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Research: Alligators, nutria and trophic cascades in coastal wetlands

The rate of loss of Gulf Coast marshes in general, and the Louisiana coastline in particular, is now a national issue, particularly following the 2005 hurricanes in the region. We suggest that current management paradigms for marsh restoration may focus too exclusively on plants and sediment, with a bottom up view of coastal wetlands.  Top down processes also merit consideration and may expand the array of potential tools for coastal management and restoration.  Here we propose an alligator trophic cascade hypothesis incorporating a top-down approach: that alligator hunting, by reducing the density and mean size of alligators, removes a natural control on the primary herbivores in wetlands, enabling the run-away consumption of coastal marshes.  We present current evidence to support this hypothesis.  Mammalian grazing can directly remove plant biomass and make plants less tolerant to flooding and salinity, therefore increasing erosion of sediments.  Both muskrat and nutria have been implicated in this process, with the larger, non-native nutria of greater current concern. Annual aerial surveys beginning in 1998 indicated that 321 km2 to 415 km2 of Louisiana’s 14,164 km2 coastal wetlands were severely damaged by nutria.  Adult alligators eat muskrat and nutria, but the role of alligators as potential controllers of mammal populations, and thus as controllers of marsh damage, has received minimal consideration. We are exploring these possible relationships.  If the hypothesis is supported, reducing the alligator harvest or closely controlling the size of the animals being harvested may prove a valuable management tool in conserving coastal wetlands. 

This work has been completed and is now published here.