Impacts of the introduced crabs, Carcinus maenas and Hemigrapsus sanguineus, in northern New England

Two introduced crab species are presently found in northern New England, Carcinus maenas, which has been in this region for over 100 years, and Hemigrapsus sanguineus which was first reported in New Hampshire in 1998. Carcinus maenas is a generalized predator and its introduction has had negative effects on several native mollusc species but the community wide impacts of its introduction have been relatively neglected. Hemigrapsus sanguineus is just beginning to establish populations in this region, and its effects on the resident community are unknown. Both monitoring and experimental approaches were used to retroactively infer the impacts of C. maenas and to predict the impacts of H. sanguineus on the rocky intertidal community of northern New England. The temporal and spatial patterns of C. maenas\u27 recruitment were documented in anticipation that this species may decline as H. sanguineus\u27 population increases. Microcosm experiments indicated that the two crab species had similar consumption patterns, but that H. sanguineus caused a significantly greater decline in barnacles than C. maenas in both short and long term field experiments. Temporal variation in the abundance of ephemeral algae was documented twice yearly at three coastal and one estuarine location. These baseline data provide pre-Hemigrapsus variability in abundance of Enteromorpha sp. and other annual algae species that can be used to compare post-invasion levels of these palatable algal species. Temporal variation in community structure prior to the establishment of H. sanguineus was also documented at two coastal and one estuarine location. Both non-metric multidimensional scaling and cluster analysis of the community data indicated that there was low temporal variability between each tidal height at a particular location. Carcinus maenas was the dominant crab at all locations, but by the end of the study period, H. sanguineus had been found at all three study locations. If H. sanguineus exhibits similar behavior in northern New England as it did in the microcosms and in southern New England, the rocky intertidal community could soon be dominated by a new introduced crab species that has a higher per capita impact than the old introduced crab species