Resource space partitioning by the Bryozoa of a Fucus serratus L. Community

The spatial distributions of five Bryozoa species encrusting Fucus serratus L. plants in Strangford Lough, Northern Ireland, have been examined in two cross-sectional samples yielding 80 plants and 1326 sampling units drawn from 13 environmentally diverse sites. The co-occurences of Flustrellidra hispida (Fabricius), Alcyonidium hirsutum (Fleming), Electra pilosa (L.), Membranipora membranacea (L.), and Celleporella hyalina (L.) on individual Fucus serratus plants were consistent with independent settlement onto each plant, but their joint distributions within the plants were markedly non-random. Measurements of behavioural interactions between species showed that competitive abilities declined through the sequence Flustrellidra, Alcyonidium, Electra, Membranipora, which parallels the sequence of competition (Levins' α) coefficients calculated from spatial distribution data. Competitive ability varied with position on the plant. Flustrellidra and Alcyonidium reversing their relative competitive superiority between basal and distal regions of the fronds. Flustrellidra and Electra were strongly associated under all conditions. Alcyonidium was also frequently present with this pair, though its numerical abundance was inversely related to that of Flustrellidra. Celleporella tended to co-occur with Electra whilst avoiding Flustrellidra and Alcyonidium. Membranipora numbers were inversely related to those of the other species. Analyses of competition coefficients showed that Flustrellidra, Alcyonidium and Electra each peaked in competitive ability against Membranipora at different points along the frond, such that the latterwaas effectively excluded by direct as well as by diffuse competition. Membranipora populations had a greater dependence on the segment area available for colonization than had any other bryozoan species.