"We know the climate is getting warmer," said Robert Colwell of the University of Connecticut, whose research appears in the journal Science.

"If things continue as projected, there will be a 3 degree Celsius (5.4 degree Fahrenheit) warming in the Costa Rican tropics within the next century," he said.

Colwell and colleagues predict that as the climate in the tropics warms, thermal bands will move up the mountains by about 600 meters (yards) in elevation. "The current climate at 100 meters will be at 700 meters," he said in a telephone interview.

Colwell and colleagues analyzed data on nearly 2,000 species of plants, insects, and fungi in Costa Rica. His team thinks about half of these species would have to move to completely new territory, well beyond the upper ranges on the mountainside.

"If species are stressed by the heat, they will do better in their accustomed climate zone. We expect ranges to move up the mountain as has been documented already in Europe and the United States," he said.

NO MORE ROOM

As a result, lowland populations in the tropics may soon experience decreases in biodiversity and species richness, in part because no other species are adapted to the climate.

And species at higher elevations may run out of room to climb higher.

Colwell said the projection contradicts the assumptions of many researchers, who believe species in the tropics would not be affected as harshly by climate change.

"The current conventional wisdom even among scientists is that tropical species will be OK despite global warming because in ancient times -- 5 to 50 million years ago -- the climate was warmer and there were tropical forests," he said.

"We argue this is not so clear. That there needs to be much more research done to see if this is the case," he said.

In a separate study in the same journal, researchers at the University of California-Berkeley compared recent changes in small mammal populations at Yosemite National Park in California to a study done in 1918.

Not surprisingly, they found mammals like shrews, mice and ground squirrels have moved to higher elevations or clustered themselves in smaller, more hospitable regions.

They saw shifts in about half the species studied.

So far, the population movements have not changed the biodiversity in the park, the researchers said, but they have changed the populations of animals interacting with one another.

"These kinds of changes in community composition have been going on forever," James Patton, who led the field work in Yosemite, said in a statement. "It is the speed with which these changes are taking place that gives one pause."

(Editing by Eric Beech)