Quantifying ecological filters: the relative impact of herbivory, neighbours, and sediment on an oligohaline marsh

The objective of this experiment was to quantify the relative importance of herbivory, interactions with neighbours and sediment addition as filters in controlling the species present in an oligohaline marsh in southeastern Louisiana, USA. We planted 16 species of wetland plants in 3×3 m plots either inside or outside herbivore exclosures, with or without presence of established vegetation, and with or without added sediment. These species, representing 12 families and diverse morphologies, included herbaceous and woody plants. At the end of the growing season above‐ and below ground biomass were measured. In cleared plots (no neighbours) inside herbivore exclosures, 12 of the 16 species grew well, indicating that they could tolerate the physical conditions in the habitat. Competition significantly reduced the biomass of four of these species (A. calamus, C. occidentalis, P. hemitomon, P. cordata), suggesting that it is the strongest species‐specific filter operating in the marsh. These results confirm the general consensus that competition from existing plants is the strongest filter operating in high biomass habitats, and further imply that competition has the potential to remove 33% of the species from the community. Facilitation occurred for one species, R. corniculata. Herbivory had the largest general effect of reducing biomass, as determined in the full ANOVA model, but on a species‐specific basis, significantly reduced the biomass of only two species (T. distichum, T. domingensis). These results support the emerging view that herbivory is an important filter in coastal wetlands. Although several other studies have argued for the importance of sedimentation in controlling plant species composition in wetlands, we found no effect of sediment addition. Competition and herbivory appear to be two critical biological filters that control plant composition of coastal wetlands, and they must be considered in future studies and restoration efforts along the Gulf of Mexico.