Asymmetric interactions between species of seaweed fly

1. This paper describes the colonization patterns of, and interactions between, three common species of British seaweed fly: the coelopids, Coelopa frigida and C. pilipes; and the sphaerocerid, Thoracochaeta zosterae. 2. All three species exhibited similar temporal patterns in abundance on wrack beds in the field: adult flies were more abundant in the first 4 days after wrack deposition and then again towards the end of the wrack bed cycle, coinciding with the emergence of the next generation. 3. Two forms of resource were presented to the flies, minced and chopped seaweed (Fucus serratus), under controlled environmental conditions. The species interacted so as to produce a dominance series, with C. frigida being the strongest species and T. zosterae the weakest. All pairwise interactions were extremely asymmetric, adding support to the growing awareness that this phenomenon is common in interactions between insects. 4. On chopped seaweed only, C. pilipes facilitated C. frigida, more than doubling the latter's emergent population size. This facilitation, in conjunction with the inhibition of C. pilipes by C. frigida, produced a contramensal (+, -) interaction. Suggestions are made as to how the facilitative effect may have been produced, but the exact mechanism is not yet understood.