Habitat Usage, Dietary Niche Overlap, and Potential Partitioning between the Endangered Spotted Turtle (Clemmys guttata) and Other Turtle Species
Habitat loss and degradation affect populations in isolation while also modifying the intensity of interspecific interactions, which may be especially relevant for species of conservation concern coexisting with more common species. We explored habitat preferences and potential interspecific resource competition in the endangered Spotted Turtle (Clemmys guttata). For habitat data, water quality measurements were recorded once per trapping session, while canopy cover and vegetation data were recorded once per field season. We also investigated niche overlap in habitat and food resources between C. guttata and other, more common, turtle species. Our data indicated that the abundance of C. guttata was negatively correlated with dissolved O2 and pH levels and positively correlated with body of water depth and canopy cover. The investigation of niche overlap revealed that Mud Turtles (Kinosternon subrubrum) and Painted Turtles (Chrysemys picta), but not Snapping Turtles (Chelydra serpentina), were negatively correlated with the abundance of C. guttata despite differences in characteristics of bodies of water used by each species. We used structural equation modeling to better understand if the relationships between C. guttata and other turtles were due to direct interactions or simply reflected environmental influences. We then used stable isotope analyses to compare similarities in δ13C and δ15N isotopes as proxies for diet overlap between C. guttata and other turtles and ultimately found that all species had similar, broad diets. The inverse relationship between abundance of C. guttata and other species, paired with the overlap in dietary niche space, suggests there is potential for interspecific interactions to negatively impact the abundance of C. guttata within turtle communities on the Atlantic Coastal Plain. The results of this study highlight that management efforts may need to consider that species of conservation concern coexist with more common species, especially as habitat loss decreases the breadth of habitat available.
The path analysis shows the relationship between habitat parameters associated with PC1 (which was strongly positively loaded with salinity, depth, dissolved O2, canopy cover, and pH) and three turtle species: Chrysemys picta, Kinosternon subrubrum, and Clemmys guttata on the Atlantic Coastal Plain. The solid and dashed lines represent direct and indirect effects, respectively, and black lines indicate positive effects while gray lines indicate negative effects. The numbers associated with each line represent the direction and magnitude of each effect, with the strength of the interaction increasing as the values approach 1.
Biplots of δ15N and δ13C for four turtle species at all sites with ellipses around each species (filled squares/solid black line = Kinosternon subrubrum, filled triangles/solid gray line = Chrysemys picta, open circles/gray dashed line= Chelydra serpentina, filled diamonds/ lack dashed line = Clemmys guttata). There is a large overlap in isotopic compositions for all species, resulting in no significant differences in isotopic niche space between species (see text).
Bivariate SIBER (Stable Isotope Bayesian Ellipses in R) plots of ellipses estimating isotopic niche based on the δ13C and δ15N compositions with all species, excluding C. serpentina. The black circles represent the mode, while the three ellipses from the center outwards show where 50%, 75%, and 95% of the data lie, respectively. The numbers indicate the site numbers, while the letter codes indicate species names (MUDT = Kinosternon subrubrum, PATU = Chrysemys picta, SPTU = Clemmys guttata). Together, these bivariate data of the isotopic compositions create ellipses which represent the relative sizes of the isotopic niche of each turtle species at all of our sites on the Atlantic Coastal Plain. Despite the lack of any significant differences in isotopic niche, these ellipses allow us to see some degree of niche overlap among C. guttata at all sites.
Contributor Notes
Associate Editor: J. M. Davenport.