Climate change is warming the tropics, too. Average temperatures have increased by 1.4 degrees Fahrenheit (0.78 degree Celsius) in the last 30 years, making them as warm as at any point in the past 2 million years. That increased warmth, however, is not good news for tropical plants and insects, according to a new study in Science.
Ecologist Robert Colwell of the University of Connecticut and his colleagues surveyed more than 1,900 species of plants and insects from sea level to nearly 10,000 feet (3,050 meters) above, along the forested slopes of a volcano near the La Selva Biological Station in northern Costa Rica. The goal was to determine the ranges of currently extant species.
Based on these ranges—and potential further warming of as much as 5.4 degrees F (3 degrees C)—more than half of these plants and insects would need to relocate 2,000 feet (610 meters) farther up the mountainside to maintain the temperatures they enjoy in their present range. And for those species—ranging from trees and epiphytes (rootless plants) to predatory ants and leaf-chewing moths currently thriving in the lowland—such a migration could leave a vacuum in its wake.
"Because lowland tropical forests are already the warmest forests on Earth, there are no replacement species waiting in the wings to replace these lowland species, as there are for many places at higher latitudes," Colwell explains.
In the absence of mountainsides to serve as a cool refuge, those plants and insects that cannot face higher temperatures may disappear as it would require migrations of hundreds or even thousands of miles to find a suitable cooler climate—crossing habitats utterly changed by human impacts. "For lowland tropical species whose geographical range lies far from mountains, for example in the middle of the Amazon," Colwell says, "the prospect for extinction cannot be dismissed."
But other scientists argue that it is possible—and even probable—that such plants might retain the ability to survive in warmer climes. "In the past 45 million years, temperatures have fluctuated often, yet we see no signs of large extinctions in the lowland tropics like those that happened in the temperate regions during glaciation," says ecologist Richard Condit of the Smithsonian Tropical Research Unit in Balboa, Panama. "There is evidence from other places that species or genera or families retain climatic tolerance for very long times." Nor is there evidence about the highest temperatures tropical species can tolerate, given that these are the warmest conditions on Earth, he adds, and it may be that shifts in rainfall patterns will play the deciding role.
"There is a lingering notion that because species live in hot climates, they can withstand even hotter climates," counters study member and plant ecologist Catherine Cardelús of Colgate University. "This, however, is not necessarily true." Only time or experiments, such as taking plants from several elevations and growing them at the same altitude or measuring how plants and insects respond to higher temperatures in the laboratory, will tell.
In the meantime, if these plants and insects must move to escape warmer conditions, then mountains like the volcano near La Selva may provide the nearest and only refuge. "The most likely exit will be tropical mountains where cooler conditions can be found within short distances," argues team member and entomologist Gunnar Brehm of Friedrich Schiller University of Jena in Germany. "That is why we think it is extremely important to protect mountainsides. They already harbor a striking diversity [of life], but can play the Noah's Ark in the future."