Bay Nature magazineWinter 2009


Planning for the Pond Turtle’s Protection

October 1, 2009

Scientists are working on the first draft of a conservation strategy for California’s only native freshwater turtle, the western pond turtle. The state Department of Fish and Game (DFG) commissioned the U.S. Forest Service’s Redwood Sciences Laboratory to write the plan, expected by June 2010.

The as yet unknown effects of climate change pose a particular challenge for ecologists, especially since relatively little is known about the ecology of our sun-basking native turtle. One thing scientists do know is that a turtle’s gender is determined by the temperature of the egg about 20 days after it is laid. But no one knows the pond turtle’s threshold temperature. Climate change makes it even more important to find out, says Don Ashton, the lead ecologist preparing the strategy report. Even a change in vegetative cover can alter eggs’ temperature by providing more or less shade, which could skew sex ratios toward one gender or another.

To discover the gender threshold, Sonoma State University biology professor Nick Geist recently collected 55 turtle eggs and incubated them in batches at different controlled temperatures. When the 44 surviving hatchlings reach four months old, he will determine their gender, and from that deduce the tipping point temperature.

But science, or collaboration, is never simple. Geist collected the eggs on land owned by a conservation agency, which required that he return the hatchlings. That’s how his study evolved into a DFG-approved “head start” pilot program. That means taking the eggs from their natural environment, raising them in captivity, and releasing them when they are large enough to be less vulnerable to predators. Such head start programs have been helpful in Washington state, where western pond turtles were nearing extinction, but they were not so successful for sea turtles, in the 1970s, when the resulting clutches had skewed sex ratios, says Ashton.

Refining head-start methods is helpful in case it becomes necessary for pond turtles, he says. “But it is an absolute last resort, and doesn’t help if you don’t deal with habitat problems facing breeding adult turtles.”

DFG considers western pond turtles a species of special concern, but the jury is out on whether the species is in decline, since no comprehensive population surveys have been done. Many biologists extrapolate declines based on the number of turtles sent to San Francisco markets for soup during the late 19th and early 20th centuries, combined with subsequent habitat loss. California Academy of Sciences biologist Galem Rathbun agrees some populations have declined, especially in paved urban areas, but says the turtles are doing well in most of California due to the proliferation of nutrient-rich ponds for cattle and sewage treatment.

About the Author

Writer Aleta George trained as a Jepson Prairie docent in 2009. In addition to writing Bay Nature's Ear to the Ground column, she has written for Smithsonian, High Country News, and the Los Angeles Times.

Read This Next

Native Son