Effects of the American Alligator on Salt Marsh Ecosystems

The American Alligator (Alligator mississippiensis) is an apex predator throughout most of the Coastal Southeastern United States. A study was recently performed to research the top-down trophic effects of estuarine alligators. Alligators that occur in salt marsh ecosystems commonly prey on Blue Crab (Calinectes salpidus), which in turn prey on herbivorous molluscs. The researchers were interested in seeing how alligator predation may effect every part of this food chain. To perform the experiment mesocosms were constructed and populated with spartina grass, periwinkle snails, ribbed mussels, and blue crabs at a density that reflects a salt marsh bank. Alligators were captured in salt marshes surrounding Sapelo Island, Georgia. Half of the treatments had an alligator placed in them and the other half were used as controls. Trials were conducted for three days and crab behavior was monitored at three hour intervals. At the end of the trial the alligators were found to consume 3.5 blue crab on average. Crabs that underwent predator treatment were also found to be much less active than in control treatments. In control trials probability of mortality was 47% higher in snails and 64%  higher in mussels than in the alligator treatments. This supports the hypothesis that alligators impose top-down control on salt marsh ecosystems where they are present. This work is valuable to understanding the intricacies of southeastern salt marsh ecosystems. It also shows what may happen to an ecosystems when alligators or other apex predators are removed by human activities.



source
"Impacts of a Large Bodied Apex Predator (Alligator mississippiensis) on salt marsh food webs."
James C. Nifong and Brian R. Silliman
Journal of Experimental Marine Biology and Ecology
http://www.sciencedirect.com.nuncio.cofc.edu/science/article/pii/S0022098113000063?np=y