UC Davis alumni find high levels of plastic ingestion in Arboretum turtles

UC Davis alumni find high levels of plastic ingestion in Arboretum turtles

Photo Credits: A turtle swimming in the waters of the UC Davis Arboretum and Public Garden. (Justin Han / Aggie)

While plastic ingestion has been well documented in sea turtle populations, this study is the first to highlight the issue in freshwater turtles

Littering can have profound impacts on wildlife, causing complications for their health through ingestion or entanglement. Plastic ingestion has been well-documented in all seven species of sea turtles, but the issue is not well understood in freshwater turtles, according to Dr. Greg Pauly, the curator of herpetology and the co-director of the Urban Nature Research Center at the Natural History Museum of Los Angeles County. 

“Plastic ingestion by freshwater turtles: a review and call to action,” co-written by Pauly, was published in Scientific Reports on March 23, 2021, and details results from a field study and a literature review on plastic ingestion by freshwater turtles. This paper was written by Dr. Adam Clause, a postdoctoral researcher at the Natural History Museum of Los Angeles County, Dr. Aaron Celestian, the associate curator of Mineral Science at the Natural History Museum of Los Angeles County, and Pauly. This study is the first to highlight freshwater turtles ingesting plastic—which may be an underappreciated threat to the world’s declining populations of freshwater turtles, according to Pauly. 

Pauly, who earned a B.S. in Evolution and Ecology from UC Davis in 1999 and then returned for a postdoc from 2009–2011, started working with Arboretum turtles in 1998 as an undergraduate and has continued studying that population for several years. Dr. Adam Clause began working with turtles in the Arboretum during his time at UC Davis, where he graduated in 2010 with a B.S. in Evolution and Ecology.

“It’s been fun to work on those turtle populations off and on for the last 20 years,” Pauly said. 

The scientists’ field study examined 65 non-native red-eared slider turtles from the Arboretum and found that 7.7% had plastic inside their stomachs or intestines. One of the ingested plastics was likely a nitrile exam glove. Although the turtle specimens were collected before the pandemic—in 2011 and 2012—this result highlights the potential impacts of increased personal protective equipment (PPE) litter in the environment, according to Pauly. Other ingested plastics likely included a white plastic shopping bag and a piece of disposable cutlery.

“I wasn’t surprised that we were finding these [plastics] because I see animals by these things all the time,” Celestrian said. “It was definitely not a good thing.”

One challenging part of this research was identifying the contents inside the turtles’ stomachs and intestines, as it can be difficult to distinguish plastic from organic material from what the turtles eat, such as crayfish exoskeletons, according to Celestian. To identify the plastic contents, Celestian used Raman Spectroscopy, a chemical analysis technique that details chemical structure, phase and polymorphy, crystallinity and molecular interactions of materials. 

“I have a spectrometer here in the lab called the Raman Spectrometer, which is very good at looking at very small particles and identifying what materials those particles are, even if they’re not recognizable,” Celestian said. “That’s what my main role was, to figure out these bits and pieces of plastics.”

In the scientists’ associated literature review, they found that dietary studies to identify plastic ingestion in freshwater turtles are rarely conducted. Over the past 10 years, nearly 50% of relevant turtle dietary studies have involved only marine turtles, according to Pauly. Marine turtles averaged 15.3 dietary studies per species during this period, whereas non-marine turtles averaged 0.3 dietary studies per species (across the 352 species of non-marine turtles), according to the study. 

The results from the field study and literature review together demonstrate that some freshwater turtle species are ingesting plastic, but researchers have little understanding of how widespread this issue might be because freshwater turtles are not getting the same amount of research attention as their marine counterparts, according to Pauly.

“There’s an increasing awareness of plastic ingestion in lots of marine species, but across the board freshwater environments are much less studied for plastic ingestion,” Pauly said. 

This is not the first study Pauly has conducted on turtles in the Arboretum. Pauly and other researchers at the Natural History Museum of Los Angeles County previously published a study in 2019 that found the living conditions and health of the two primary species found in the Arboretum, the native western pond turtles and the non-native red-eared slider turtles, is greatly improved by removing large percentages of invasive red-eared slider turtles. 

The 177 red-eared sliders removed from the Arboretum during the 2019 study were the turtles examined in this new project, which began as an investigation into understanding the dietary overlap between western pond turtles and red-eared sliders. However, when Pauly and Clause began to dissect turtles and look at their stomach contents, Pauly said they were shocked to find plastics in some of these individuals. 

“That completely changed the direction of the study to then start thinking about, ‘[…] what’s the number of turtles that might be ingesting plastic, what impact that it’s having on that population, and more broadly, is this a common issue for turtles globally?’” Pauly said. 

Right away, Pauly knew this was a noteworthy discovery, and after looking in the scientific literature, the scientists realized how poorly studied this issue is with respect to freshwater turtles. In their paper, the scientists issued a “call to action” for other researchers to look into this issue for freshwater turtles, Clause said.

Scientists are still unsure why the freshwater turtles consume plastic in the first place, according to Clause.

“Two of the most likely explanations are that [turtles] are either confusing it with a natural food item, or they’re just eating it accidentally while they’re eating normal things in their environment,” Clause said. 

UC Davis’ Arboretum and Public Garden works with multiple departments on campus to reduce pollution in the Arboretum Waterway. Prevention is the first step, according to Nina Suzuki, the waterway steward at the UC Davis Arboretum and Public Garden. Suzuki and other Arboretum employees work with many departments to prevent litter and other pollutants from reaching the Arboretum waterway. Some of these measures include education and signage, improved waste collection bins, promoting reusables and compostables, reducing single-use plastic at campus food service outlets, regular street sweeping, physical barriers at storm drain outlets and regular staff maintenance of the waterway and Arboretum. 

“I think this kind of research is important because it reminds us all that our actions have consequences beyond what we typically think of as our sphere of influence,” Suzuki said via email. “Most people who walk through the Arboretum probably haven’t ever looked a turtle in the eye. They probably also haven’t ever dropped a piece of plastic trash into the Arboretum Waterway. And yet, our use of disposable plastics in everything from beverage containers to utensils to toys, has resulted in tiny bits of plastic making their way into places we would never have expected, like the stomachs of turtles.”

The turtle species living in the Arboretum are commonly studied for research and in UC Davis courses, such as Wild Davis, a course taught by Dr. Laci Gerhart-Barley, an assistant professor of teaching, focusing on urban ecology and urban naturalism. 

Since the turtles in the Arboretum illustrate a number of important aspects of urban ecology, such as how invasive species like the red-eared slider turtles impact native species, Gerhart-Barley includes them in the class. As detailed in Pauly’s previous study, the dumping of non-native animals into an ecosystem, such as the Arboretum, can have very negative impacts on native species as well as the non-native species. 

“That’s an ongoing point of concern with long term management of those invasive turtle species,” Gerhart-Barley said.

Gerhart-Barley plans to include this new study in her Wild Davis course as an example of how human activity in the Arboretum can impact the animals in ways that are less visible; in this case, through the ingestion of plastic, she said. 

“That’s one of the problems with ingesting things is the animal might not look sick, or might not look harmed,” Gerhart-Barley said. “It’s not like the plastic copings around their neck or it’s really visible that there’s a problem. They could appear to be completely normal and still be really negatively impacted.”

Beyond understanding the reasons why turtles ingest plastic, this study brought up many more questions that need to be answered regarding plastic consumption—beyond just freshwater turtles, according to Celestian. 

“This is just the first study of its kind,” Celestian said. “I think more studies are definitely warranted to see how, what the level of pollution is versus what the health of the turtles might be.” 

According to Pauly, the goals of this study were twofold: to highlight the issue of turtles ingesting plastic in an urbanized waterway in California and to emphasize that this is an underappreciated problem for freshwater turtles globally.

“We know across the globe, we know that urbanization is a major threat to freshwater turtles as a group are experiencing—some of the highest rates of extinction of any urban group,” Pauly said. “[Plastic ingestion] is potentially a new emerging threat that freshwater turtles globally might be facing.”
Written by: Margo Rosenbaum science@theaggie.org

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