Two
heads are better than one: development of a rapid assessment technique
for diamondback terrapin (Malaclemys terrapin) populations using headcounts
Leigh
Anne Harden, Shannon E. Pittman, J. Whitfield Gibbons, and Michael E.
Dorcas
Although diamondback terrapins appear to be declining
throughout much of their geographic range, more information is required
to evaluate population trends. Unfortunately, sampling terrapin populations
is both time and labor intensive. We have initiated studies to examine
the efficacy of using headcounts in tidal creeks as a rapid-assessment
technique for monitoring terrapin populations. In 2005, as part of a 24-year
study of terrapins at Kiawah Island, SC, we began headcount surveys in
conjunction with regular aquatic sampling. Headcount surveys consisted
of recording the number of terrapin heads we observed from a boat going
up (run 1) and down (run 2) tidal creeks. These surveys were conducted
before aquatic sampling (i.e., low tide) as well as other times (e.g.,
high tide). We found a significant relationship between the number of
heads seen and the number of terrapins captured (R2 = 0.543). The number
of heads seen in run 1 combined with run 2 provided the strongest correlation
with the number of terrapins captured. We also examined the effect of
other variables such as day of year, time of day, cloud cover, and creek
location on the number of heads seen. We plan to refine our model by including
other locations along the east coast. The development of a refined model
will allow rapid assessment of terrapin populations and effective monitoring
of population trends, thereby improving implementation of appropriate
conservation measures.
Ecological
effects of major injuries in diamondback terrapins: Implications for conservation
and management
Kristen K. Cecala, J. Whitfield. Gibbons, and Michael E. Dorcas
Many turtle species frequently suffer major injuries due to attempted predation or anthropogenic factors. Diamondback terrapins (Malaclemys terrapin) are one species known to be affected by anthropogenic activity, but we know little about the causes of injuries. In declining diamondback terrapin populations, learning more about causes and results of injuries can be helpful in developing sound management plans. We examined patterns of limb loss and major shell injuries in a population of terrapins studied for 24 years at Kiawah Island, South Carolina to infer the cause of injuries and possible predators on terrapins. The rate of shell injuries has increased temporally, possibly as a result of increased watercraft activity. Because no differences in rates of limb loss were found between males and females, we conclude that limb loss likely results from aquatic encounters (i.e., limb loss does not appear to be the result of terrestrial predation during nesting). We also found that males experience reduced body condition when injured, and that terrapins with a major injury have lower survivorship than uninjured terrapins. We conclude that, in addition to protecting nesting habitats, measures to protect terrapins from watercraft activity may increase the survivorship of adult terrapins.
Transmitter attached with epoxy
Terrapin with attached transmitter and micro-datalogger
Movements and temperature variation of diamondback terrapins inhabiting
a salt marsh in Kiawah Island, South Carollina
Leigh Anne Harden, Nick DiLuzio, J. Whitfield Gibbons, and Michael E. Dorcas
Though the strong site fidelity of diamondback terrapins has been documented, little is known about their daily and tidal movements within a salt marsh creek. Where are terrapins at low tide? High tide? What available creek habitats are terrapins using on a daily basis? With help from Dr. Michael Dorcas, Leigh Anne Harden and Nick Diluzio of the herp lab investigated these questions this past May by conducting a week-long radiotelemetry study on diamondback terrapins (Malaclemys terrapin). Using a jon boat, they tracked 3 male and 3 female terrapins inhabiting an intertidal creek adjacent to the Kiawah River. Each terrapin was equipped with a transmitter and a micro-datalogger to record carapace temperatures every 30 minutes. Similar micro-dataloggers were placed in various creek locations (water surface, mud, grass) to record temperatures of the salt marsh habitats available to terrapins. With this pilot study, we hope to gain more knowledge about the movements, habitat use, and thermal biology of diamondback terrapins in a constantly changing environment.. Information of this kind may be used for terrapin habitat conservation initiatives and may be a factor in estimating population sizes. (pdf)
Radio tracking terrapins
Michael E. Dorcas, J. Whitfield Gibbons, and John D. Willson
Twenty-one years of mark-recapture data from >2800 captures of >1400 diamondback terrapins (Malaclemys terrapin) in a South Carolina salt marsh were used to examine ecological factors critical for management and conservation of this species. Decreasing population sizes in several tidal creeks in the vicinity of Kiawah Island, SC indicate that this species is declining in the region. Adult survivorship and the proportion of young individuals in the population have declined steadily since the inception of the study in 1983, suggesting that the decline results from a combination of adult mortality and reduced juvenile recruitment. Recovery from declines is hampered by high site-fidelity and limited emigration by terrapins from other creeks. Remaining turtles are larger than those captured in previous years and the sex ratio of the population has shifted in favor of females (the larger of the sexes). These demographic changes are consistent with decline due to mortality of smaller (younger and male) turtles in recreational and commercial crab traps. We discuss possible implications of bycatch-related terrapin declines and suggest management options for this species. (pdf)
To learn more, check out the PowerPoint presentation presented by Mike Dorcas and Whit Gibbons at the Powdermill Turtle Biology Meetings.
This
website created by Leigh Anne Harden and Kristen Cecala.
Some photographs by J.D. Willson, Tom Luhring, and Whit Gibbons.
Questions? Contact Michael Dorcas