Twelve percent of Mexico's spiny lizard population has been driven to extinction over the last quarter-century by increasing local temperatures, a phenomenon that is linked to global warming, researchers said Thursday.
The results suggest that, if warming continues, nearly 40% of all lizard populations globally and 20% of all lizard species could become extinct by 2080, the authors said.
Lizards may not be cute and cuddly animals, but they are a valuable link in the global food chain, consuming large amounts of insects and serving as food for larger species. If they disappeared, the viability of other species could be threatened, the team reported in the journal Science.
The researchers "deliver a disturbing message," biologist Raymond B. Huey of the University of Washington and his colleagues wrote in an editorial accompanying the report. "Climate-forced extinctions are not only in the future, but are happening now."
Because lizards are cold-blooded animals, it might seem at first blush that higher temperatures would be beneficial. But that's not the case, said evolutionary biologist Barry Sinervo of UC Santa Cruz, the lead author of the report. "These lizards need to bask in the sun to warm up, but if it gets too hot they have to retreat into the shade and then they can't hunt for food," he said at a news conference.
The team found that, at the sites where extinctions were occurring, the number of hours per day the animals could spend in the sun had dropped sharply. "They would barely have been able to emerge to bask before having to retreat," Sinervo said.
The effects were most pronounced during the spring breeding season, when it is most important for the animals to eat lots of food to be able to produce offspring. If they can't produce the normal number of young, the population collapses.
Lizards that bear their young alive are at greater risk — perhaps twice the risk — than those that lay eggs because they have evolved lower body temperatures, Sinervo said. "We are literally watching these species go extinct before our eyes," he said.
Sinervo did not set out to study extinctions. Instead, he was planning to use a Eurasian lizard to determine the role of skin color in evolution. But when he revisited sites where the lizards had been earlier, there were far fewer of them.
He then became aware of a similar problem with Sceloporus lizards in Mexico. Ecologist Jack W. Sites Jr. of Brigham Young University, while he was a graduate student during the late 1970s and early 1980s, had documented the spiny lizard populations at more than 200 sites in Mexico from the Rio Grande Valley south to Mexico City.
But when researchers returned to the sites, "the habitat was still there, but the lizards were hard to find or gone," Sites said. "If it's sunny, the lizards aren't hard to find. When you go back and don't see lizards, the alarm bells go off."
Sinervo then mobilized a larger team that studied other sites around the world with similar results, they reported. He also developed a mathematical model which correlated lizard populations with temperatures and, thus, the number of hours that the animals could spend in the sun each day. The model identified five other sites where the team found that extinctions had occurred.
Although most lizards evolve too slowly to adapt to the changes, some are able to survive by moving to higher altitudes where it is cooler. But the team found that this displaces other lizard species that normally live at those altitudes, leading to their loss.
Reducing carbon dioxide emissions could prevent some of the future extinctions, Sinervo said, but the amount already present is sufficient to cause a loss of 20% of the lizard population and 6% of lizard species worldwide by 2050.
The researchers are convinced that their conclusions are correct but, Sites said, "This is one time when I would love to be proven wrong."