Coastal species continue to face environmental changes as the density of humans and their impacts on coastlines increase, and understanding how species adapt to these changes is important for conservation management. The Diamondback terrapin (Malaclemys terrapin) is a long-lived, sexually dimorphic generalist that exhibits high site fidelity and thus is a useful estuarine indicator species. We applied stable isotope analysis to elucidate temporal and spatial shifts in their resource use from the Greater Florida Everglades to their southernmost range in Key West. We measured isotopic composition (δ13C, δ15N) of terrapin blood and scutes as well as resources across three study sites, comprised of mangrove forested creeks and island habitat types. We tested for effects of terrapin size and habitat on the isotopic composition of blood and scute tissues from terrapins and examined the aforementioned effects and their interaction on the isotopic values using linear models. We then evaluated the δ13C and δ15N values from their resources to determine whether changes in the prey items were driving changes in terrapins. Finally, we evaluated differences in niche breadth within and between sites over time by analyzing tissues with both short (blood) and long (scute) turnover rates.
Results/Conclusions
Small males in creek habitats showed greater enrichment in 15N values relative to larger females (Δ15N = 0.48, p=0.009), but males in island habitats were less enriched in 15N than females (Δ15N = -0.89, p=0.016). Terrapin blood samples were consistently depleted in 13C compared to scute samples, and small terrapins were consistently depleted in 13C compared to large terrapins. Terrapin blood samples were enriched in 15N relative to scute samples (Δ15N = 0.42, p=0.001), and this pattern was most parsimoniously explained by a single parameter across all sites. These differences between tissue types indicate potential shifts in either food resources and/or habitat use through the time period recorded in scute tissue. Analysis of the resources showed prey items from island sites typically were less depleted in 13C, and δ15N values for each prey item varied differently among sites. Isotopic niche breadth of the blood samples were more constrained than those from scutes revealing shifts in foraging over time. These tissue comparisons represent a powerful tool for estimating long term foraging strategies and to identify potential shifts due to disturbance events.