We Now Know a Bit More About What Makes California Native Turtles Sick


The western pond turtle, California's only native freshwater turtle, has been on the decline for decades, and biologists have been scrambling to find ways of making life better for the species. A recent study fills in a scientific blank about the fetching reptile.

Known to many biologists as Emys marmorata (though there are plenty of conflicting names out there as scientists wrestle with turtle taxonomy), the western pond turtle's decline is likely due in large part to conversion of the state's freshwater ponds to residential and agricultural use, as well as water pollution in what ponds we've left. But the threat that's gotten the most attention lately is competition from introduced freshwater turtles species, especially including the red-eared slider.

Red-eared sliders, native to the southeastern regions of the United States, have aggressively outcompeted western pond turtles since they were first introduced to the state. The newcomers have displaced western pond turtles from many of the best remaining bodies of suitable fresh water throughout the state. But a new study suggests that one concern of wildlife biologists about the red-eareds' effect on western pond turtles may not be as big a problem as feared: it looks like western pond turtles aren't catching illnesses from their exotic relatives.

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Once widespread throughout California, western pond turtles, which generally max out at just eight inches in carapace (shell) length, have taken a demographic nosedive since the 19th century. Direct human impacts such as eating the turtles and destroying their habitat were the biggest problems earlier in California's history. Simple human presence still poses a problem for the turtles, which will go so far as to abandon favored basking sites if traffic on nearby roads is too high.

But the introduction of red eared sliders to California, mainly as disposable pet turtles, has become a serious threat to the western pond, and which biologists have been monitoring worriedly for some time.

A very popular pet turtle, red-eared sliders (Trachemys scripta elegans) are commonly "released to the wild" by short-sighted owners when they grow too large and cumbersome to be kept conveniently. With the burgeoning of the animal rights movement, the release problem has mounted as turtles for sale in live animal food markets are often purchased by people intending to release them en masse. Once in the wild, red-eared sliders favor the same kind of habitat and food preferred by the western pond turtle.

The problem for western pond turtles is that red-eareds are bigger, with carapace lengths of a foot or even more. As the exotic turtles can thus outmass the natives by a considerable amount, the sliders can easily displace the pond turtles by eating up the food supply or even just shoving them out of the way.

Red-eared sliders in a pond in San Francisco | Photo: Chris Clarke

Herpetologists had long suspected that the newcomers might also be contributing to the western pond turtles' decline by acting as vectors for disease organisms. Some studies have been made of diseases crossing turtle species lines in captive populations. But until recently, no researchers had looked at disease in wild populations of the western pond turtles in California.

In the study, published late last year in the journal Diseases of Aquatic Organisms, researchers from the U.S. Fish and Wildlife Service, the California Department of Fish and Wildlife, the UC Davis School of Veterinary Medicine, and the Hubbs-SeaWorld Research Institute examined populations of western pond turtles and red-eared sliders at 10 study sites throughout California. The scientists tested turtles at all the sites for four pathogens: Salmonella, the same group of bacteria that poses the well-known public health risk to humans; Ranavirus, which infects several species of turtles including red-eared sliders; Mycoplasma, an odd bacterium that causes respiratory ailments in both western pond turtles and red-eared sliders; and Herpesvirus, a common virus in reptiles that in freshwater turtles seems mainly to cause serious liver damage.

The results were a two-pronged bit of good news. The first prong: None of the turtles sampled showed signs of infection with Salmonella, Ranavirus or Herpesvirus.

The researchers did find turtles infected with Mycoplasma. But the group didn't find clear evidence that western pond turtles who lived near red-eared sliders had any greater risk of Mycoplasma infection than pond turtles with no slider neighbors. If pond turtles were catching Mycoplasma infections from red-eared sliders, you'd expect greater rates of infection where the two species shared ponds. No such evidence turned up.

Instead, rates of Mycoplasma infection in both species seemed linked to geography, with the likelihood of infection increasing the father south in California the sampling took place. Northern California turtles were Mycoplasma-free, while about a quarter of turtles sampled in Southern California were infected.

What that geographic distribution means isn't certain, though climate and water quality come to mind as likely factors. But at least the study offers a tentative hope that for all the damage the invasive sliders are doing to California's western pond turtles, they at least seem not to be making them sick, at least at first glance. We'll take our good news where we find it.

For ongoing environmental coverage in March 2017 and afterward, please visit our show Earth Focus, or browse Redefine for historic material.
KCET's award-winning environment news project Redefine ran from July 2012 through February 2017.

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