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

Abstract

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.

Of mudsnails, terrapins, and flukes: Use of trematodes as a field‐based project in parasitology research

“…Byers et al (2011) and Eugene (2019) found that both cyst abundance and cyst prevalence were positively correlated with the sizes of nearby nesting female terrapin populations. However, Prowant and Burke (2020) reported that cyst counts were not correlated with head counts, another newly developed terrapin census technique. Because participant counts of cysts were not always accurate, we suggest that groups focusing on determining intensities should be provided more training on counting of cysts (particularly cysts on the operculum of mudsnails).…”

“…Thus, records of “infected” snails are not correct in some reports (e.g. Demetriades, 2020; Prowant & Burke, 2020).…”

“…Byers et al (2011) was the first to test whether the abundance of cysts on mudsnails could be used reliably to estimate populations of terrapins, and found that cyst abundance was significantly positively associated with size of nine diamondback terrapin populations in Georgia. Since then, Prowant and Burke (2020) applied the technique to populations of terrapins on Long Island, NY, but did not find that cyst prevalence or abundance correlated well with terrapin population estimates based on head counts. However, it is important to note that head counts have not been evaluated extensively as a survey technique for terrapin populations (Harden et al, 2009).…”

“…C . Geographic distribution showing the range of the host, the diamondback terrapin Malaclemys terrapin (dashed line) and sites where Pleurogonius malaclemys has been sampled from mudsnails or within terrapins (from northernmost record in Buzzards Bay, MA, to Long Island, NY [Eugene, 2019]; NY and NJ [Chodkowski et al, 2016; Prowant & Burke, 2020]; NJ [McDermott, 1951; Demetriades, 2020]; NC and SC [Blakeslee et al, 2012]; NC [Hunter, 1961]; GA [Byers et al, 2011])…”

Advances in the Biology and Conservation of Turtles

“…Making turtle censuses easier yet accurate can facilitate conservation. Two of the papers [6,7] focus on interesting new techniques (visual head counts and surveys of parasitic trematodes) for surveying diamondback terrapins (Malaclemys terrapin), a declining species listed as Vulnerable by the IUCN [8]. Like many turtles, terrapin population trends are difficult to measure with precision or accuracy, and these new techniques offer easier methods, which are even usable by citizen scientists.…”