THE IMPACT OF VERTEBRATE HERBIVORES ON WETLAND VEGETATION IN ATCHAFALAYA BAY, LOUISIANA

Delta islands in the Atchafalaya and Wax Lake deltas in Atchafalaya Ba y, Louisiana, are in an extremely dynamic successional phase. These is lands initially supported large marshes dominated by the pioneering pl ant species Sagittaria latifolia and Sagittaria platyphylla. A general decrease in vegetated areas has occurred in the delta island marshes in the Atchafalaya Delta since about 1980, while in the Wax Lake Delta portion of the complex the vegetation still flourished. The Atchafala ya Delta provides an interesting setting for the study of herbivory be cause of the complex interaction of biotic and physical factors operat ing in this delta. We hypothesized that grazing by herbivores has a ma rked effect on vegetation in these developing marshes. To test this hy pothesis, exclosures were erected on islands in both deltas in Septemb er 1985 and January 1986. Each set of exclosure treatments included an openly-grazed control area, an ungrazed area, an area allowing nutria grazing, and one allowing waterfowl grazing in each site. Results of the experiment, based on field sampling of vegetation, indicated decre ases in plant biomass and changes in plant species composition in graz ed treatments. Waterfowl and nutria reduced biomass about equally, but there was a more marked effect in the openly grazed areas. These find ings may be extrapolated to sediment diversion areas along the Mississ ippi River.