threat to migratory species because it can happen anywhere on the breeding grounds, winter grounds, or the stopover sites in between. Obstacles to migration -- ranging from hydroelectric dams for fish to cell towers for bats to barbed-wire fences for antelope -- impede or stop an animal's journey. Not surprisingly, in a world of growing affluence and technological sophistication, more and more of the passageways for wildlife are being obstructed or destroyed. And then there are the political hurdles conservationists face in their efforts to protect migratory species. Migratory animals cross borders with abandon, yet those borders demarcate independent nations, states, agencies, institutions, and cultures that somehow must coordinate their conservation efforts if the species is to prosper. A Swainson's thrush migrating from its wintering grounds in Brazil to its breeding grounds in Manitoba will pass through (or over) ten different countries and more than forty states, provinces, departments, and other major subnational jurisdictions, not to mention countless cities, towns, and villages. The newest threat to migration is global climate change. It holds the potential to erase key habitats for certain species, such as the nesting beaches of sea turtles. It may also disrupt the carefully timed movements of predators and prey. For example, in the Netherlands, scientists have attributed a recent, sharp decline in the population of pied flycatchers to the fact that caterpillars, which the birds feed to their nestlings, are appearing earlier and earlier in the spring. Now, by the time the flycatchers have returned from their wintering grounds in Africa, set up their territories, and hatched their eggs, the peak of the caterpillar emergence has passed. Across the world, then, animal migrations are diminishing in number and splendor. The irony is that just as the phenomenon of migration is slipping away, we are entering a golden age for studying it, thanks to advances in science and technology. Transmitters weighing less than a dime can now be attached to creatures as small as a thrush, while larger transmitters capable of communicating with satellites have already been attached to everything from great white sharks to polar bears to whooping cranes. Almost every aspect of migration inspires awe: the incredible journeys migratory animals undertake and the hardships they face along the way; the complex mechanisms they use to navigate across the land and through the skies and seas; the increasingly sophisticated tools with which scientists study them; and, not least, the perseverance of the people striving to save these animals in the face of a more congested, inhospitable world. It all adds up to one of the most daunting yet rewarding challenges in wildlife conservation.David Wilcove is professor of ecology, evolutionary biology, and public affairs at Princeton University, where he studies the conservation of biodiversity. His book, No Way Home: The Decline of the World's Great Animal Migrations, was published this year by Island Press.David S. Wilcove mail@the-scientist.comLinks within this article:B. Grant, "Cataloguing life," The Scientist, December 1, 2007. http://www.the-scientist.com/article/display/53881/A. Gawrylewski, "Opening Pandora's locks," The Scientist, October 1, 2007. http://www.the-scientist.com/2007/10/1/46/1/A. Harding, "Iraq's marshes return," The Scientist, August 1, 2006. http://www.the-scientist.com/article/display/24069/P. Woodworth, "What price ecological restoration?" The Scientist, April 1, 2006. http://www.the-scientist.com/article/display/23277/L.M. Silver, "A nasty mother," The Scientist, July 1, 2006. http://www.the-scientist.com/article/display/23827/D.S. Wilcove, No Way Home: The decline of the world's great animal migrations, Island Press. http://www.islandpress.org/books/detail.html/SKU/1-55963-985-7

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