Cattle Grazing of a Celtic Sea Saltmarsh Affects Invertebrate Community Composition and Biomass, but not Diversity

Large grazers can regulate biodiversity and ecosystem functioning through both direct and indirect effects on biological communities. For this reason, grazing by domestic livestock is frequently implemented as a conservation tool in many parts of the world and remains most popular in northwest Europe. Saltmarsh invertebrates support multiple ecosystem functions; therefore, understanding the effects of grazing on saltmarsh invertebrate fauna is important to enable the most apt decisions pertaining to grazing management on these highly valuable ecosystems. This study examined the effects of cessation of cattle grazing on the ground-invertebrate community in Ballyteige Burrow saltmarsh on the Celtic Sea coast of Ireland. We found that cessation of cattle grazing resulted in a shift of saltmarsh invertebrate community composition, with increases in Gastropoda and Amphipoda, and decreases to some Diptera and Coleoptera species. Cessation of grazing did not detectably affect invertebrate abundance, but biomass increased by 53%. This was driven by higher abundance of the amphipod Orchestia gammarellus, a species with important roles in nutrient cycling, sediment bioturbation and sustaining fish of high fishery value. We did not find cessation of grazing to affect invertebrate richness or evenness. Although there is mounting evidence of the irreplaceable ecological value that invertebrates provide, conservation assessment of saltmarshes in Europe tends to be carried out without regard to resident invertebrate fauna. Therefore, grazing management of saltmarshes in Ireland and more widely need to give due attention towards understanding the role of invertebrates as a part of the biological diversity that maintains healthy saltmarshes.