Animal space use and movement can be highly context-dependent within and among individuals, and understanding this variation is important for elucidating the ecological roles and importance of apex predators. Crocodilians are dominant apex predators in numerous tropical and subtropical freshwater and estuarine ecosystems. We quantified movements and home ranges of American alligators (Alligator mississippiensis) across a range of environmental conditions in space (from freshwater marshes to coastal estuaries) and time (e.g. seasonal variation) in the southern Everglades. In addition, we monitored possible responses to experimental freshwater releases in marsh habitats. Starting in January 2013, we fitted satellite tags (Spot 5, Wildlife Computers) to alligators in the Shark River Estuary of Everglades National Park (n = 12) and freshwater marsh in the South Florida Water Conservation Area 3A and 3B (n = 18). Using the Douglas filtering method for Argos satellite data, we filtered locations based on location class and animal speed. We retained all class 3 (4699 records; accurate within 250 m) and class 2 (2609 records; < 500 m accuracy) locations for analyses. We used 95% and 50% kernel density estimators to quantify home range and core area size, respectively. We also calculated weekly and monthly movement rates.
Results/Conclusions
In the marsh, alligators exhibited relatively small and stable home ranges that did not appear to vary across seasons or with water releases into the marsh. In the estuary, however, we documented high variation among individuals in movements. Some individuals occupied small and stable ranges, similar to those in the marsh while others exhibited considerably greater range size, and more directional movements. While some alligators moved hundreds of meters into the marsh and back to estuarine channels, most alligators appeared to restrict their movements to channels. Identifying the relevant temporal and spatial scale of movements for alligators will allow for the assessment of the potential scale of effects for mobile aquatic apex predators on ecosystem function in forthcoming experimental work. Highly localized movements in the freshwater marsh may have implications for the role of alligators as physical ecosystem engineers. Cross-ecosystem movements by estuarine animals may facilitate nutrient transport by large mobile apex predators across environmental gradients (e.g., salinity) in oligotrophic zones.