Dec. 4, 2008 -- Turtles are notoriously slow, but their behavior is changing fast in response to climate change. From Nebraska to South Carolina, turtles are nesting up to three weeks earlier in the springtime, according to long-term data from several research groups.
The implications of the shift are reverberating through the ecosystem in surprising ways -- leading to overpopulation in some places and a skewed sex ratio in others. Those changes could spell trouble for the entire food chain.
"If [the trend] continues, it's just going to get ugly," said Fred Janzen, an evolutionary biologist at Iowa State University in Ames. "I'm definitely concerned."
Janzen has been tracking painted and snapping turtles along the Mississippi River between Iowa and Illinois for more than two decades. Year after year, he used to watch as turtle moms started building nests around June 1. Then, in the late 1990s, females began to lay their eggs about a week earlier.
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"I thought, 'Wow, that's early,'" said Janzen. "This was really a change. And it kind of stayed that way. For almost a decade, it's been getting earlier and earlier."
Climate records show that winters in the area have grown substantially warmer during the same time period -- by about 4 degrees Fahrenheit. Last year was the only exception: Temperatures were exceptionally cold, and turtles nested later than they had during the past decade.
Until now, scientists didn't know what determines when turtles build their nests. Janzen's data have not yet been published, but other work supports his conclusions that warmer weather is causing turtles to nest earlier.
At the Great Rivers Field Station in Brighton, Ill., for example, herpetologist John Tucker has documented a 20-day shift over the last 15 years in when red-eared sliders start laying eggs. Since 1998, he said, the average annual temperature in the region has risen by 1.6 degrees Celsius.
"I can't associate [the shift] with any other environmental variables," Tucker said. Colleagues in South Carolina and Nebraska have documented similar trends.
For some turtle species, including sliders, getting a head start on egg-laying means that moms are often able to squeeze in three (instead of the usual two) nests before the egg-laying season ends around the Fourth of July. That means turtles are laying 33 percent more eggs each season, Tucker reported in August in the journal Chelonian Conservation and Biology.
Already abundant along the Mississippi River, sliders in the area are now competing with the region's sport ducks for aquatic vegetation.
Earlier nesting dates are having another unexpected result in some turtle populations: a male boom. Temperature determines whether turtle eggs hatch male or female. And since turtles are now laying their first clutch of eggs earlier, when the ground is colder, more eggs are hatching male.
Those males, in turn, are pestering females for attention. Tucker has evidence that the stress of all that extra attention is taking is toll on female sliders -- leading to lower body weight.
The humble turtle may not be a charismatic bellwether species, like polar bears are. But turtles may yet become a spokes-creature for global warming, said Tucker.
"Turtles are a model," he said. "It's not like we're watching glaciers fall apart in the Arctic. But you don't have to go far to see the effects of climate change. Some guy like me can find these effects going on in our backyard."
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