Population Modeling of Diamondback Terrapins


Hi everyone!  My name is Sarah Gilliand, and I’m a rising senior majoring in Biology and minoring in Mathematics.  This summer I’m going to be continuing my research project, “Population Modeling of Diamondback Terrapins,” in Professor Drew LaMar’s quantitative biology lab.  Diamondback terrapins (Malaclemys terrapin) are a species of turtle found along the U.S. coast from Massachusetts to Texas.  They are the only North American turtle that lives in brackish water, and can often be found in creeks and marshes near Williamsburg.

Up and down the east coast terrapins enter crab pots and are unable to escape.  When one terrapin enters, others will follow it.  Due to the limiting size of the trap openings, adult males (consistently smaller at full size than females) and juveniles of both sexes often find their way into the traps as by-catch and drown.  This summer, I’m going to be surveying the terrapins present in creeks near Williamsburg.  Because of the effects of crap potting, I expect to find a low ratio of breeding males to breeding females, which would affect the number of offspring that can be produced.

I’ve been working in Professor LaMar’s lab for the past two years, and we’ve constructed a mathematical model for terrapin populations.  This model divides the populations into seven stage classes based on the life history traits of terrapins: female hatchlings, female juveniles, female immature breeders, female mature breeders, male hatchlings, male juveniles, and male breeders.  Unlike previously existing terrapin models, this model is nonlinear.  Because of this, the reproductive contributions of both male and female individuals are taken into account, instead of just those of the females.

The data I collect this summer will be added to data from previous surveys by other W&M students, and the numbers can be plugged into the model to give us an idea of what the future holds for our local terrapins.  If, as I’m predicting, we do find higher numbers of breeding females than males, I’ll be able to use the model to show the effects of crab potting and whether or not the terrapin populations are declining.