In a rare study on the impact of global warming in the tropics, University of Connecticut ecologist Robert Colwell and colleagues worked their way up the forested slope of a Costa Rican volcano to collect data on 2,000 types of plants and insects.

"Half of these species have such narrow altitudinal ranges that a 2,000 feet uphill shift would move these species into territory completely new to them," said a summary of their article released on Thursday.

Many species would be unable to relocate at all, as most tropical mountainside forests have become "severely fragmented" by human activities.

Tropical lowland forests —the warmest on earth — would meanwhile be challenged by the absence of replacement species. Flora and fauna unable to move uphill could also perish, unless it turns out they they can bear higher temperatures. "Only further research can estimate the risk," the summary said, "but Colwell's report indicates that the impact of global climate change on some tropical rainforest and mountain species could be significant."

In another article, Science reports this week on a similar uphill trek by squirrels, mice and other small mammals in Yosemite National Park in California, one of the oldest wilderness parks in the United States.

Comparing a landmark 1918 study against fresh data about Yosemite's wildlife numbers, it found that small mammals have moved to higher altitudes, or reduced their ranges, in response to warmer temperatures.

"These kinds of changes in community composition have been going on forever," said James Patton, a UC Berkeley professor emeritus of integrative biology who led the field work for the second study in Yosemite.

"The only thing that makes this different is that it has probably happened in our lifetime. It is the speed with which these changes are taking place that gives one pause," he added.