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Research: Effects of disturbance and fertility upon the vegetation of a Louisiana coastal marsh: an evaluation of Huston’s general model of diversity

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Fire, herbivory, nutrients, general disturbances, and sedimentation are some of the major factors controlling marsh structure and composition. These factors are not routinely manipulated in robust replicated factorial design experiments. In order to perform this type of experiment, a large research facility, Turtle Cove Experimental Marsh (TCEM), was constructed in the wetlands behind Turtle Cove Environmental Research Station. TCEM developed through a collaborative effort between the research laboratories of Dr. Paul Keddy (Edward G. Schlieder Endowed Chair for Environmental Studies) and Dr. Gary Shaffer (Southeastern Wetlands Restoration Laboratory).

There are two main objectives of the project:

1. to explore the effects of multiple disturbance and fertility regimes upon plant community structure

2. to set targets and provide guidelines for wetland restoration.

The main theoretical framework for this experiment is provided by Huston’s general model of diversity (Huston 1979). All of the treatments can be arranged along Huston’s two orthogonal axes, the rate of disturbance and the rate of recovery from disturbance (i.e. fertility). Mammalian herbivores such as nutria are excluded from 3 main plots (40 x 60m), while 3 main plots remain open to grazing. Within both types of these main plots, 3 x 3m plots have been receiving the factorial combinations of fertility and disturbance treatments since early 2002.

Initial results show that nutria, the principal vertebrate herbivore of the marsh, may limit biomass production and increase species richness. Prescribed fire does not appear to have a place in management of the area - it seems to promote heavy, localized herbivory in burned areas thereby, reducing the amount of organic matter incorporated in the soil. The sediment + fertilizer treatment, which simulated a proposed freshwater diversion, significantly increased biomass production with no reduction in species diversity. Huston’s (1979) general model of diversity appeared to be applicable to the Manchac marsh.

Huston, M.A. 1979. A general hypothesis of species diversity. The American Naturalist 113:81-101.