| The extent of biological invasions, their role on the feeding of native fishes and their impact on community stability were investigated in Alsea Bay and Yaquina Bay, two estuaries on the central Oregon coast, USA. Most nonindigenous species (NIS) introduced in these intermediately invaded estuaries are considered byproducts of culturing introduced Atlantic and Pacific oysters. Secondary potential vectors of NIS in Yaquina Bay are external fouling of ship hulls and ballast water. Native benthic invertebrates and native fishes dominate in density, catch per unit effort (CPUE) and richness in both estuaries. Three of the 11 benthic NIS of invertebrates in Yaquina Bay and one of the eight NIS in Alsea Bay are among the 10 most dominant benthic invertebrate species. The NIS of invertebrates are concentrated in habitats with above average water temperature, salinity, and macrophyte density at high-tide. The CPUE of fishes and decapod crustaceans are associated with above average water temperature, salinity and macrophyte density but are not consistently correlated with invertebrate density in sediments. Biological invasions have caused significant prey shifts in intertidal food webs of Yaquina Bay. Diets of two species of native juvenile flatfishes (Pleuronectes vetulus and Platichthys stellatus) included mainly polychaetes, crustaceans and bivalves and each of these taxa are represented in the diet by native species and NIS in each estuary. Both flatfish species are generalist predators and had no consistently higher selection for either native species or NIS. Prey selection experiments indicated that two native and two introduced amphipod prey (Corophium spp.) are acceptable prey for juvenile English sole. Thus, predator-prey coevolution plays no significant role on prey selection. Interspecific prey selection may depend on prey exposure, water visibility, substratum type, and species diversity of available prey. Modeling of functional-group interactions for the intertidal benthic community of Yaquina Bay suggested reduced community response to invasions or removal of fish predators as indicated by the community tendency to zero overall-feedback. However, the increased risk of stability decline of invaded community models implies that further human-mediated biological invasions should be avoided. |
