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Research: Filters controlling the plant species pool of coastal marshes

What controls species richness in a given ecological community?

You begin with a large pool of species available. Abiotic conditions then filter out unsuitable species from the species pool based upon species traits. Dominant competitors can directly exclude weaker species, or preclude establishment, by limiting resources. Disturbance may increase species richness by reducing the success of dominant competitors and creating gaps for weaker species, or, more frequently, disturbance may decrease species richness.

The objective of this experiment is to examine the relative importance of species availability, competition, herbivory (a major disturbance) and nutrients in controlling the species composition of marshes. It is part of a larger study on the dynamics of an oligohaline marsh along Pass Manchac in southeastern Louisiana.

The experiment has a split-plot factorial design with three replicates. To test for effects of herbivory, main plots (40 x 60 m) are either open or fenced exclosures. To test the species pool effect, twelve herbaceous and four woody wetland species were transplanted, and seeds from thirteen species were sown within 3 x 3 m subplots. Most of these species did not occur in the study area and represent additions to the local species pool. To test for effects of competition, herbicide was first applied to selected subplots to remove competition while others were left undisturbed within the established vegetation. Throughout the growing season, herbicide plots were continually weeded and herbicided. To test for the effects of flood-related nutrient addition, sediment was manually spread on a subset of plots a week after planting.

We measured plant mortality, seedling establishment, and percent coverage May to September 2003. Above-ground and below-ground biomass was harvested September to November 2003. Biomass was cleaned, dried and weighed. All data will be analyzed to test which factors (herbivory, species availability, competition, sediment) influence species richness in an oligohaline marsh.