Tracking Alligator Movements in South Carolina to Understand Population Trends

USGS Cooperative Research Unit Corner

Tracking Alligator Movements in South Carolina to Understand Population Trends


Researchers in the South Carolina Cooperative Fish and Wildlife Research Unit based at Clemson University have teamed up with the South Carolina Department of Natural Resources (SCDNR) to conduct a study that will identify causes of alligator population fluctuations. The findings will be used to help predict population outcomes of management decisions in the northern portion of the species' range.

The American alligator (Alligator missippiensis) is an iconic species of ecological and economic significance in the southeastern United States. Alligators are ecosystem engineers capable of altering both habitat structure and function. They create depressions in wetlands, called ?alligator holes', which increase habitat diversity and provide aquatic refuge for other species in times of drought helping to maintain species richness. Today, alligators are also used as an indicator species to reflect ecosystem health in on-going efforts to restore wetlands or hydrologic regimes.

However, historically alligators were recognized for their commercial, rather than ecological value. Widespread, unregulated commercial harvest spanning several centuries induced drastic alligator population declines throughout their range, prompting legal harvest closure in 1969. Alligator populations quickly rebounded under their protected status, enabling states to gradually re-open tightly regulated harvest programs beginning with Florida in 1988 and, most recently, South Carolina in 2008.

Given the checkered past of alligator harvest, many state wildlife agencies are investing in research efforts that can both fill gaps in basic alligator ecology and inform harvest decision-making. The lack of ecological knowledge is particularly acute for populations at the northern extent of the alligator's range, including South Carolina that has the newest alligator harvest program. Climate and habitat differences in southern areas like Florida and Louisiana compared to northern portions of the alligator's 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 swamp, tidal marsh, and impounded freshwater wetlands. Moreover, alligators are mobile, opportunistic predators that are known to readily adjust their habitat use patterns for feeding or breeding opportunities. Therefore, researchers were faced with a more basic question: what is the most efficient way to gather accurate population counts on a species that is always on the move?

Their approach was to conduct an intensive population monitoring effort that overlapped with a movement study. The monitoring study is designed to repeatedly survey (i.e. count) alligator populations on a small number of locations during the alligator's peak activity period from April to mid-September. Within the same surveyed areas, the researchers fitted 24 adult male alligators with satellite transmitters, programmed to acquire eight daily location fixes (at 3-10 meters accuracy) over the same time period.

The simultaneous studies are designed to disentangle phenomena that would be difficult to tease apart otherwise. The repeated surveys within each year will provide descriptions of how alligator abundance changes seasonally. Combining the survey count data with movement patterns in the same area will help the researchers understand if fluctuations in population counts are due to individuals moving to or from the surveyed habitat or due to changes in environmental conditions that affect the researcher's abilities to count alligators. For example, alligators are much easier to detect at low water levels because there is less water available for them to submerge within. Identifying where and when major movements are likely to occur will help researchers prioritize which habitat types to survey at what times, in order to get the most accurate population estimates on which to base management decisions.

The research group just completed the second of the three-year study and have already keyed in on some interesting findings. First, both the count and movement data show that alligator habitat use is strongly tied to irrigation activities in impounded wetlands (managed to provide food for wintering waterfowl). Reducing the wetland water levels concentrates valuable prey items like fish and crabs, which creates an easy feeding opportunity for alligators. The satellite telemetry data has documented several marked alligators traveling to draining wetlands from miles away. Water level manipulations, therefore, have the potential to influence population counts within a very large radius.

Second, it is clear that alligators do not obey political boundaries. Nearly every alligator that was fitted with a transmitter within an area closed to harvest has ventured into harvested areas. This has important implications for evaluating the effectiveness of reserve or sanctuary networks. Lastly, movement patterns were very different among individual alligators. Several alligators remained mostly within the area where they were marked, whereas others used different habitat types, such as tidal marsh, relatively frequently. This is consistent with other recent alligator studies that have found individuals may "specialize" in certain prey items between marine and freshwater food webs.

Next year will mark the final year of data collection for the study, and hope to implement a newly designed monitoring program beginning in 2017.

This research is funded by SCDNR, the United States Geological Survey, and the Yawkey Foundation I. This article was written by Abby Lawson, a PhD Candidate in the USGS South Carolina Cooperative Fish and Wildlife Research Unit at Clemson University. The principal investigator is Patrick Jodice, Unit Leader of the USGS South Carolina Cooperative Fish and Wildlife Research Unit.

November 16, 2015