This 25-minute talk was given to a regional Society of Wetland Scientists meeting in 2006 in Vicksburg. Its sub-title is The Potential for Biological Control of Nutria by Alligators.
More about the biology behind the talk
For more than a decade, wetland scientists in Louisiana have documented the damage caused to wetlands by the introduced herbivore, nutria (Myocastor coypus). Experiments at Turtle Cove show that nutria reduce plant biomass by approximately one-third, which may not only reduce survival of plants, but reduce the accretion of peat that slowly raises the elevation of wetlands. For several decades, a different group of scientists have documented the diets of alligators (Alligator mississippiensis). They have shown that while small alligators eat mainly crustaceans and fish, larger alligators feed heavily upon nutria.
A study of the stomach contents from 100 large (1.5-3.0 or 4.5-9 ft) alligators from southeastern Louisiana in 1987, concluded that “Mammals become important to alligators of about 1.5 m in length, and there is a general shift from muskrats to nutria as alligator length increases.” Oddly, there seems to have been reluctance to draw an obvious conclusion from the two types of studies combined – that large alligators might provide a natural biological control upon nutria, and large alligators might provide an important (and free) benefit to wetlands.
To explore this possibility further, we reviewed the existing stomach content data, and confirm that nutria form a large part of the diet of large alligators. To explore the possible effects of alligators upon nutria populations, we took an existing model for population growth of nutria, and added in feeding by alligators over a range of body sizes, seasons and population densities. The model showed that alligators might indeed reduce nutria population sizes. The surprise from the model was the broad range of conditions under which alligators appeared to be capable of eliminating nutria.
There is therefore evidence that alligators can provide a free ecological service to the state of Louisiana by protecting wetlands from predation by nutria. This is consistent with results from other ecosystems where predators protect plants from herbivores. The value of this free service likely exceeds 4 million dollars a year, the amount currently paid to trappers to carry out the same task. Hunting and trapping may not only reduce alligator density, but more importantly, preferentially remove the largest alligators – the very size class that has the greatest feeding rate upon nutria.
Future cost-benefit analyses for alligator management need to consider not only the economic gain from hides, meat and eggs, but also the possible economic losses from increased nutria populations and damage to coastal wetlands. Future research is needed to better quantify the alligator-nutria relationships over a range of habitat and scales. There is a particular need for studies that include plants, grazers and alligators. Owing to the size of the experiments that would be needed, this will likely require collaboration among a large group of managers and scientists.
Paul Keddy, Ph.D. 