Examining the Potential for Overcompensation by the European Green Crab in Response to Control Efforts

Overcompensation, or the hydra effect, is a phenomenon where an increase in mortality results in an increase in the growth rate of a given population. This increased growth can lead to a population becoming unstable or even exceeding previous levels. Under certain conditions, overcompensation has occurred as the result of efforts to control invasive species. The European green crab, Carcinus maenas, is native to Western Europe but is now found throughout the world. Eradication is increasingly proposed as a method to reduce the effects C. maenas and other destructive non-indigenous species, but so far few attempts have been made to evaluate the overcompensatory potential of invaders targeted for removal. Populations that overcompensate can still be reduced/ eradicated from discrete areas, but require a larger percentage of the population be removed. Failure to remove a sufficiently large percentage could lead to rapid population rebounds and thus have severe ecological and economic consequences. Therefore, it is imperative to determine if a species might increase as a result of harvest. How a species reacts to harvest will depend greatly on the age classes targeted and on the density-dependent interactions in a population. For example, species that exhibit strong negative interactions between adults and juveniles are more likely to overcompensate when removal efforts target adults. Due to environmental and logistical limitations, the majority of C. maenas caught during removal efforts are adults. In an effort to examine the impacts of adult C. maenas on juvenile conspecifics, experiments were performed during the summer of 2010 in Bodega Harbor, CA. The experiments examined 1) cannibalism rates by C. maenas with and without alternative prey in intertidal enclosures, 2) survivorship of tethered juvenile C. maenas at locations with varying adult densities, and 3) impacts of the presence of banded adults on the foraging rates of juveniles in laboratory flow tanks. While adults were found to consume juvenile conspecifics, cannibalism rates were low and adults preferentially consumed alternative prey. Additionally, adult density had no significant effect on juvenile survivorship, although adult densities at all sites were relatively low. Surprisingly, foraging rates of juveniles significantly decreased in the presence of adult conspecifics. Further analysis of adult impacts, specifically survivorship at extremely high adult density and further replication of the foraging work, are necessary to understand how populations of C. maenas will respond to adult removal.