Global warming role reversal

Science has a way of forcing us to reexamine some of our basic assumptions about nature. Consider the following statement: Animals that thrive in high temperatures are more likely to survive global warming than those that are less tolerant to heat. While this conclusion may seem obvious, a Stanford (Calif.) University study finds that the opposite may be true.

An experiment performed by postdoctoral fellow Jonathon H. Stillman examines the effect of climate change on porcelain crabs--inch-long invertebrates that inhabit coastal areas along the Pacific Ocean. The experiment focused on four species: two from the chilly coast off Cape Arago, Ore., and two from Puerto Penasco, Mexico, located on the Gulf of California. He discovered that those in the cool Pacific Northwest have the ability to adjust to larger increases in habitat temperature than crabs living in the warm coastal waters of Mexico.

"This is definitely counterintuitive," he indicates. "You would expect heat-tolerant organisms to be the most resilient to global warming, but it turns out they may have a harder time surviving as their habitat temperatures increase." Moreover, a half-degree jump in the Earth's temperature could be enough to wipe out countless porcelain crabs.

"The results were surprising," Stillman admits. It turned out that coldwater crabs were able to change their upper thermal tolerance limit much more readily than those from hotter climates. In fact, the top survivor in the experiment was Oregon's P. eriomerus, which lives in the coolest habitat of the four species studied. At the other extreme, Mexico's P. gracilis had the poorest showing.

"During the hottest summer low tides, P. gracilis experiences habitat temperatures right at the edge of its thermal range," Stillman observes. "Because this species has a limited capacity to adjust that range, it will be impacted by global warming-related increases in habitat temperature."

Stillman concludes that species with the greatest capacity to change their upper tolerance limits, such as P. eriomerus, were the least heat-tolerant: "On the other hand, creatures such as P. gracilis, which have evolved the greatest tolerance to high temperatures, have a lower capacity for thermal acclimation--and it's those species that will be the most susceptible to global warming. Perhaps it's an evolutionary tradeoff: To survive those really high temperatures in the summer, these animals have given up their ability to adjust their thermal limits."

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