Inter-annual and inter-populational differences in diamondback terrapin diets

Background/Question/Methods

Understanding the role that predators play in shaping prey communities is a critical component of understanding community structure.  Diamondback Terrapins (Malaclemys terrapin) forage in near-shore brackish marshes along the U.S. Atlantic coast from MA to TX.  Terrapin populations probably reached high densities prior to heavy human exploitation.  A single short-term exclosure study in South Carolina showed that terrapin predation on invertebrates can alter prey community composition, thus their impact under high densities may have been considerable.  However, nearly all that is known about the role of terrapins as predators is based on short-term diet studies, conducted almost entirely in the southeastern third of their range.  We used fecal analysis and feeding trials to examine the diets of adult female during six nesting seasons at Jamaica Bay Wildlife Refuge (JB), New York. Based on studies of other terrapin populations we predicted that JB terrapins would consume primarily gastropods, bivalves and crustaceans

Results/Conclusions

We found that while bivalves and crustaceans constitute 57% of JB terrapin diets, there was very little evidence of gastropod consumption. We also found a much higher than expected abundance of vegetation in their diets, especially sea lettuce (Ulva rigida). However, our correlation test indicated that sea lettuce might be incidentally ingested when terrapins foraged for soft-shelled clams (Mya arenaria). We found dramatic differences between even geographically close populations, for example, although Eastern Mud Snails (Ilyanassa obsoleta) are common in both JB and nearby Oyster Bay, JB terrapins almost never ate them, while they made up as much as 75% of diets of Oyster Bay terrapins.  We similarly found that terrapin diets vary dramatically from year to year; it is unclear what drives these changes.  Our results indicate that diets should not be generalized from a single study site or short term analyses.