The world's great animal migrations

When Sarah Rayner hiked into Mexico's El Rosario Butterfly Sanctuary earlier this year, she thought she knew what to expect. The Baton Rouge biology teacher was making a long-awaited trip to see the millions of monarch butterflies that migrate there each winter. But nothing had prepared her for the moment when she entered a sun-dappled oyamel grove and was suddenly surrounded by butterflies — in the sky, on every tree, even alighting on her head and shoulders.

Slideshow: World's great animal migrations

Spotting even a single wild creature in its natural habitat is memorable. So it's that much more inspirational to see multitudes — whether in herds, flocks, or colonies — all gathered together and moving forward for a common purpose. Sure, it takes some planning to get the timing right, but experiencing an animal migration is often the highlight of a trip, if not its sole purpose.

Animal migrations happen all over the world, usually for a creature's survival. Whale sharks off Mexico's Caribbean coast follow the climatic patterns that sustain their supply of food and water. Others, including green sea turtles in Costa Rica, travel vast distances each year to return to ancestral breeding or birthing grounds. These creatures migrate en masse not only because of their communal instincts, but because it provides safety from predators.

Increasingly, however, migrating animals are facing greater threats than beasts of prey. According to David Wilcove, a professor of evolutionary biology at Princeton University and author of "No way home: The decline of the world's great animal migrations," climate change, man-made obstacles like roads and dams, and exploitation of natural resources are putting the species involved in these animal migrations at serious risk.
"Great migrations are best viewed as irreplaceable treasures," Wilcove writes, "increasingly scarce reminders of a time when humans did not dominate the earth."

Coordinating conservation efforts with the demands of economic development on our crowded planet can be a Herculean task. Yet there are glimmers of hope, among them, the decision in June 2011 by the Tanzanian government to cancel plans for a major road through the northern Serengeti that would have cut off a critical portion of the wildebeest and zebra migration.

Perhaps the best chance these animals have at maintaining their way of life is for us to appreciate and experience firsthand their extraordinary journeys.

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