Some effects of herbivory and 30 years of weir management on emergent vegetation in Brackish Marsh

Weirs are low-level dams used in Louisiana's coastal marshes to improve habitat quality for ducks and furbearers; however, some ecologists question whether weir management inhibits soil drainage and thereby negatively affects the emergent plant community. We compared the emergent plant communities of unmanaged marsh and marsh that was weir managed since 1958 at Marsh Island, Louisiana to test for management effects. Two data sets were analyzed: 4 permanent transects sampled 8 times between 1958 and 1988, and 44 random transects sampled in 1987. If weir management inhibited soil drainage, differences between weir managed and unmanaged marsh should have been similar to differences between naturally poorly drained and better drained marsh. Poorly drained marsh had lower species richness and diversity than better drained marsh, and poorly drained and better drained marsh differed in the importance of 4 major plant species. Neither species richness, species diversity index, major plant species, nor vegetative cover differed between weir managed marsh and unmanaged marsh in either data set. Analyses indicated that 4 minor species were negatively affected, and 2 were positively affected by weir management, but those results were not consistent in both data sets. Species richness and diversity index were negatively related to the frequency of nutria (Myocastor coypus ), but species richness was positively related to the frequency of muskrats (Ondatra zibethicus ). Incorrect conclusions would have been reached if variation in herbivore activity had not been noted. Although management may affect other marshes differently, any differences in emergent plant community structure between weir managed marsh and unmanaged marsh at Marsh Island are less than observed differences between naturally well-drained and poorly-drained marsh. The lack of a management effect was attributed to the lack of substantial soil drainage even in unmanaged marsh.