Cold-blooded and other animals that are unable to regulate their internal temperature may have a hard time tolerating global warming, according to an analysis by biologists from the University of California, Berkeley, and San Francisco State University.
A meta-analysis of studies that measured the ability of animals to deal with extremes of heat and cold found that, on average, most ectotherms are not very flexible. Terrestrial ectotherms, such as lizards and insects, are even less adaptable than fish and crustaceans, they found.
As Earth warms, these animals will be "living at temperatures much closer to their limit," said the study's lead author, Alex Gunderson, a postdoctoral fellow at UC Berkeley and SF State's Romberg Tiburon Center. This means that they are less likely to be able to survive yearly temperature swings, which themselves may become more extreme with climate change.
"Because animals have some ability to acclimate to higher temperatures, scientists hoped that they might be able to adjust their physiology to keep up with global warming," he said. "We found by compiling these data in the first large-scale study of hundreds of different animals that the amount they can actually adjust is pretty low. They don't have the flexibility in heat tolerance to keep up with global warming."
As a result, these animals have limited options as temperatures increase worldwide, he said. They can either move northward or to higher and cooler elevations, alter their behavior to spend more time in the shade, or evolve more thermal tolerance than they have today. Global temperature averages are rising so fast, however, that animals may not have time to evolve the physiological ability to tolerate higher heat.
"Our results suggest that their ability to acclimate to increasing temperatures will not buffer them from the changes that are occurring and that they are going to have to depend on behavioral or evolutionary change to persist," Gunderson said.
He and co-author Jonathon Stillman, a UC Berkeley assistant adjunct professor of integrative biology and SF State professor of biology, will publish their analysis online May 20 in the Proceedings of the Royal Society B. The study was funded by the Berkeley Initiative for Global Change Biology and SF State.
Fish better off than lizards
Gunderson and Stillman looked at 112 published studies on temperature tolerance representing 232 species in 394 populations of ectotherms, ranging from amphibians, lizards and insects to fish and crustaceans. Most of these studies involved exposing animals to low or high temperatures for a week or more and then testing their ability to withstand extremes of cold or heat until they are unable to move: a death sentence in the wild.
The general trend is that animals kept at higher temperatures are able to withstand higher extremes. The analysis revealed, however, that once animals had acclimated to higher temperatures, they generally had a lower margin of safety. That is, those kept at 20 degrees Celsius might be able to withstand 30 degrees, but when acclimated to 25 degrees they might succumb at 32 degrees.
Their results also disproved a hypothesis that animals living at higher latitudes might be better at acclimating than tropical species because they see more temperature fluctuations throughout the year. While this is true for cold tolerance in many species, Gunderson and Stillman found, it's not true for heat tolerance.
"Ectotherms living in Canada, for example, do not have more ability to adjust to heat extremes than animals living in the tropics," Gunderson said.
While the study did not pinpoint specific vulnerable species, it did conclude that aquatic ectotherms - fish, crabs, lobster and shrimp - had twice the ability to physiologically acclimate to higher temperatures than terrestrial animals, in particular lizards and insects. Animals living in areas today that are close to their heat tolerance limit would be most at risk, they said.
Gunderson noted that animals worldwide are already adjusting to global warming or going extinct, and the current study was designed to discover why. Is it inflexible behavior, a lack of physiological plasticity or an inability to evolve quickly enough?
"This study will allow researchers to integrate quantitative estimates of acclimation into models of how animals will fare under various climate scenarios," said Stillman, "paving the way for models that incorporate physiological responses to thermal extremes, which may be one of the strongest forces driving evolution today and one that is expected to increase with climate change."
"Ectotherms are very much at the mercy of their environments," Gunderson said. "Now we see that their ability to acclimate to hotter temperatures is unlikely to keep them in the game. They will have to depend on other strategies."