Stable isotopes of C and N reveal habitat dependent dietary overlap between native and introduced turtles Pseudemys rubriventris and Trachemys scripta

Abstract

Habitat degradation and species introductions are two of the leading causes of species declines on a global scale. Invasive species negatively impact native species through predation and competition for limited resources. The impacts of invasive species may be increased in habitats where habitat degradation is higher due to reductions of prey abundance and distribution. Using stable isotope analyses and extensive measurements of resource availability we determined how resource availability impacts the long term carbon and nitrogen assimilation of the invasive red-eared slider turtle (Trachemys scripta elegans) and a native, threatened species, the red-bellied turtle (Pseudemys rubriventris) at two different freshwater wetland complexes in Pennsylvania, USA. At a larger wetland complex with greater vegetative species richness and diversity, our stable isotope analyses showed dietary niche partitioning between species, whereas analyses from a smaller wetland complex with lower vegetative species richness and diversity showed significant dietary niche overlap. Determining the potential for competition between these two turtle species is important to understanding the ecological impacts of red-eared slider turtles in wetland habitats. In smaller wetlands with increased potential for competition between native turtles and invasive red-eared slider turtles we expect that when shared resources become limited, red-eared slider turtles will negatively impact native turtle species leading to long term population declines. Protection of intact wetland complexes and the reduction of introduced species populations are paramount to preserving populations of native species.

Figure 1. Species accumulation curves of aquatic vegetation for the two study sites in 2010.

After 4 months of vegetative surveys the number of new species being found at Fort Mifflin (FM) had leveled off while at Silver Lake Nature Center (SLNC) the number of new species had not leveled off. Additional sampling at FM would likely not have found many new species while at SLNC additional sampling would likely result in higher species richness.

Figure 2. δ¹³C (x-axis) and δ¹⁵N (y-axis) results for blood plasma sampled from red-bellied turtles (closed squares) and red-eared slider turtles (open squares) at the Silver Lake Nature Center (SLNC, bottom row) and Fort Mifflin (FM, top row) for the years 2008, 2009 and 2010.

Error bars represent the standard error of the mean. At SLNC there were significant differences for δ¹³C and δ¹⁵N across all three years. At FM no significant differences were found in δ¹⁵N values and in 2008 and 2009 no significant differences in δ¹³C values were found.