Abstract
Invasive freshwater turtles are an abundant and widespread threat to freshwater ecosystems. They alter native ecosystems through predation, competition, and as a disease or parasite vector. Invasive turtles may also have performance advantages that allow them to outcompete native turtles and facilitate their establishment where introduced. Although the impact of invasive turtles on native turtle populations is well documented, few reviews have compared and generalized the impacts of invasive turtles; by understanding the current impacts and status of the literature on invasive turtles, we could better predict the potential effects of non-native turtles, inform management practices, identify knowledge gaps and avenues for future research. I conducted a meta-analysis using Web of Science and Google Scholar to answer several major questions: (1) which population-level impacts (changes to abundance, growth rate, disease prevalence, habitat use, energy expenditure, reproduction rate) are highest in magnitude, (2) is competition or predation more detrimental to native species, (3) do invasive turtles exhibit higher performance (faster weight gain, food consumption at faster rates, greater thermotolerances) than native turtles, and (4) do the performance differences between invasive and native turtles vary among different native turtle species? I found that invasive turtles had a significant negative impact on native species (g=-1.14, p=0021) though such results varied between biotic interaction type (competition vs. predation) and population-level impact types. Specifically, studies of predation significantly affected native species, but effects through competition were only marginally significant. I also found invasive turtles significantly increased the avoidance behavior of their prey and competitors and altered the development of amphibian species. Although the effect sizes were negative, I did not find a significant difference in the resource use and growth/mass of native species in the presence of invasive turtle competitors. Invasive turtles also exhibited a significantly higher performance than native turtles (g=-0.77, p<0.0001); invasive turtles grew and gained more weight relative to native turtles and exhibited higher physiological tolerances to temperature than natives. Invasive turtles did not differ in their feeding kinematics (which should facilitate prey capture) or resource use compared to native species, though there was weak evidence that native species exhibited more avoidance behaviors in response to perceived predation. My meta-analysis identified significant knowledge gaps on the impacts of invasive turtles. Most studies examined the influence of Trachemys scripta elegans on native species, and I found no quantified and comparable studies examining impacts to native communities and ecosystems. Furthermore, very few quantified and manipulative field studies have documented the effects of invasive turtles. My work reaffirms the negative impact invasive turtles, particularly, T. s. elegans, have on native amphibian species as a predator and how specific performance advantages have contributed to its establishment across the globe. Future meta-analyses with higher sample sizes should further explore the negative impact of invasive turtles through competition with native turtle species. This work also highlights the need for empirical field studies on the community and ecological effects of invasive turtles.