Student Research: Shedding Light on the Invasive Species Quandry
For a long time Lauren Wyman has been fascinated by the function of ecological systems. So when it came to her senior thesis, the Princeton University ecology and evolutionary biology major posed a question: What happens to these systems when they are disrupted by invasive species?
“You hear about invasive species all the time, but people don’t know the extent to which they can impact everything from National Parks to fisheries,” Wyman said. “I thought my research could help get closer to solving a large, global problem.”
Wyman collecting data with a fellow field assistant. Photo courtesy of Tyler Kartzinel.
In May 2013, Wyman set out to study how different combinations of invasive lizard species affect certain populations of native lizards and orb-weaving spiders in the Bahamas. By conducting fieldwork on sixteen Bahamian islands over the course of two weeks, Wyman hoped to not only learn more about the function of these ecosystems, but to also shed some light on the larger invasive species quandary in the process.
The seeds of Wyman’s thesis were first planted during the fall of her junior year, when she met with Robert Pringle, assistant professor of ecology and evolutionary biology. Wyman was drawn to Pringle’s passion for the study of species interactions and she was also intrigued by the ambitious scope of his research.
“Lauren came to me with an itch to understand how food webs are organized, and how major environmental perturbations like species invasions affect the number of species that can coexist in an ecosystem,” said Pringle. “She was really excited about the theoretical dimensions of these questions and wanted to address them in a realistic way.”
A curly tailed lizard marked with red paint—an indicator that this individual had been counted in the population survey. Photo courtesy of Lauren Wyman '14.
Pringle, who would go on to become Wyman’s thesis adviser, had the perfect study system. In 2011, while still a post-doc at Harvard University, he and fellow researchers selected 16 islands to use as experimental model ecosystems. Each island was assigned to one of four different treatments and had its own native population of brown anoles, a small lizard species.
Pringle and his colleagues wanted to know what would happen if two new lizard species were added to these ecosystems, so they introduced the curly-tailed lizard and the green anole in various combinations throughout the 16 islands. Some islands received the curly-tails, another group received the green anoles, and a third group received both; the remaining received none, to serve as controls.
According to Wyman, these three lizards play interesting and complex roles in their respective island food webs. Brown anoles, for instance, feed on several different arthropods, including orb-weaving spiders. They typically live on the ground, but they are known to move into higher vegetation when confronted with ground-dwelling predators. Meanwhile, the curly tailed lizard is a ground dweller and preys on the brown anole. The green anole lives in treetops and competes with its brown cousin for insects and space within the canopy. The overarching aim of the experiment is to understand how the processes of predation and competition interact to shape the island food webs.
One of Wyman’s sticky traps: comprised of a plate coated with super glue used to collect insects over a 24 hour period. Wyman identified the insects caught in the trap in order to obtain an understanding of the insect communities on each island. Photo courtesy of Lauren Wyman '14.
Wyman’s two-week research trip to the Bahamas from May 1 to May 14 was funded by the Becky Colvin Memorial Award, which is presented annually by the Princeton Environmental Institute (PEI) and the Department of Ecology and Evolutionary Biology to support field research projects critical to the senior thesis. In this experimental environment, Wyman was able to explore the primary question of her thesis: How do invasive lizard communities affect native brown anole and orb-weaving spider populations?
“Lauren thinks big and she's intellectually ambitious,” Pringle said. “I think it's striking to see a senior in college undertaking this kind of large-scale experimental work. She is doing is varsity-level field biology.”
During her first week in the Bahamas, Wyman traveled by boat with a team of researchers to five different islands every day for three days straight. The goal was to count as many lizards as possible, marking each one with colored paint while also taking note of the lizard’s species, gender, and perch height. Using a complex algorithm, Pringle was able to quantify the population density of each lizard species on every island.
“Counting the lizards was relatively easy. Capturing them, on the other hand, was extremely difficult,” Wyman said. “They are crazy fast! Probably the fastest things I’ve ever seen. And they’re smart.”
During her second week in the Bahamas, Wyman worked alongside David Spiller, project scientist at the University of California-Davis and a research pioneer in the field of small-island ecosystems in the Caribbean. Over the course of several days, Wyman and Spiller traveled to each of the 16 islands, counting and collecting spiders while also noting the height of their webs in the canopy. This was important because it allowed her to study how the lizards were making an impact on the spider population, whether through a change in density or by forcing them to move higher in the vegetation.
Lauren Wyman with David Spiller, project scientist at the University of California-Davis. Photo courtesy of Lauren Wyman '14.
While her fieldwork was indeed ambitious and taxing, Wyman said she had been well prepared by a 2012 PEI-sponsored internship during her sophomore year, in which she spent 10 weeks in Peru studying the impact of chytrid fungus on local frog populations.
“In a lot of ways the field work I did there was very similar to what I wound up doing in the Bahamas. I had to learn how to catch frogs, how to handle them safely, how to take notes, and how to conduct lab work with live animals,” Wyman said.
While working through the results of her thesis research, Wyman has noted some interesting observations. For instance, on islands where only the curly tailed lizard was introduced, spiders were found perching much higher than on all of the other experimental treatments. However, this was not the case on islands with both curlies and green anoles. In addition, she noticed that neither brown anole nor spider densities were different between the various islands.
“This means our experimental islands are large enough for a lot of different species to co-exist at the same time,” Wyman said. “This is a departure from many previous studies which have seen the curly tailed lizard driving the brown anole extinct, or the brown anole driving the spiders extinct.”
In writing her thesis, Wyman came to several interesting conclusions about the study. First, the introduced lizards had no impact on the density of brown anoles, but they did significantly affect the height at which they perch. Wyman said this impact on height is intuitive. In the presence of the curly-tailed lizards, brown anoles move up in the canopy.
“There might not be an effect on brown anole density because our islands were large, which meant that predator density was relatively low,” Wyman said. “Also our islands supported a rich arthropod community, so the curly-tailed lizard may have supplemented their diet with cockroaches and other ground-dwelling arthropods, decreasing predatory pressure on the brown anole.”
Second, the introduced lizards had no affect on spider density but did have a slight affect on spider height. Wyman’s focal spider species, E. cazieri, perched significantly higher on “curly-only” islands in comparison to all other experimental treatments.
“There is likely an interaction effect between the curly-tailed lizards and the green anoles in determining spider height,” Wyman said. “That the introduced lizards affected height but not density suggests that behaviorally mediated interactions such as avoidance are more important than consumptive interactions in our system. ”
Wyman said her observations and conclusions are indicative of the study’s larger potential applications.
“There’s an interesting conservation side to this as well as some really fun science,” Wyman said. “This type of research helps us understand how big we need to make places like nature preserves and parks, because the more we understand about the impact of invasive species the better we can understand the parameters of protection.”
After graduation, Wyman, who is from Beachwood, Ohio, will work at the UN World Food Program in Panama City, Panama through a Princeton in Latin America fellowship. In the long term she said she would like to work for a nongovernmental organization that studies the intersection of conservation and development in the developing world.
“This kind of science, where you manipulate a system to better understand how it works, is incredibly exciting,” Wyman said. “I think it’s important for us to understand as much as we can about ecosystems so we as humans can coexist with the rest of the natural world for centuries to come. Hopefully, I’m contributing to that future in some way.”