Grazers vs Grazers: Large Mammal Herbivores Influence Salt Marsh Invertebrate Communities

Large mammals cause disturbances in the environments in which they graze by dramatically changing the physical structure of habitats. The response of plant and animal communities to megafaunal herbivory is highly variable among ecosystems, geographic location, and species of interest. Past research has often only focused on the response of a single taxa, and effects of grazing in North American salt marshes is relatively understudied. I conducted field studies on three barrier islands on the coast of Georgia, USA. Each island served as a site for one of three grazing treatments (cattle, feral horses, and artificial grazing by clipping). Treatment and control plots at the three sites were sampled for vegetation metrics and invertebrate abundance and diversity. All three grazing types altered invertebrate community composition, and each represented a different level of grazing intensity. Cattle grazing had a particularly negative impact on plant characteristics and the invertebrate community, while artificial grazing produced some opposite effects. Leaf chlorophyll content and toughness and invertebrate taxa such as snails (Littoraria irrorata) and Chaetopsis flies had varied responses, but plant height and katydid (Orchelimum fidicinium) density were consistently reduced by the three grazing types. To further investigate this, I performed two katydid feeding experiments and found that a more nutritious diet may not necessarily be more palatable. I also found that the response of planthoppers (Prokelisia spp.) to grazing correlates to the response of leaf chlorophyll content. The indirect effects of grazing on plant nutrition may therefore play an important role for herbivorous invertebrates. The responses of predatory invertebrates such as spiders are likely more dependent on the direct effects of grazing on vegetation biomass and architecture. The responses of different taxonomic groups and invertebrate communities to large mammal grazing and the underlying mechanisms should be considered when making livestock and land management decisions that aim to promote ecosystem functioning.