Multiple indirect effects of an invasive mussel change native bivalve recruitment and survival
Indirect species interactions are important to ecosystem structure and function, but are largely overlooked in the context of non-native species, especially in marine and estuarine invasions. In southern California, introduction of the Asian nest mussel, Arcuatula senhousia, has been linked to profound changes in native bivalve assemblages. Arcuatula occupies a unique ecological niche as a prolific habitat-modifying bivalve that is highly susceptible to predation. Thus, we performed three field experiments to test the hypothesis that Arcuatula affects native bivalves through multiple mechanisms, including resource competition and predator-mediated interactions. First, to evaluate preemptive competition, we manipulated Arcuatula density and measured native bivalve recruitment. Second, to assess the potential for exploitative competition between Arcuatula and native adult bivalves, we manipulated Arcuatula density in the presence of one of two native clam species (Laevicardium substriatum and Chione undatella) in caged plots and measured their growth and survival. Third, to determine the role of predators in mediating interactions between Arcuatula and native clams, we paired native clams with Arcuatula in uncaged plots and measured clam proportional survival and predator density.
We found that increasing Arcuatula density reduced the diversity and evenness of native bivalve recruits and, for a few abundant taxa, altered recruit density and size, possibly by competing for space or water-borne food. When paired with shallow-dwelling native clams, Arcuatula reduced adult native bivalve survival but not growth, likely indicating rapid competition for surface sediment space (e.g., overgrowth competition). In the third experiment, at very modest densities Arcuatula attracted native predators (primarily drilling gastropods) and indirectly increased the mortality of native clams, especially for the poorly-defended L. substriatum. We infer that Arcuatula has the potential to change native bivalve assemblages by (1) altering the diversity, density, and size of native bivalve recruits by preemptively exploiting benthic space, (2) reducing the survival of shallow-dwelling adult native bivalves by competitively dominating surface sediments, and (3) increasing the mortality of poorly-defended native bivalves through apparent competition. Thus, Arcuatula influences a functionally similar native assemblage through both abiotic and biotic indirect effects. Different responses among species and life-history stages to preemptive, exploitative, and apparent competition with Arcuatula may contribute to the fundamental changes in southern California bivalve assemblages observed over the past 50 years.