Mechanisms and consequences of an exotic crab species invasion
Rocky intertidal habitats in southern New England were dramatically altered after green crabs, Carcinus niaenas, were introduced to the region in the early 1800\u27s. Roughly 10 years ago, another exotic crab species (the Asian shore crab, Hemigrapsus sanguineus) became established in the same habitats. This investigation focuses on (1) how the Asian shore crab successfully invaded rocky intertidal habitats of southern New England during the 1990\u27s, and (2) how the Asian shore crab is affecting other resident organisms (including C. maenas) in invaded habitats. The first goal was achieved by studying guilds of co-occurring crabs in native (Tanabe Bay, Japan) and invaded (Long Island Sound, USA) habitats of H. sanguineus. Analyses revealed that patterns of resource utilization by H. sanguineus were very similar in both locations. Furthermore, there was little overlap in resource use between H. sanguineus and other co-occurring crabs in invaded habitats. Such ecological factors likely contributed to the invasion success of H. sanguineus in southern New England. The second goal was achieved by using a mulifaceted approach (field surveys, laboratory studies, and manipulative field experiments). While adult green crabs are larger than adult H. sanguineus (and are immune to predation by H. sanguineus ), 99% of the green crabs sampled in rocky intertidal areas were less than the maximum size of H. sanguineus. Small green crabs are at risk of being preyed upon by adult H. sanguineus and, in repeated field experiments, densities of C. maenas \u3c10 mm were reduced via interspecific predation. Predation pressure on green crab recruits has likely increased in rocky intertidal areas due to the invasion by H. sanguineus and, as a result, green crabs \u3c35 mm carapace width appear to be disappearing from these habitats. The decline of green crabs, coupled with elevated densities of H. sanguineus , may be affecting other resident species in rocky intertidal habitats of southern New England. This was demonstrated for the commercially important, native blue mussel, Mytilus edulis. A variety of field and laboratory experiments suggest that juvenile blue mussels are at greater risk of predation now than they were prior to the arrival of H. sanguineus .