UC Davis alumni conduct groundbreaking study to understand competition between invasive, native turtle species

UC Davis alumni conduct groundbreaking study to understand competition between invasive, native turtle species

Photo Credits: ADAM CLAUSE / COURTESY

Native western pond turtles benefit from removal of invasive red-eared slider population in UC Davis Arboretum

    Today, UC Davis Arboretum visitors can see two species of turtles: the native western pond turtles and the non-native red-eared slider turtles. Both live together and swim peacefully through the algae-coated waters — sometimes, if caught at the right moment, visitors might witness their adorable faces peeking out of the water or sunbathing on logs and rocks. Behind the scenes of this seemingly harmonious lifestyle between the two turtle species, however, is a competition for food and habitat space.

After a group of former UC Davis students studied this relationship in the wild and co-wrote a paper, they found that the living conditions and health of both turtle species can be greatly increased by removing large percentages of the invasive red-eared slider turtle species.

    “Nobody has ever studied this competition, which is why we wanted to do it,” said Greg Pauly, a wildlife, fish and conservation biology graduate who is now the curator of herpetology and co-director of the Urban Nature Research Center at the Natural History Museum of Los Angeles County.

Although red-eared sliders are only native to the central U.S., they have been introduced all over the world when abandoned as pets. This has contributed to sliders becoming one of the most invasive species in the world, according to the Global Invasive Species Database under the International Union for Conservation of Nature. The western pond turtle is currently undergoing review for listing under the U.S. Endangered Species Act, according to Max Lambert, a postdoctoral fellow at UC Berkeley and a UC Davis graduate in wildlife, fish and conservation biology.

“People buy [red-eared sliders] when they are hatchlings, when they are beautiful and cute,” Pauly said. “People take them home and do not realize they will live up to 30-40 years and can grow as large as dinner plates. These spur-of-the-moment decisions become big investments, and people regret it and abandon the turtles in urban and rural waterways. Now we have the turtles found in all but two states in the United States and on every continent except Antarctica.”

    Despite the spread of sliders before this study, there was no data on how red-eared sliders impact the native western pond turtles because the relationship between these species has never been studied in the wild, Lambert said. Experiments like this have been conducted in labs, but this paper demonstrates how non-native species can impact native and imperiled species in the wild, which makes this study important. 

    “These turtles are imperilled everywhere they live,” Lambert said. “The non-native turtle is pushing the native turtle away from best spots.” 

Along with Lambert and Pauly, UC Davis alumni Jennifer McKenzie, Robyn Screen, Adam Clause, Benjamin Johnson and Genevieve Mount — along with Brad Shaffer, a professor in the evolution, ecology and biodiversity department — all contributed as co-authors of the study. Their first paper was written in 2003, as the project began when the co-authors were undergraduates at UC Davis. The turtle removal and observations, however, occurred over a period of several months in 2011 and 2012.

Over four months, researchers removed about 90% — or about 177 turtles — of the red-eared slider population to see if the pond turtles changed their habitat use, according to Lambert.

To capture the red-eared slider turtles, the researchers used traps resembling giant funnels in which the turtles swim in without being able to escape. To get turtles smart enough to avoid the traps, researchers jumped in the water with giant nets, Lambert said. Without a place to relocate these turtles, they were euthanized and donated to the UC Davis veterinarian school. Since the diet of these turtles consists of eating human trash, sliders were also donated to a Los Angeles County Museum of Natural History study on urban diets and a UC Davis museum also conducting a study on diet.

After the turtle trapping, the research team spent several months monitoring the basking behaviors of the turtles and tracking their weight. After completing the study, the team found that sliders do have an impact on the western pond turtle, Lambert said. 

Overall, the western pond turtles were healthier and better able to use their habitat freely. The turtles increased their weight by about 40 grams, which is 5-10% of their bodyweight. Female turtles also increased the number of eggs they were able to produce. This body mass data directly shows how competition between the species was present, Pauly said, which was the first time this had been documented in the wild.

“The western pond turtles had a more even habitat use and were more evenly distributed out,” Lambert said. “They do not like being near other turtles, and they were forced to be super dense, so the pond turtles could spread out to be where they wanted to be.”

Additionally, the remaining non-native red-eared sliders benefited from their decreased population as well. 

“The non-native turtles got much bigger,” Lambert said. “They gained 5-15% of their body weight, which is a high increase for one year. There was much more food to eat, and less mouths to feed.”

Additionally, the invasive species could choose better habitats to live in that better suited their species. They abandoned large areas and resided in smaller spaces closer to people, which they actually prefer, Lambert said. The turtles were also able to bask more and do so in better locations — meaning they could rest on logs and rocks out of the water without having to compete.

“They were negatively impacting themselves because there were so many of them,” Lambert said. “Decreasing their populations relieves them of competition amongst themselves and best habitat use.”

The Davis Arboretum is not the only place that suffers from the issue of turtle overpopulation. When people abandon their pets, it subjects them to unfair treatment, according to Lambert. The turtles have to live in unpleasant and overcrowded conditions and in unfamiliar places where they have to fight among each other for food and habitat space. To fix this issue, Lambert said individuals need to stop buying and abandoning these animals in the wild.

Since this is the first time scientists have studied the relationship between these turtles in the wild, Lambert said that more people need to be monitoring the habitats around the world to understand the relationship between these two specific native and invasive species.

“There are lots of sliders across the world, and nobody is monitoring the success of the removals,” Lambert said. “You can take data before and after on native species because you want to know if the invasive species is hurting environment if you are going to invest lots of time and money into it.”

The research team said they were lucky to be able to use the Arboretum to conduct the study. 

“We do really important conservation research right where we live,” Lambert said. “UC Davis is a great place for research opportunities early on in students’ school experiences and careers.”

Written by: Margo Rosenbaum — science@theaggie.org