Host plant quality and vegetation architecture interact to affect arthropod community structure and diversity.
ABSTRACT- Although host plant quality and architecture have long been known to affect dependent herbivore populations, few studies have examined how such changes in basal resources may affect a community of arthropods representing diverse trophic levels and feeding groups. Because host plant quality is often confounded with changes in vegetation structure, it is important to examine arthropod community responses to plant architecture independently of changes in host plant nutrition. In tidal marsh habitats, cordgrass (Spartina alterniflora) regularly receives allochthonous inputs of nitrogen, and is also subject to changes in vegetation complexity via the removal of dead biomass (or thatch) from low elevations on the marsh. We therefore designed a two by three factorial experiment with three levels of nitrogen addition (no, low, or high nitrogen subsidy) and two levels of habitat structure (thatch present or absent) to test the separate and combined effects of nutrients and habitat complexity on cordgrass arthropod communities. We found that nitrogen addition, the presence of thatch, and their interaction positively affected arthropod species richness and abundance. The effects of nitrogen and thatch manipulation on the community were not restricted to herbivorous arthropods, but also increased populations of predators and detritivores. Furthermore, we found that arthropod community composition shifted in response to the different levels of nitrogen fertilization and the presence of thatch. Fertilization and thatch therefore not only affected arthropod species richness and abundance, but also changed the structure of the arthropod community. These results are important for two reasons. First, because thatch accumulation is related to differences in elevation, the spatial location of cordgrass on a landscape indirectly influences the associated community via changes in the retention of dead biomass. Second, while natural processes such as tidal flooding can be a source of allochthonous nitrogen, tidal marshes commonly experience nitrogen deposition via anthropogenic sources. If changes in nutrient status affect arthropod community composition, then nitrogen inputs in marsh ecosystems could alter food web structure.
Key words: arthropod, community, plant, quality