Poor Correlation between Diamondback Terrapin Population Estimates Using Two New Estimation Methods

Reliable estimates of animal and plant population sizes are necessary to track trends in populations through time. Diamondback terrapins are an ecologically unique keystone species that are globally declining. Conservation efforts for this species rely on accurate estimates of population sizes; however, diamondback terrapin population size estimates are difficult to measure with precision or accuracy. Terrapin collection methods are often labor-, time-, and cost-intensive. The present study compares two recently developed rapid assessment methods for measuring diamondback terrapin abundances. Since mark–recapture or similar data were unavailable, we could not test the accuracy of either method directly; instead, we compared the two methods. If the methods produce similar estimates of terrapin population size, this would increase confidence in these methods. We measured the abundance of diamondback terrapins at 77 sites in Long Island, New York, using headcount surveys and surveys of parasitic trematodes that can be used as a proxy for terrapin abundance. We used random forest analyses to test whether the variation in diamondback terrapin abundance measured using headcount surveys could be explained by either the prevalence or the abundance of trematode parasites. The most variation explained by any of the models was 7.77%, indicating that trematode prevalence and abundance could not explain the variation in terrapin abundance measured using headcounts. This poor correlation between terrapin census methods indicates that one, or both, of the census measures are inaccurate, at least in the habitats found across Long Island, NY. A technique that accurately estimates the abundance of diamondback terrapin populations is critical to understanding their population fluctuations and trends. The only way to evaluate the status of the species is to have information on population numbers and trends across the species’ range, which might not be possible without a more accessible survey method.

Visual Head Counts: A Promising Method for Efficient Monitoring of Diamondback Terrapins

Determining the population status of the diamondback terrapin (Malaclemys terrapin spp.) is challenging due to their ecology and limitations associated with traditional sampling methods. Visual counting of emergent heads offers a promising, efficient, and non-invasive method for generating abundance estimates of terrapin populations across broader spatial scales than has been achieved using capture–recapture, and can be used to quantify determinants of spatial variation in abundance. We conducted repeated visual head count surveys along the shoreline of Wellfleet Bay in Wellfleet, Massachusetts, and analyzed the count data using a hierarchical modeling framework designed specifically for repeated count data: the N-mixture model. This approach allows for simultaneous modeling of imperfect detection to generate estimates of true terrapin abundance. Detection probability was lowest when temperatures were coldest and when wind speed was highest. Local abundance was on average higher in sheltered sites compared to exposed sites and declined over the course of the sampling season. We demonstrate the utility of pairing visual head counts and N-mixture models as an efficient method for estimating terrapin abundance and show how the approach can be used to identifying environmental factors that influence detectability and distribution.

Visual Head Counts: A Promising Method for Efficient Monitoring of Diamondback Terrapins

Generating a range-wide population status of the diamondback terrapin (Malaclemys terrapin spp.) is challenging due to a combination of species ecology and behavior, and limitations associated with traditional sampling methods. Visual counting of emergent heads offers an efficient, non-invasive and promising method for generating abundance estimates of terrapin populations across broader spatial scales and can be used to explain spatial variation in population size. We conducted repeated visual head count surveys at 38 predetermined sites along the shoreline of Wellfleet Bay in Wellfleet, Massachusetts. We analyzed the count data using a hierarchical modeling framework designed specifically to analyze repeated count data: the so-called N-mixture model. This approach allows for simultaneous modeling of imperfect detection to generate estimates of true terrapin abundance. We found detection probability was lowest when skies were overcast and when wind speed was highest. Site specific abundance varied but we found that abundance estimates were, on average, higher in unexposed sites compared to exposed sites. We demonstrate the utility of pairing visual head counts and N-mixture models as an efficient method for estimating terrapin abundance and show how the approach can be used to identifying environmental factors that influence detectability and distribution.

Do diets vary over large spatial or temporal ranges? A test using interannual and interpopulation data on Diamondback Terrapin (Malaclemys terrapin) diets

Animal diets may vary spatially or temporally as resource availability vary. Diets of species with extensive geographic ranges often span multiple habitats, thus their diets may vary accordingly. Temporal diet variation is rarely explored because most diet studies are short term; this is problematic for long-lived species where individuals may persist as prey availability changes. We analyzed diet variation in Diamondback Terrapins (Malaclemys terrapin (Schoepf, 1793)), which inhabits nearly 70 000 km of United States Atlantic coastline, spanning 16.5°N latitude and 27.4°W longitude and four Köppen climatic zones, and Bermuda. We explored spatially or temporally Diamondback Terrapin diet variation, including populations from Atlantic salt marshes, an Everglades mangrove swamp, the Texas Gulf Coast, and a Caribbean golf course pond. We found remarkably high levels of similarity, indicating that although diets vary according to local prey availability, they are broadly similar at lower taxonomic resolution. Even short-term studies may be sufficient to accurately characterize diets of Diamondback Terrapins. These results are surprising given the geographic range sampled in this study and indicate that Diamondback Terrapin diets are conservative, reflecting local prey availability. Such diets apparently allow Diamondback Terrapins to exploit their extensive range and may allow Diamondback terrapin populations to persist as local prey species wax and wane.