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Non‐Additive Impact of Blue Crabs and Spot on Their Prey Assemblages

Martin, Thomas H., Wright, Russel A., Crowder, Larry B.
Ecology 1989 v.70 no.6 pp. 1935-1942
Callinectes sapidus, Leiostomus xanthurus, Ulva, algae, crabs, estuaries, field experimentation, fish, habitats, predation, predators, prediction, North Carolina
We designed a field experiment to examine predator interactions–in particular, the effect of each predator on the growth and survival of the other, and to examine the effects of predation on prey assemblages–in particular, predation effects by each predator alone as well as together. Blue crabs (Callinectes sapidus) and the fish, spot (Leiostomus xanthurus), co—occur in Southeastern estuaries of the USA, and share many prey taxa and habitat types. We predicted that the blue crabs and spot would suffer both interference and exploitation competition when held together in enclosures. We also predicted that, as a consequence of the competitive interactions, their joint impact on prey assemblages would be different from that predicted based on the impact when held alone. We installed mesh enclosures in an existing earthen pond after allowing it to be filled with water from Bogue sound, North Carolina, and to be colonized by the naturally occurring species assemblage. A factorial design was used to allow us to test for non—additive effects of the two predators on their prey assemblage. Contrary to our predictions, we found that spot survival was enhanced in the presence of blue crabs. This enhancement was probably effected by removal of the alga Enteromorpha intestinalis by the crabs. The alga was positively affected by spot. The positive effect of spot on Enteromorpha, and the negative effect of crabs, was probably responsible for differences in densities and distributions of prey taxa within the enclosures. Significant interaction terms in our analysis suggest that community response to either predator is not independent of the other. Our data suggest that we cannot expect to explain adequately the effects of multi—species predator assemblages on their prey by combining information obtained through single—predator experiments.