A little climate change goes a long way in the tropics

In hot places, even minor warming could rev up metabolism in animals that don’t generate their own heat

While biologists worry primarily about global warming’s effects on life in the Arctic, a seemingly paradoxical result of climate change may be sneaking up on metabolically sensitive creatures in the tropics.

HOT, HOTTER Temperatures have risen more in the Arctic than in the tropics, but the metabolic effects of temperature change may be disproportionately more disturbing to some residents of warmer climates, such as this caiman lizard. Tim Vickers/Wikimedia Commons

Temperatures in the Arctic have climbed upward faster than in low latitudes. Yet a quirk of physiology could mean that slight upticks in the tropics are delivering a disproportionate jolt to some residents, says Michael Dillon of the University of Wyoming in Laramie. Insects, soil bacteria, frogs, salamanders, lizards, snakes and other animals that warm and cool with their surroundings could be getting a major kick in the metabolism from even slight rises in temperature.

For these organisms — the ectotherms — warming up more means speeding up the metabolic pace of life. And Dillon points out that temperature drives metabolism exponentially, delivering way more of a buzz at higher temperatures than at low ones.

Combining established principles of physiology with some 500 million temperature records, Dillon and his colleagues calculated how much the temperature changes in four broad regions may have pepped up local ectotherms. Since 1980, the changes calculated for metabolic rates in the tropics are at least double those in the Arctic, Dillon and his colleagues report in the Oct. 7 Nature.

“It’s taken a while for people to start applying the thinking of climate change to the tropics,” says tropical ecologist Catherine Cardelús. At first biologists tended to dismiss tropical organisms as already highly adapted to heat, says Cardelús, of Colgate University in Hamilton, N.Y. But a growing number of studies are showing that ecology is far from that simple.

Dillon speculates that sped-up ectotherms in the tropics might become more vulnerable to starvation if resources can’t keep pace, he speculates. “If you’re burning more energy, you need more energy,” Dillon says. Food webs may shift. Soil respiration may increase. Mosquitoes may breed faster.

Also, in the tropics, Dillon says, “the potential for big impacts on a global scale is the highest there simply because the biodiversity is the highest.”

He and his colleagues started with ideas from decades of physiology research that show exponential increases in metabolism as temperature climbs. Warming an ectotherm from 15° Celsius to 20° C might not increase metabolic rate much, Dillon says. But dial up the heat from 25° C to 30° C — the same number of degrees — and “metabolic rate could just shoot through the roof.”

To see how that effect might play out with climate warming, the team sorted through weather records to find stations that have taken at least six daily readings year-round since 1961. Data from these 3,186 stations showed Arctic temperatures rising by roughly 1.5 degrees Celsius on average since 1980 while tropical sites barely sloped upward, failing to reach even a 0.5 degree change overall.

To see what those shifts do to metabolism, the researchers tweaked a basic physiological equation to create four versions, one each to represent a generalized single-celled organism, an invertebrate, an amphibian or a reptile. For each of the millions of temperature records, researchers calculated metabolic rates for each of the four groups.

Based on these calculations, warming could hit low-latitude ectotherms harder than high-latitude ones. Arctic temperature increases could double those in the tropics, and metabolic changes in each region’s ectotherms would still be about equivalent.

Dillon cautions that plenty of real ectotherms have some ability to moderate their temperatures through behavior, such as darting under rocks to escape the sun. These measures have limits though, especially for small creatures, and actual biology — not just temperature — needs investigating.

“You can’t assume that the temperature patterns tell you what the effects on biology will be,” he says.

Susan Milius is the life sciences writer, covering organismal biology and evolution, and has a special passion for plants, fungi and invertebrates. She studied biology and English literature.

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