by Carl Zimmer
The world is etched with invisible paths, the routes taken each year by uncountable swarms of geese, elk and salmon, of dragonflies, zebras and leatherback turtles.
Their migrations speak to us in some unfathomably deep way. Birders flock to stopover sites like Cape May, N.J., to watch birds on their journeys to the far north in the spring and back to the tropics in the fall. Eco-tourists head for the Serengeti to train binoculars on herds of wildebeest that stretch to the horizon. American schoolchildren watch monarch butterflies hatch from chrysalises in their classrooms and then see them off on their trip to Mexico.
But in his new book "No Way Home," David Wilcove, a Princeton biologist, warns that "the phenomenon of migration is disappearing around the world."
Despite their huge numbers, migratory species are particularly vulnerable to hunting, the destruction of wild habitat and climate change. Humans have already eradicated some of the world's greatest migrations, and many others are now dwindling away. While many conservation biologists have observed the decline of individual migrations, Dr. Wilcove's book combines them into an alarming synthesis. He argues that it is not just individual species that we should be conserving - we also need to protect the migratory way of life.
As a scientist, Dr. Wilcove finds the disappearance of the world's migrations particularly heartbreaking because there is so much left for him and his colleagues to learn. What are the cues that send animals on their journeys? How do they navigate vast distances to places they have never been? How do some species travel for days without eating a speck of food?
Scientists will never be able to answer those questions for migrations that have been wiped out. The journeys of tens of millions of buffalo on the Great Plains will remain a mystery.
But today, scientists are inventing new ways to learn about the surviving migrations. They can tag dragonflies with tiny monitors and analyze the chemistry of feathers to discover the hidden wintering grounds of birds.
Unfortunately, a lot of what they are learning is about all the threats a human-dominated world poses to migrations.
Animals are particularly susceptible to hunting as they migrate, because they swarm in vast groups at predictable times and places. The survival of migratory animals depends on all the habitats along their journey. And a migratory bird's numbers may dwindle if the forests where it winters are cut down, or if its summering grounds are destroyed, or if its stopovers are eradicated.
At least the birds enjoy the luxury of flying; when salmon in the Pacific Northwest swim from the oceans into rivers to reach their spawning grounds, they now must struggle past chains of dams. Redfish Lake in Idaho was named for the color it turned when it filled with thousands of sockeye salmon that had just swum the 900 miles from the sea. This year only four sockeye reached the lake.
In "No Way Home," Dr. Wilcove also describes threats that have only recently come to light. Cowbirds can devastate migrating songbirds in the United States by parasitizing their nests, for example. Cowbird mothers throw out the songbirds' eggs and lay their own instead. It turns out that fragmenting forests are an excellent habitat for cowbirds.
In years to come, Dr. Wilcove warns, global warming may come to have a huge effect on migrations, by dismantling ecosystems and leaving migrating animals without the food they depend on.
It is difficult to come up with a strategy to preserve a phenomenon as multifaceted as an annual migration. If a species of tree that lives only in part of Florida is endangered, the solution is straightforward: try to conserve that little patch of habitat. But migratory animals don't respect international borders. The preservation of their migrations demands that countries to work together to find solutions. Dr. Wilcove points to some good models - Tanzania and Kenya's conservation of the Serengeti plains, and the United States and Canada's efforts to protect the sandhill crane.
But a bird like the red knot, which summers in the Arctic and winters in Tierra del Fuego, the southern tip of South America, stopping along the way to refuel in North and South America, will require an unprecedented level of cooperation.
It is, Dr. Wilcove writes, a worthy fight: "It all adds up to one of the most daunting yet rewarding challenges in wildlife conservation."
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