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Abstract
Diamondback terrapins, hereafter referred to as terrapins, are the only estuarine turtle species native to North America. However, terrapins are also occasionally found in marine habitats, such as seagrass beds, and yet little is known about how they use those marine habitats. We sampled epidermis from terrapins (Malaclemys terrapin macrospilota) inhabiting a seagrass-dominated coastal bay in Northwest Florida and compared resource use among terrapin sexes and life-history stages using the isotopic niche and mixing models. Terrapins were generalist foragers, and their diets varied by sex and life stage, as has been reported elsewhere. Basal resource mixing models indicated the terrapin food web was based primarily on Thalassia testudinum for adult females (50.0%) and Spartina alterniflora for adult males (49.7%) and juvenile females (42.2%). Dietary mixing models indicated the adult female diet included a relatively high proportion of Thalassia testudinum (31.3%), suggesting a strong reliance on seagrass dominated prey and not necessarily large consumption of seagrass, followed by lower proportions of gastropods (26.6%) and crustaceans (19.1%). Primary diet items for juvenile females and adult males included relatively equal proportions of echinoderms, gastropods, crustaceans, ascidians, and porifera. Body and head size of terrapins may drive differences in diet, as interpreted from mixing model results. Although mangroves are expanding their range northward along the Gulf of America coast and have become established at our study site, it does not appear that terrapins are foraging within these newly established mangrove forests. Finally, the terrapin niche, particularly for adult females, may overlap with the sea turtle niche in seagrass-dominated bays. Whether sea turtles impact terrapin populations, including through direct predation, is unknown.
Highlights
Coastal wetlands provide foraging resources for a wide variety of species including birds (Beerens et al, 2011), fish (Cintra-Buenrostro & Fierro-Cabo, 2022), dolphins (Sievers et al, 2019), and turtles (Lamont et al, 2021)
Among the three trophic discrimination factor (TDF) that we tested, we found the values measured in juvenile loggerhead sea turtles (Caretta caretta) (Reich et al, 2008) resulted in most (92%) of the terrapin isotope values being encompassed by the potential prey, so these TDF values were applied in our analysis (Supplementary Table 2)
Joseph Bay encompass a significant amount of Thalassia habitat (Lamont et al, 2023a, 2023b), and our basal resource and diet mixing model results suggest adult females may be foraging in that habitat, not just passing through it as they move among salt marsh patches
Summary
Coastal wetlands provide foraging resources for a wide variety of species including birds (Beerens et al, 2011), fish (Cintra-Buenrostro & Fierro-Cabo, 2022), dolphins (Sievers et al, 2019), and turtles (Lamont et al, 2021). 1 U.S Geological Survey, Wetland and Aquatic Research Center, Gainesville, FL 32653, USA changes affect key taxa that are both sensitive to perturbations and occupy important network positions in the food web (Loesser et al, 2024; McCann et al, 2017). Adding to this complexity, resource use of many species, including those that inhabit wetlands, is affected by life stage (Duffy et al, 2010; Rothman et al, 2008; Sanchez-Hernandez et al, 2019) and body size, with larger bodied individuals typically eating larger prey (Cohen et al, 1993; Scharf et al, 2000; Owen‐Smith & Mills, 2008). Their populations are believed to be declining throughout their range and are listed as Vulnerable on the IUCN Red List of Threatened Species (IUCN, 2018)
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