Do changes in size class composition influence ontogenetic niche shifts in American alligators?

Background/Question/Methods

The American alligator (Alligator mississippiensis) is a large-bodied, apex predator in freshwater wetland habitats within the southeastern United States. Over their lifespan, an alligator’s body size can increase by four to five orders of magnitude, enabling consumption of larger prey and greater tolerance for saline habitats. Morphological and physiological differences among size classes are thought to drive ontogenetic shifts in habitat selection, though social dominance behaviors are also thought to influence spatial segregation of size classes. Alligators are also a harvested species in the region and typically hunters target the larger, more dominant, individuals. Therefore, anthropogenic drivers may affect relative abundance of alligator size classes (hereafter composition) and hence, size-class specific habitat occupancy patterns. We examined data collected in coastal South Carolina from nightlight surveys during 1971-1994 (pre-harvest) and 1996-2014 (post-harvest). Surveys were conducted on rivers, lakes, and freshwater impoundments; and location, habitat class, and size class recorded for each alligator observation. Our objective was to determine if population size class composition and habitat use differed among these periods.

 

Results/Conclusions

Our preliminary findings indicate that alligator densities increased from 0.5 ± 0.4 alligators/km to 2.8 ± 2.0 (SE) between 1971 and 2014 across all habitat types. The proportion of juvenile size classes (< 1.8 m) decreased from 0.49 ± 0.37 to 0.35 ± 0.28, whereas the proportion of adults increased from 0.51 ± 0.40 to 0.65 ± 0.51 over this same time period. At a finer scale, despite the implementation of harvest programs, we also determined that the proportion of the largest size class (> 3 m) detected on nightlight surveys increased from 0.08 ± 0.10 to 0.14 ± 0.09. Further, we did not detect differences over time in the prevalence of juvenile or medium size classes (1.8 to 3 m) in the most productive habitats (e.g., freshwater impoundments and brackish marsh). The decrease in juvenile size classes we detected may reflect reduced reproductive activity mediated by removal of adults through harvest programs. However, the increase in the largest size classes over time in combination with a lack of change in habitat selection patterns in smaller size classes suggests that social dominance likely continues to influence spatial segregation of size classes.