Stable Isotopes Reveal Demographic Variation and Individual Repeatability in Diet of an Omnivorous Turtle

Omnivores consume a wide range of food sources across multiple trophic levels, but resource use can vary among and within populations owing to both extrinsic and intrinsic factors. Understanding the specifics of the dietary niche is important for many applications in ecology and conservation, yet the details of individual variation in foraging ecology remain incomplete for many species. The Eastern Box Turtle (Terrapene carolina) is a generalist omnivore that consumes a variety of plant, animal, and fungal foods, but information about its diet comes primarily from examination of gut or fecal contents that potentially overlooks important sources of variation. We used stable isotope analyses of turtle claws and potential foods to examine sources of among- and within-individual variation in diet for populations in two distinct environments. Turtles consumed foods across 1.6–2.4 trophic levels at Coastal Plain and Sandhills sites, respectively, but carbon and nitrogen isotopes (δ13C and δ15N) did not vary between populations. Instead, isotope values varied among individuals according to life cycle stage and body size, with δ13C and δ15N higher in adults than in juveniles and increasing with body size. Individuals were also consistent (i.e., repeatable) in isotope values over periods of up to 7 yr, suggesting possible individual specializations or preferences in food sources. Isotope mixing models identified mushrooms (74%) as the largest food source for adults, followed by lesser amounts of plant (15%) and animal (11%) sources. Juveniles consumed relatively equal amounts of plant (36%), mushroom (33%), and animal (31%) food sources. Observations of foraging support mushrooms as the primary food for adult turtles owing to seasonal increases in consumption during late summer and fall. Such dietary niche variation suggests that not all individuals are equivalent in their ecosystem functions via trophic interactions and may require land managers to consider different foods as complementary resources required to complete the life cycle and to maintain individual variation in foraging niches within T. carolina populations.