The impact of a coastal invasive predator on infaunal communities: Assessing the roles of density and a native counterpart

Our understanding of the influence of many predatory invaders in areas of increasing overlap with native counterparts remains elusive. In Atlantic Canada, the European green crab, Carcinus maenas, overlaps with the native rock crab, Cancer irroratus, but surprisingly little is known about their effects in the light of their potential interactions. In this study we used short-term cage inclusion experiments to assess the impact of low and high green crab densities upon infaunal communities. Then, we re-assessed this impact by running identical manipulations combining green crabs with rock crabs of comparable size and at similar overall predator densities. Our results indicate that both low and high green crab densities accounted for severe declines in infaunal organisms with respect to ambient cages. Polychaetes, the group best represented in this trial, accounted for most of such decline with densities at least 50% lower in the green crab inclusions. A similar (~ 50%) decline in infaunal density was observed when green crabs and rock crabs were combined at low predator densities. However, at high predator densities their impact on polychaetes, molluscs and total infauna was less severe and non-significant with respect to ambient cages. Our results indicate that between-predator interactions have serious indirect effects on benthic prey and contrast previous results on the role played by these crab species. We propose a re-assessment of the role played by native counterparts while searching for management alternatives to minimize the impact of invasive predators in areas heavily invaded.