The effects of herbivory on neighbor interactions along a coastal marsh gradient

Many current theories of community function are based on the assumption that disturbances such as herbivory act to reduce the importance of neighbor interactions among plants. In this study, we examined the effects of herbivory (primarily by nutria, Myocastor coypus) on neighbor interactions between three dominant grasses in three coastal marsh communities, fresh, oligohaline, and mesohaline. The grasses studied were Panicum virgatum, Spartina patens, and Spartina alterniflora, which are dominant species in the fresh, oligohaline, and mesohaline marshes, respectively. Additive mixtures and monocultures of transplants were used in conjunction with exclosure fences to determine the impact of herbivory on neighbor interactions in the different marsh types. Herbivory had a strong effect on all three species and was important in all three marshes. In the absence of herbivores, the impact of neighbors was significant for two of the species (Panicum virgatum and Spartina patens) and varied considerably between environments, with competition intensifying for Panicum virgatum and decreasing for Spartina patens with increasing salinity. Indications of positive neighbor effects (mutualisms) were observed for both of these species, though in contrasting habitats and to differing degrees. In the presence of herbivores, however, competitive and positive effects were eliminated. Overall, then, it was observed that in this case, intense herbivory was able to override other biotic interactions such as competition and mutualism, which were not detectable in the presence of herbivores.