At the same time, a little bit of warming may actually move certain organisms, particularly insects, in the high latitudes closer to their optimal temperature, the researchers say.
"In the tropics, most of the organisms we have studied, from insects to amphibians and reptiles, are already living at their optimal physiological temperatures," said Curtis Deutsch, UCLA assistant professor of atmospheric and oceanic sciences and co-author of the study. "When warming starts, they do less well as they move toward the hottest end of their comfort range. Even a modest increase in temperature appears rather large to them and negatively impacts their population growth rates."
Why should we be concerned with the fate of insects in the tropics?
"The biodiversity of the planet is concentrated in tropical climates, where there is a tremendous variety of species," Deutsch said. "This makes our finding that the impacts of global warming are going to be most detrimental to species in tropical climates all the more disturbing. In addition, what hurts the insects hurts the ecosystem. Insects carry out essential functions for humans and ecosystems — such as pollinating our crops and breaking down organic matter back into its nutrients so other organisms can use them. Insects are essential to the ecosystem."
At least for the short term, the impact of global warming will have opposing effects. In the tropics, warming will reduce insects' ability to reproduce; in the high latitudes, the ability of organisms to reproduce will increase slightly, Deutsch said. If warming continues, the insects in the high latitudes would eventually be adversely affected as well.
"Our results imply that in the absence of any adaptation or migration by these populations in the tropics, they will experience large declines in their population growth rate," Deutsch said. "This could lead to a fairly rapid population collapse, but organisms are adaptable; the question is, what will their response be? They could migrate toward the poles or toward higher elevations, for instance."
"We don't think this is restricted to insect species," Deutsch said. "Data on turtles, lizards, frogs and toads show patterns that are very similar to what we find for insects. They will do much worse in the tropics than in the high latitudes."
Scientists have measured in laboratories how sensitive different species are to changes in temperature. For insects, the data is comprehensive and includes information on how temperature affects the population growth rate for species, Deutsch said. He and his colleagues — who included Joshua Tewksbury, assistant professor of biology at the University of Washington, and Raymond Huey, professor of biology at the University of Washington — studied the data, then went to climate models and analyzed what the predicted temperature change in various regions implied about species' future growth rate.
According to climate predictions, more rapid rates of warming of the Earth's surface will occur in the higher latitudes, especially in the polar regions, than at the equator, Deutsch said.
"You would think a larger warming in Alaska would have a greater impact on the organisms living there than a much smaller increase in, say, Panama or Costa Rica," he said. "We found the opposite will be true. A 1-degree temperature change in Panama will not be felt the same way by an organism as a 1-degree temperature change in Alaska."
The range of temperature tolerance that an organism has is largely dependent on how much temperature variability it experiences. In the tropics, the amount of temperature variability is very small; there is little difference between summer and winter, while in Alaska, the seasons are dramatically different.
To live in their environments, organisms in the tropics should have a relatively narrow tolerance for temperature change, while in the high latitudes, organisms should be able to tolerate a much wider variation in temperature.
"The magnitude of the impact of global warming depends largely on what we do to slow it down," Deutsch said.
Stuart Wolpert | EurekAlert!
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