In Warming World, Critters Run to the Hills

A heat wave is sweeping the planet, and animals and plants are making a break for cooler climes. Or so scientists have always assumed. It's been hard to tie a species' migration directly to climate change, particularly with human activity destroying ecosystems every year. But researchers have now gathered more evidence for that link by compiling data from 54 scientific papers that collectively map the habitat ranges of more than 2000 species during the past 4 decades. On average, the team finds, creatures move both up mountains and farther away from the equator at a speed that keeps pace with the rate of climate change and at a pace that is far faster than previously predicted.

In 2003, ecologist Camille Parmesan of the University of Texas, Austin, led a study that analyzed all the known papers that had tracked the ranges of species year by year up through 1999. This "meta-analysis," along with several similar ones, found that
animals and plants were increasingly moving farther north and up hills. But based on what he called a "back-of-the-envelope calculation," ecologist Chris Thomas of the University of York in the United Kingdom suspected that the migration rate might actually be much faster than these papers had predicted. And with an extra decade of data from 2000-09—the warmest decade on record—available to him, he and colleagues revisited the topic to do their own meta-analysis. ="#climatechange">

The new study has plenty of limitations, Thomas acknowledges. The scarcity of papers meant that most of those included targeted only Europe and North America, few were from the Southern Hemisphere, and no marine species were included. "We're prisoners of the data," he says. But by analyzing 54 papers that met their criteria, the researchers found that, on average, organisms move up hills at 12.2 meters per decade, twice the rate previously described in the literature. And they move away from the equator at 17.6 kilometers per decade, which is three times the rate previously described.

The researchers also calculated how far a species would have to move in a given region of the world to stay at the same temperature. The actual migration rates, on average, closely follow the rate of warming year by year in that region—strong evidence, the researchers say in their paper published in Science today, of a direct link to climate change.

Thomas was surprised that the species moved so fast over ground, where they have to move 50 to 60 kilometers on average to find a habitat 0.5°C cooler. "That's quite a long distance across human-dominated landscapes," he points out. In addition, he had assumed that the uphill rate would be faster, where species need to move less than 100 meters to drop 0.5°C, but the uphill migration rate was slower, a finding the researchers can't explain. Either species are now stuck on tops of mountains, Thomas speculates, or those in the Northern Hemisphere might be moving around mountains laterally to the colder north face.

Sifting through the data, the authors were also surprised to find there was no difference between taxonomic groups: plants move at the same rate as insects, and birds are no faster than mammals. But when they looked at individual species, they found that within these taxonomic groups, some species move much faster than others, such as the comma butterfly, which moved northward 220 kilometers in 2 decades. And 22% of species, including the Cirl bunting, even move in the opposite direction toward warmer temperatures, suggesting that they are more flexible to changing climates than others, Thomas says.

Parmesan, who was not involved in the current study, is now working on another meta-analysis that will attempt to discover any common traits shared by species that lag behind the trend, as these might be more tolerant of climate change. She praises the authors' findings. "We're all coming up with the same message," she says. "It's nice to see."

Conservation biologist Wendy Foden of the International Union for Conservation of Nature in Cambridge, U.K., calls the findings "hopeful" because they show that creatures, on average, are "keeping track of climate change themselves" and are able to adapt. Still, she worries that "we don't know if species can keep this up if the planet continues to warm at faster rates."