Many ecosystems are characterized by diverse assemblages of species that require similar resources for survival and growth. Understanding how these species partition shared resources is key to evaluating both coexistence and the relationship between biodiversity and ecosystem functioning. For example, four mussel species - Aulacomya, Perna, Mytilus, and Xenostrobus - co-occur on New Zealand rocky shores. I conducted surveys at four locations on the coast of the South Island and documented patterns of distribution and abundance of the four species along tide-height gradients. I also quantified stable isotope (δ13C and δ15N) values for each species at low, intermediate, and high elevations on the shore. Mussels are basal species on rocky shores, mediating carbon fluxes from nearshore pelagic to intertidal benthic ecosystems. I quantified these carbon inputs by measuring annual rates of growth in carbon associated with each species at each tide height and location.
Results/Conclusions
Mussels exhibited distinct patterns of zonation, with Perna and Aulacomya most abundant on the low shore, Mytilus most abundant on the mid-shore, and Xenostrobus most abundant on the high shore. Cores through the mussel bed revealed vertical changes in species composition from the top to the bottom of the bed, which was often composed of a combination of “canopy” and “understory” mussel species. Especially in locations where mussel beds were thicker, Mytilus and Perna tended to live on the surface of mussel beds, whereas Aulacomya and Xenostrobus tended to live closer to the substratum. Species exhibited distinct carbon and nitrogen stable isotope signatures - Aulacomya and Xenostrobus had lower δ13C and δ15N values than Mytilus and Perna - suggesting that different species are targeting different food sources. On average, Perna mediated just over half of the carbon inputs into these systems; the remaining half was contributed by the other three mussel species. However, the relative role of Perna declined higher on the shore, whereas the roles of Mytilus and Xenostrobus increased. A diverse assemblage of coexisting basal species allows maintenance of carbon inputs across the intertidal zone.