Barry Sinervo crosses the world every summer to go on a lizard quest. The herpetologist at the University of California, Santa Cruz, travels to the lowlands in countries such as Finland, France, and Slovenia in search of Lacerta vivipara, the European common lizard. Lately, says Sinervo, the lizard is becoming harder and harder to find. As global temperatures rise, that could soon become true for lizards worldwide. New research by an international team of biologists, including Sinervo, suggests that 20% of all lizard species could go extinct by 2080 if climate change is left unchecked.
The global nature of the problem first struck Sinervo several years ago when he began comparing his field notes on lizard extinctions in France with his colleagues' observations from Mexico. As fast as lizards were disappearing from Europe, they were disappearing from Mexico even faster. Since 1975, Mexico has lost 12% of its lizard species. Lacking any other readily recognizable reasons why lizards were dying off, the researchers speculated that global warming played a part. Lizards are highly sensitive to air temperature, and many will die if they search for food for too long while it's hot out. Yet reducing foraging time to stay cool might be enough to drive some lizard species to extinction, Sinervo thought.
Sinervo and his colleagues recently went to the Yucatán to test this theory. With thermal sensors, PVC pipe, plastic caps, and automobile primer paint, they created "electronic lizards"—temperature sensors painted to have light reflectivity similar to that of a lizard—and placed them at two sites where lizards could still be found and two where they'd gone extinct. After recording temperatures for 4 months, the researchers found that, in line with Sinervo's theory, daytime temperatures at the sites where lizards had gone extinct would have severely reduced the amount of time lizards could forage, whereas temperatures at the sites where lizards still lived were milder enough to allow them plenty of time to seek food.
Sinervo then developed a model for predicting how much hotter than average daytime temperatures need to be for local lizard populations to die out. To test this model against existing historical data on local climate and population, he asked scientists from 12 countries in North America, South America, Europe, Africa, and Australia to see if their own local lizard populations—comprising 34 taxonomic families—fit the pattern of increased daytime temperatures leading to extinctions. They did. The model also predicted that in places where global warming's effects have so far been less dire, such as South Africa, lizards would be faring better. Aaron Bauer, a biologist at VillanovaUniversity in Pennsylvania, who frequently works in South Africa, confirmed that the model—published in tomorrow's issue of Science—fit there, too. "The model is very, very simple, and it's almost surprising how well it fits the data," Bauer says.
Extrapolating their model out to the year 2080, based on current rates of carbon emissions, the researchers expect that many places along the equator could see as much as 40% of their local lizard populations go extinct, and up to 20% of lizard species worldwide may vanish. Especially vulnerable are lizards that give live birth, as they tend to have lower tolerance for high temperatures.
"There are several species in Mexico, for instance, that we're going to watch wink out of existence," Sinervo says. "As a scientist, it's really depressing to see the thing you really want to study disappear before you even get a chance to see it."
Sinervo adds that 20% might even be on the conservative side: Lizards that are able to move to higher elevations to escape the heat could wind up displacing, and causing the extinction of, other lizard species.
He and his colleagues concluded that it's likely too late for many lizard species to avoid extinction, even if governments soon managed to drastically reduce carbon emissions. Rapid action to protect the climate, however, could avoid the worst-case scenario.
"I would hope that our predictions for 2080 get modified," Bauer says. "Lizards really are important components in their ecosystems."
Walter Jetz, an ecologist at YaleUniversity who was not involved in the project, says the new model is one of the first to convincingly demonstrate a link between heat-related behavior patterns and actual extinctions. "The study provides a strong—and disheartening—baseline assessment of one of the challenges faced by lizards in a rapidly changing world," he says. Jetz isn't sure of the team's 2080 prediction, however, noting it's unlikely the study's relatively few number of lizards surveyed can accurately predict the fate of lizards worldwide.
Eric Pianka, a zoologist at the University of Texas, Austin, is skeptical of the model itself, questioning its applicability to nocturnal lizards. Habitat destruction and other kinds of human involvement play at least as great a role in lizard extinctions as global warming, he adds. "I certainly hope their dire predictions do not come true," Pianka says. "A world without lizards would be a sorely impoverished placed."