The Impacts of a Nonindigenous Marine Predator in a California Bay

Coastal marine ecosystems worldwide are being altered rapidly by the invasion of nonindigenous species. Unlike terrestrial and freshwater systems, the impacts of an invading species have never been quantified on multiple trophic levels for a marine food web. We measured the impact of the nonindigenous green crab, Carcinus maenas, on a coastal marine food web in central California and found that this predator exerted strong ‘‘top-down’’ control, significantly reducing the abundances of several of the 20 invertebrate species monitored over a 9-yr period. Densities of native clams, Nutricola tantilla and Nutricola confusa, and native shore crabs, Hemigrapsus oregonensis, showed 5-fold to 10fold declines within 3 yr of the arrival of green crabs. Field and laboratory experiments indicated that green crab predation caused these declines. We also tested for indirect responses of invertebrates and vertebrates to green crab predation. There were significant increases in the abundances of two polychaete taxa, Lumbrineris sp. and Exogene sp., and tube-building tanaid crustaceans, Leptochelia dubia, most likely due to the removal of cooccurring green crab prey. However, we observed no changes in shorebird abundances (13 species) over a 9-yr period suggesting that green crabs have had no ‘‘bottom-up’’ effect on shorebirds, which subsist on benthic invertebrate prey. We predict that such bottom-up control will occur as the local effects and geographic range of green crabs increase. The 2-yr temporal scale of direct and indirect responses of the invertebrates in this low energy, soft-substrate system was also in agreement with the results of perturbation experiments by others on rocky shores, which showed that most direct and indirect responses also occur within a 2-yr time frame.