Variability in Estuary Habitat Use by American Alligators—But What About Shark Encounters?

An estuarine habitat is very important for a lot of species due to freshwater and marine conditions, ecological reasons recreational reasons, and commercial importance. Salinity levels fluctuate, the tides go through cycles, and there is input from upstream freshwater flows. All of these factors determine the density population of the American Alligator within the estuary. Other factors that contribute are reproductive cycles and temperature.

 

Figure 1: An American Alligator eating a shark.

Alligator mississippiensis¸ or the American Alligator, is a primarily freshwater species. It is spatially and temporally heterogeneous largely due to a salinity gradient that fluctuates. This species hangs around the estuary near the Atlantic Ocean in Southern Florida. The density of alligators in the downstream zone was lower than that of the midstream zone during the dry season when salinity increases due to reduced precipitation. Conversely, the density of the large size alligators was higher in the downstream zone than in the midstream zone because of the wet season, probably due to a decrease in salinity concentration. The authors’ results indicated high adaptability of alligators to the fluctuating habitat conditions. Use of estuaries by alligators is likely driven in part by physiology and by reproductive cycles, and their results supported the alligators’ opportunistic use of estuary habitat and ontogenetic niche shifts.

The American Alligator is an intriguing candidate for this study. The researchers were curious to the drive of alligators to and from the estuary. They found that typically the only alligators present in the estuary were of the larger class size because of their smaller surface area to volume ratio whereas the small class has a higher surface area to volume ratio. Alligators have a limited osmoregulatory capability in saline environments. Alligator nests were not found in moderate-high salinity areas, which suggests that salinity has a strong effect on the nesting sites of alligators. Therefore, altered salinity by changes in freshwater flows into estuaries likely affects the location of alligators, hence why they travel upstream during droughts and travel downstream during the wet season. The researchers used a long-term monitoring program to determine trends in relative density of alligators in the Shark River estuary.

 

Figure 2: An American Alligator eating what presumably looks like a large fish or a small shark.

Large alligators may make more frequent excursions to the downstream zone during the wet season because of lower salinity and higher temperature. As a mobile top predator, the alligator transfers energy and nutrients by their movement and foraging activities across ecosystems. In this study, the alligator connects freshwater, estuary, and near-marine zones by their movement and foraging. During the dry season, it is noted that the adult male alligators spend twice as much of their time in an upstream zone. The larger alligators are more tolerant to salinity concentration differences. Because of their tolerance, they have more foraging options, especially in the Shark River.

Recent surveys indicate the declining density of alligators in multiple wetland compartments in southern Florida, even up to a 99% decline in the southern Everglades. Historically, population counts were higher, but recently, due to human and nature involvement (droughts), the density has decreased. Human development has disturbed these estuaries and the many life forms that call these places home. There are efforts to improve the conditions in the southern estuaries to more natural patterns. If restoration is achieved, it is expected that alligator density will increase and that more medium and small class size alligators will be observed as salinity becomes more favorable, whereas in this study mostly large sized alligators will observed.

Typically all class size alligators consume estuary/marine species including mud crabs, horseshoe crabs, blue crabs, grass shrimp, and fish, but juvenile alligators consumed less frequently than larger alligators. Large-sized alligators do travel downstream into the estuaries where other predators are present. Alligators will eat anything that typically fits into their mouths which begs the question, what if there is a competing predator. With this in mind, do alligators come across sharks and what happens?

It is reported that alligators and sharks do come into contact, and the outcome is interesting regarding encounters of sharks with alligators and then again with crocodiles. “Alligators are opportunistic” and surely they wouldn’t pass up a “big chunk of protein that’s swimming by.” There is evidence of alligators eating lemon, nurse, and bonnethead sharks, as well as an Atlantic stingray. “Gators will eat almost anything that will fit in their mouths” says a researcher at the Northwestern State University of Louisiana, though the researcher points out that he has never found shark remains in the stomach of an alligator. This may be due to the highly acidic stomachs of the alligators, which could dissolve the cartilaginous skeletons of sharks with little trace. The literature was searched for crocodile encounters, and the conclusion was quite the opposite. The crocodile, more often than not, was the prey in many cases—great white sharks preying on American crocodiles in Columbia and tiger sharks eating estuarine crocodiles in Australia.  

Check out this short 1 minute video on the shark/alligator ancient rivalry

References

Fujisaki, I., Hart, K., Cherkiss, M., Mazzotti, F., Beauchamp, J., Jeffrey, B., & Brandt, L. (2016). Spatial and Temporal Variability in Estuary Habitat Use by American Alligators. Estuaries & Coasts, 39(5), 1561-1569. Doi:10.1007/s12237-016-0084-2

https://www.newscientist.com/article/2148317-alligators-versus-sharks-who-wins-this-ultimate-showdown/