A comparison of nest predation between mainland and isolated nesting areas of the diamondback terrapin in coastal Virginia
Diamondback Terrapin (Malaclemys terrapin) populations face numerous threats such as habitat loss, road mortality, drowning in crab pots, and predation of adults and eggs. Currently the Virginia Department of Game and Inland Fisheries classifies the Commonwealth’s Diamondback Terrapin population as Tier II (“High risk of extinction or extirpation”). However, there is a lack of sufficient data concerning the degree to which specific threats (e.g. nest predation) impact terrapin populations in Virginia. This project was established to compare nesting success and nest predation between nesting sites classified as “isolated” or “mainland” to determine their relative contribution to population recruitment. Isolated sites were less than 2 ha, completely surrounded by saltmarsh, and generally low in elevation. Conversely, mainland sites were of comparable size but located in larger tracts of land (>2ha) and were not completely enveloped by saltmarsh. We predicted nest predation would be greater in mainland sites than isolated sites. During summer 2014, preselected nesting sites were repeatedly surveyed by foot for recent nesting activity. Confirmed nest were repeatedly monitored for presence/absence of predation.
We found that terrapin nests located in isolated nesting areas were predated, on average, three days later than nests located on the mainland (T=2.21, P=0.047); however, overall predation rates were similar (35% at both locations). Mean clutch size for terrapins in both areas combined was determined to be 10.2 eggs per clutch, which is similar to other studies. The number of days between the first discovery of the nest and the day it was first rechecked was not significantly different between locations (T=1.2, p=0.238), which suggests that the observed difference was not an artifact of biases in our sampling technique. Overall, these results suggest that predators may be able to more easily locate nests on mainland sites; however, it has little bearing on overall nest predation rates. This study further suggests that nesting females are likely to have equal success (concerning predation risk) on either location and therefore mainland areas are likely to more “attractive” in terms of reduced risk of tidal inundation.