Much of the grassland habitat of the Bay checkerspot butterfly is already destroyed, and studies suggest that climate change will push the insect to extinction. Moving the species is an option, but one that is not without risk. Conservationists have tried to save the butterfly by saving the remaining patches where it survives. But thanks to global warming, that may not be good enough.

Climate scientists expect that the planet will become warmer in the next century if humans continue to produce greenhouse gases like carbon dioxide. The California Climate Change Center projects the state’s average temperature will rise 2.6 to 10.8 degrees Fahrenheit. Warming is also expected to cause bigger swings in rainfall.

Studies on the Bay checkerspot butterfly suggest that this climate change will push the insect to extinction. The plants it depends on for food will shift their growing seasons, so that when the butterfly eggs hatch, the caterpillars have little to eat. Many other species may face a similar threat, and conservation biologists are beginning to confront the question of how to respond. The solution they prefer would be to halt global warming. But they know they may need to prepare for the worst.

One of the most radical strategies they are considering is known as assisted migration. Biologists would pick a species up and move it hundreds of miles to a cooler place. Assisted migration triggers strong, mixed feelings from conservation biologists. They recognize that such a procedure would be plagued by uncertainties and risk. And yet it may be the only way to save some of the world’s biodiversity. “Some days I think this is absolutely, positively something that has to be done,” said Dr. Jessica Hellmann of the University of Notre Dame. “And other days I think it’s a terrible idea.”

Conservation biologists are talking seriously about assisted migration because the effects of climate change are already becoming clear. The average temperature of the planet is 1.6 degrees Fahrenheit higher than it was in 1880. Dr. Camille Parmesan, a biologist at the University of Texas, reviewed hundreds of studies on the ecological effects of climate change this month in the journal Annual Review of Ecology, Evolution, and Systematics. Many plant species are now budding earlier in the spring. Animals migrate earlier as well. And the ranges of many species are shifting to higher latitudes, as they track the climate that suits them best.

This is hardly the first time that species have moved in response to climate change. For over two million years, the planet has swung between ice ages and warm periods, causing some species to shift their ranges hundreds of miles. But the current bout of warming may be different. The earth was already relatively warm when it began. “These species haven’t seen an earth as warm as this one’s going to be in a long, long time,” said Dr. Mark Schwartz, a conservation biologist at the University of California, Davis.

It’s also going to be more difficult for some species to move, Dr. Schwartz added. When the planet warmed at the end of past ice ages, retreating glaciers left behind empty landscapes. Today’s species will face an obstacle course made of cities, farms and other human settlements. Animals and plants will also have to move quickly. If a species cannot keep up with the shifting climate, its range will shrink. Species that are already limited to small ranges may not be able to survive the loss.

In 2004, an international team of scientists estimated that 15 percent to 37 percent of species would become extinct by 2050 because of global warming. “We need to limit climate change or we wind up with a lot of species in trouble, possibly extinct,” said Dr. Lee Hannah, a co-author of the paper and chief climate change biologist at the Center for Applied Biodiversity Science at Conservation International.

Some scientists have questioned that study’s methods. Dr. Schwartz calls it an overestimate. Nevertheless, Dr. Schwartz said that more conservative estimates would still represent “a serious extinction.” Many conservation biologists believe that conventional strategies may help combat extinctions from global warming. Bigger preserves, and corridors connecting them, could give species more room to move. Conservation biologists have also been talking informally about assisted migration. The idea builds on past efforts to save endangered species by moving them to parts of their former ranges. The gray wolf, for example, has been translocated from Canada to parts of the western United States with great success.

When Dr. Jason McLachlan, a Notre Dame biologist, gives talks on global warming and extinction, “someone will say, ‘It’s not a problem, since we can just FedEx them to anywhere they need to go,’ ” he said. No government or conservation group has yet begun an assisted migration for global warming. But discussions have started. “We’re thinking about these issues,” said Dr. Patrick Gonzalez, a climate scientist at the Nature Conservancy.

The conservancy is exploring many different ways to combat extinctions from global warming, and Dr. Gonzalez says that assisted migration “could certainly be one of the options.” For now, the conservancy has no official policy on assisted migration. As Dr. McLachlan began hearing about assisted migration more often, he became concerned that conservation biologists were not weighing it scientifically. He joined with Dr. Schwartz and Dr. Hellmann to lay out the terms of the debate in a paper to be published in the journal Conservation Biology.
(NYT, 24/01/2007)