Tiny temperature increases in the tropics could have a big impact on cold-blooded insects, lizards and amphibians, according to a new study.

Weather records show the tropics have warmed about 0.5 degrees Celsius since 1980, a fraction of the roughly 1.5-degree-Celsius jump recorded in the Arctic.

But the authors of the new study -- published today in the journal Nature -- say the relatively mild warming could have outsized impacts on cold-blooded species that live near the equator.

"If you're in the tropics, most organisms tend to experience very little variation in temperature," said lead author Michael Dillon, a physiological ecologist. "They have very narrow tolerance for changes. Small changes in temperature can push them over the edge."

Cold-blooded species like lizards, amphibians and insects rely on the environment to regulate their body temperatures -- basking in the sun to warm up, or seeking shade to cool off.

As their environment warms, it drives their metabolic rates up. In practical terms, that means they need more food, oxygen and water to survive and reproduce.

The new study shows that effect is less pronounced at lower temperatures, Dillon said. Heating of 1 degree Celsius would have a smaller impact on metabolism for a cold-blooded, land-dwelling species that lives in the Arctic than on one that lives in the tropics, his study shows.

"The assumption has been that effects on organisms will be biggest in the place where the temperature has changed the most," Dillon said. "The underlying assumption is that ... no matter where you start, a change means the same thing. But with physiology, that's rarely the case."

'Non-linear' animal reactions
He and his colleagues based their analysis on nearly 500 million temperature records collected at 3,000 weather stations between 1961 and 2009, feeding that information into models that allowed them to estimate how climate shifts affected the metabolism of cold-blooded insects, lizards and amphibians around the world.

Warren Porter, a zoologist at the University of Wisconsin, Madison, said the paper was "an important reminder of how non-linear animal processes can substantially respond to even subtle changes in the environment."

The results are striking, given that the projections for warming's influence on tropical species' metabolism are "conservative," Porter added, since they are based on models that assume animals are immobile in shade 2 meters above the ground -- rather than on the ground, where it is often warmer.

Still, Dillon said it's not clear whether a faster metabolism would be a boon or a bust for the tropics' cold-blooded insects, lizards and amphibians.

That's because as an animal's metabolism revs up, it requires more food and water to produce enough energy to grow and reproduce. But that food and water isn't always available.

"If resources aren't limiting, you could guess that these elevated metabolic rates could lead to elevated reproduction or growth rates," he said. "But resources could be limiting, in which case, [a species would] run into a wall."

That means that two different species living in the same ecosystem could have very different reactions to a temperature increase.

"If we just pay attention to temperature patterns, that leads us to think we can ignore the tropics, because temperature change hasn't been very great there," he said. "But even though the temperature change has not been great, the effect on organisms may be really, really big."

Reprinted from Climatewire with permission from Environment & Energy Publishing, LLC. www.eenews.net, 202-628-6500