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

Abstract

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.

Herbivory and plant competition reduce mountain beech seedling growth and establishment in New Zealand

“…Deerinduced turf communities may also prevent mountain beech canopy regeneration through competitive exclusion of seedlings. The relationships between herbivory, browse-tolerance and resource availability need to be better understood to develop general rules in a variety of habitats including mountain beech forest (Di Tommaso and Aarssen 1989;Louda et al 1990;Taylor et al 1997;Husheer and Robertson In Press).…”

The interactive effects of fire and herbivory on a coastal marsh in Louisiana

“…Grace et al (1992Grace et al ( , 1993 found in greenhouse pot experiments that S. patens transplants were relatively weak competitors and, depending on the competition index used, ranked either sixth or fourth in competitive ability out of six common marsh species (the other species being Eleocharis macrostachya, Solidago sempervirens L., Scirpus validus Vahl, Scirpus robustus Pursh, and Sagittaria lancifolia). In a one-year field study of competition conducted at the Pearl River, Taylor et al (1997) found that when protected from herbivory, S. patens transplants were more affected by competition than those of either Panicum virgatum or Spartina alterniflora Loiseluer. Unpublished greenhouse competition experiments involving transplants of S. patens and Scirpus americanus showed Scirpus americanus to be a better competitor than S. patens (Keough, Guntenspergen, and Grace unpublished).…”

Ecological Dynamics II: The Influences of Vertebrate Herbivory on Ecological Dynamics in Wetland Ecosystems

“…As relatively small herbivores, both goose and coypu have high abilities to select for individual plant species and even plant parts Shirley et al 1981;Brackney and Hupp 1993). In the subarctic, and in more temperate climatic zones, grazing of coastal salt marshes decreased the competitive interactions for light if neighbours were grazed as well (Mulder and Ruess 1998;van der Wal et al 2000a), or decreased in general the neighbour effects (Taylor et al 1997). Competition for light was, however, strongly enhanced if neighbour plants were avoided by the grazers, but droppings increased the soil nutrient availability (Mulder and Ruess 1998).…”