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August 6, 2015

The largest American alligator satellite telemetry-tracking study worldwide is lurking in South Carolina’s Lowcountry.

Transmitters are programmed to acquire an alligator location once every three hours from April to the end of September each year. Credit: Chris Boyce

The largest American alligator satellite telemetry-tracking study worldwide is lurking in South Carolina’s Lowcountry. The American alligator is an iconic species of the southeastern U.S., often described as a “living fossil” or “ferocious predator.” As is usually the case, however, there is much more to the story.

“Alligators are ecosystem engineers, capable of altering both habitat structure and function,” says doctoral graduate student Abby Lawson, conducting research with the USGS South Carolina Cooperative Fish and Wildlife Research Unit at Clemson University. “They create depressions in wetlands, called ‘alligator holes,’ which increase habitat diversity and maintain wetland species richness.”

Though the presence of alligators is an indicator of ecosystem health, their presence is not necessarily welcomed by others. The alligator’s dual role as a public safety nuisance and a species of ecological and economic importance makes alligator management incredibly challenging.

Alligators are territorial -- an individual’s movements may be influenced by other alligators in close proximity. Credit: Chris Boyce

Alligator habitat investigator

In 2014, the late Dr. Kate McFadden of the USGS South Carolina Cooperative Fish and Wildlife Research Unit developed a study to investigate movement patterns of American alligators in certain regions of South Carolina. These areas contain several hundred square kilometers of impounded wetlands and host the highest alligator densities in the state.

USGS and other scientists have studied in-depth alligator populations in Florida and Louisiana, but basic ecological knowledge is lacking for populations at the northern edge of their range. For example, differences in climate and habitat between the southern and northern portions of the range limit the applicability of findings from other studies to South Carolina alligator management.

South Carolina alligators occupy a patchwork of diverse habitats, including rivers, lakes, wooded swamps, tidal marshes, and impounded freshwater wetlands. As a mobile, opportunistic predator, alligators seasonally adjust their habitat use for feeding. For example, some Florida alligators venture into brackish water habitats to feed on nutrient-rich blue crabs during the wet season because increased rainfall reduces salinity levels, making the water more tolerable to the gators.

Habitat use can also differ between sexes. During the summer, breeding female alligators in Louisiana select shallow, vegetated habitat for nest construction, while males remain in open waters, like rivers and lakes. How these movement patterns may translate to South Carolina’s fragmented landscape, with the state’s abrupt changes in habitat and land ownership occurring at small spatial scales, remains unknown.

Long-term surveys indicate the alligator’s core habitat in South Carolina consists of artificial freshwater wetlands that are impounded by levee systems, in which water levels are maintained to provide waterfowl habitat. Historically, this network of impounded wetlands was used for rice cultivation, until reservoir construction in the 1930’s drastically restricted the amount of freshwater from rivers to reach coastal marshes. The reduced freshwater flows caused a major transition from freshwater wetlands to brackish and saltmarsh habitats over time. Today, the remaining freshwater impoundments are valuable refuges for species with low salinity tolerances, like alligators, but our understanding of how, when, and to what extent alligators use these systems remains unknown.

Navigator: Tracking the BIG one is no walk in the park

Lawson and fellow researchers work in habitat teeming with one of the earth’s top predators — with adult males weighing in at nearly half a ton (that’s 1,000 pounds!). With capture equipment in tow, the researchers wait – patiently, until they capture a seven-foot adult male, the minimum size required to fit a transmitter for the study.

They secure the alligator while collecting body measurements and prepping the alligator for a transmitter attachment. The snout is wrapped in duct tape, seven times to be exact, while one crew member sits on the alligator’s back and another holds its powerful tail in place. Meanwhile, another team member collects biological samples that include blood, urine, and keratin tissue for multiple contaminants studies.

Promulgator: Prehistoric ectotherm meets modern technology

This movement ecology study evaluates home range, daily movement rate, and habitat use patterns of adult male alligators. Credit: Abby Lawson

Lawson's team marks their adult male alligators with Global Positioning System (GPS) satellite transmitters. The transmitters get location fixes at 3-10 meter accuracy and send the data to researchers every three hours from April 1- September 30, 2015 - 2017. The precise movement data, spanning one to three years for each individual, will allow the team to evaluate if individual movement patterns remain consistent across years or change over time, potentially in response to variable rainfall, temperature, or prey distributions.

This part of the study will also provide insights into habitat selection, movement rates, and harvest (hunting) exposure. To date, the team has marked 24 alligators from 7 to 12 feet in length, in freshwater impoundments both open and closed to harvest, in a mixture of private and public lands, subject to different management regimes, which could influence movement behaviors.

Meanwhile, Lawson and her team, in cooperation with the South Carolina Department of Natural Resources, are expanding their population monitoring efforts. The team uses nightlight surveys to gather abundance, density, and size class (in two-foot increments) distribution data. The expanded survey effort will help researchers better compare contemporary to historical data, to better identify long-term population trends.

Pairing overlapping population surveys and telemetry data may also provide insight into alligator behavioral patterns, such as social-dominance and territoriality. Lawson explains: “Because we’re unable to mark the entire population with transmitters, the population surveys will enable us to determine how many and what size of alligators our marked individuals are sharing their home ranges with. That way we can better determine space requirements for individual alligators, and how many alligators a refuge or wildlife management area could support.”

Circumnavigator: A changing world

Examining long-term population trends will enable the researchers to determine ways in which a changing climate will impact alligator populations. Climate projections predict increased temperatures and reduced rainfall for the southeastern United States. Though increased temperatures may enable faster growth rates, decreased rainfall may reduce available habitat and feeding opportunities. Drought-related reductions in alligator populations have negative implications for other wetland species as well. Alligator holes retain water during drought, which provides valuable habitat for other species like fish and amphibians.

Alligator transmitter installation and recovery. Pictured: Abby Lawson, Thomas Rainwater, John Lane (Wofford College), and Erin Weeks (South Carolina Department of Natural Resources). Credit: Phil Wilkinson

Informing management decisionsStart with science

While the researchers are identifying conservation threats, evaluating long-term population trends, and developing a monitoring program tailored to South Carolina’s unique habitat, this approach can easily be adopted by other southeastern states with alligator populations. This research will be used to develop a way to better predict the outcome of management decisions—which could include setting harvest regulations, nuisance policies, reserve networks design, and habitat restoration.

Habitat loss, combined with a growing human population in coastal communities, will likely lead to more frequent human-alligator conflicts. Therefore, understanding alligator behavioral responses and population trends to habitat fragmentation and loss is vital to effective management in a dynamic, and shrinking, natural landscape.

Note: Fieldwork for this study was through a collaborative effort between Clemson University, the Medical University of South Carolina, and Hollings Marine Lab. Funding was provided by the South Carolina DNR, USGS and the Yawkey Foundation.


Learn More

Project: Alligator Population Ecology Project

Project blog: The Abby Lawson Project

Clemson Poster: Optimizing Survey Designs for Complex Habitat

ABC News 4: Research group tracking Lowcountry's alligators to learn habits

Twitter: @AbsLawson and the project handle #CUGators

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