Chesapeake Biological Laboratory

Fear for the turtle

Diamondback terrapins face increasing threats from climate change

Diamondback terrapins face increasing threats due to the climate change, including loss of habitat and rising temperatures that impact the sex of hatchlings.

When it made the diamondback terrapin its mascot, the University of Maryland helped raise awareness about an important local species. Now, as coasts erode, habitat disappears, and waters warm, that awareness might be the terrapins’ best chance of survival amid a changing climate.

Christopher Rowe, an associate professor at the University of Maryland Center for Environmental Science’s Chesapeake Biological Laboratory, has spent 15 years working with the state reptile, but narrowed his focus to the effect of climate change on them in recent years because of the increasing threats they face.

“There is a broad range of questions that need to be addressed,” Rowe said. “Once we do address them, then we can start to say what the future might hold for terrapins in the Bay. We’re not there yet.”

Terrapins have a long history in the state of Maryland. Historically, they were a source of food for Chesapeake colonists because they were abundant and easy to catch, according to a state profile of the species.

It wasn’t until decades later in 2007 when the state made it unlawful to take or possess diamondback terrapins for commercial purposes. More recently, in 1999, the state Department of Natural Resources, put forth another regulation to protect the terrapin, this time requiring all recreational waterfront property owners who set crab pots to attach bycatch reduction devices to avoid capturing turtles.

It isn’t clear how large the terrapin’s total population is today, but what is clear is that even with these measures, the species is increasingly being threatened by climate change.

Rowe has been looking at the impact of climate change on terrapins from two angles: ecology and habitat, and the way they respond to physical changes.

Losing habitat

Several sub-species of terrapins can be found from the Gulf of Mexico to New England. The northern diamondback terrapin species can be found from North Carolina north. In Maryland, however, they face a greater threat from rising sea level than in many other areas.

“With climate change, sea-level rise is occurring and in Maryland at a pace greater than in a lot of other places,” Rowe said.

Speaking of a recent collaboration with Ryan Woodland, a Chesapeake Biological Laboratory assistant professor, he added, “We’ve seen that as sea level is coming up, some of the estuarine marshes that terrapins rely on are starting to disappear.”

The species use the marshes to feed and hibernate, and they use low-lying sandy areas for nesting. Based on data from over 500 nests in Maryland, the average height above high tide for a terrapin to make its nest is about 18 inches, Rowe said. If nests go underwater during high tides or storms, the eggs will drown.

Sea-level rise isn’t the only problem for terrapins.

Human population growth is a threat, Rowe said. Development cuts down the abundance of habitats for terrapins. Many waterfront houses have bulkheads to keep the land from eroding, but that prevents terrapins from accessing the land to nest.

Having fewer areas to nest also could leave them more exposed to predators. Already, Rowe said, the chance of survival for hatchlings is low because of potential run-ins with predators, such as large fish, crabs, mammals, and birds.

“The bigger the human footprint, the smaller the terrapin footprint,” he said.

Rowe refers to the terrapin colloquially as the polar bear of the Chesapeake Bay. Like polar bears at risk for losing habitat in the Arctic, terrapins face a similar risk in Maryland.

“When we look at loss of habitat, we’re looking at an overall reduction of marsh communities and the marsh communities are what feed the turtles,” Rowe said.

Poplar Island in the Chesapeake Bay is one place that offers a safe haven for terrapins amid all the changes. The island that eroded away over time is being rebuilt beyond its original 1,100 acres. Scientists at UMCES’ Horn Point Laboratory have been monitoring marsh growth to see if it can keep up with sea-level rise. Because the island is predator-free and brings back some of the habitat they have lost on the mainland, Poplar has been a successful refuge for terrapins, Rowe said.

While habitat seems to be the most pressing issue for terrapins now, Rowe suspects a long-term issue as climate changes will be rising temperatures. Terrapins are ectotherms, or “cold blooded,” meaning their body temperature is the same as their environment. As the water or air warms, terrapins will, too, Rowe said.

That is problematic physiologically for a few reasons. For one, body temperature sets the rate of metabolism. As they warm, terrapins will have a higher metabolic expenditure that would demand they eat more often. Terrapins typically eat snails, mollusks, and small clams.

Responding to physical changes

The sex of a terrapin hatchling, like many turtles, is dependent on temperature, and warmer temperatures produce more females. “In the future, there’s really no denying that if warming continues, sex ratios will change,” says Rowe.

“They are almost experiencing a double jeopardy situation where temperatures are going up, which means they have to eat more, but the habitats that they use for feeding are declining because of sea-level rise,” he said. “They are kind of caught in the middle. No matter how you slice it, conditions in the future will be worse for them than they are today.”

The other issue with rising temperatures is that the sex of a terrapin hatchling, like many turtles, is dependent on temperature. The temperature of the nest dictates the sex of the hatchling, and warmer temperatures produce more females.

“In the future, there’s really no denying that if warming continues, sex ratios will change,” Rowe said.

He hopes to get a project funded to forecast the turning point when there won’t be enough males to sustain the population.

Rowe has started monitoring how temperatures of nests naturally fluctuate daily. He plans to incubate eggs under those naturally fluctuating conditions at different average temperatures projected by climate change models to evaluate the sex ratios of the offspring. He hopes his work can help drive conservation efforts to address the sex ratio issue.

“Once we know the scope of the problem and when the turning point might occur,  then we can start considering strategies, like maybe we’ll want to artificially incubate eggs at male temperatures or perhaps put shades over natural nests to keep them cooler,” he said.

Looking forward

Beyond conservation, Rowe hopes his research can help him educate the public about threats to the terrapin. A general awareness of and appreciation for terrapins across the state has helped. People seem to be generally alert for turtles in roadways, for example, but there is more people could do to help terrapins, he said.

Boaters should be alert for terrapins when cruising through shallow, near shore waters, Rowe said. Only their head will appear above the water, but avoiding them could save their life.

Rowe also noted that ghost crab pots have been a threat to turtles that can become trapped inside and, unable to reach the surface to breathe, then drown. While the state urges residential crabbers to use turtle excluders that prevent adult terrapins from entering the trap, Maryland Department of Natural Resources data shows the compliance rate is around 50 percent. Rowe encouraged everyone to use the devices on their crab pots to protect turtles without deterring the crab harvest.

Additionally, he said, people can advocate for terrapins by supporting efforts to protect their habitat and lessen shoreline development, but should also start to think about climate change from a wider view.

“Terrapins are just one part of a complex system,” Rowe said. “If we conserve the habitats themselves through limiting impacts of climate change, many other species will benefit, beyond just the terrapins.”