SREL Reprint #3095

Long-Term Population Trends of American Alligators (Alligator mississippiensis) in a Man-Made Reservoir: Responses to Environmental Perturbations Arising from Management Activities

I. Lehr Brisbin, Jr., Robert A. Kennamer, and Warren L. Stephens, Jr.

Savannah River Ecology Laboratory, P.O. Drawer E, Aiken, South Carolina 29802, USA

Abstract: The expansion of urban and suburban population centers in the southeastern United States has been accompanied by increases in man-made reservoirs to meet demands for hydroelectric power, cooling of industrial effluents, and recreational real estate development. Reservoirs have provided American Alligators (Alligator mississippiensis) with a relatively new type of large, open-water habitat — one rarely encountered naturally. Using a long-term series of population studies of Alligators inhabiting a 993 ha nuclear reactor cooling reservoir in west-central South Carolina (USA), we assessed temporal trends in Alligator numbers and demography as this reservoir was subjected to various impacts typical of industrial use. The reservoir underwent shoreline habitat succession for almost 30 yr (1958-1987) while managed for stable water levels and influenced by reactor thermal effluents. Warmer portions of the reservoir attracted some of the population's larger males in winter, affected the timing of their annual reproductive cycles, and possibly limited rates of reproduction in the reservoir. Nevertheless, from the 1970s through the 1980s, the Alligator population shifted from one dominated by larger adults, with a relatively low rate of reproduction, to one more characteristic of a stable age structure, including a higher reproductive rate. Relatively low levels of contaminants, including radionuclides on the resident Alligators were almost certainly inconsequential, but bioaccumulation of mercury in the ecosystem was thought to have a greater potential for negative impacts on larger, older individuals having longer lifetime exposures. In 1991, a partial drawdown reduced the reservoir's surface area by almost half to allow for repair of the retaining dam, with refill to full pool occurring during 1994-1995. During this drawdown, Alligator movements increased, particularly among males; emigration from the reservoir was even documented for a few individuals. Reproduction and recruitment was also reduced during the drawdown and immediately following the refill, most likely due to the destruction of all emergent shoreline vegetation in the reservoir thus reducing cover available to hatchlings and exposing them to increased predation. In the summer of 2004, 9 yr after the refill was completed, surveys recorded Alligator numbers comparable to those 15 yr earlier before the drawdown and refill. Considerable numbers of juveniles were again present, confirming the resumption of successful reproduction and recruitment. Apparently, such reservoirs can not only provide adequate habitat to support healthy reproducing Alligator populations, but these animals are able to sustain their numbers through partial drawdowns and refills, as long as emergent shoreline vegetation is allowed to reestablish and provide protective cover for hatchlings and juveniles.

Keywords: Alligator mississippiensis, Crocodilians, Population Ecology, Reservoirs, Urban Ecology

SREL Reprint #3095

Brisbin, I. L., Jr., R. A. Kennamer, and W. L. Stephens, Jr. 2008. Long-Term Population Trends of American Alligators (Alligator mississippiensis) in a Man-Made Reservoir: Responses to Environmental Perturbations Arising From Management Activities. In: J. C. Mitchell, R. E. Jung Brown, and B. Bartholomew (Eds.). Urban Herpetology. Society for the Study of Amphibians and Reptiles. Herpetological Conservation Number 3: 519-527.

This information was provided by the University of Georgia's Savannah River Ecology Laboratory (srel.uga.edu).