COMPETITION FOR SPACE IN ENCRUSTING BRYOZOAN ASSEMBLAGES - THE INFLUENCE OF ENCOUNTER ANGLE, SITE AND YEAR

This field study examined competition for space among encrusting bryozoans on artificial panel substrata used to simulate the habitat found on the undersides of boulders. In all 1761 interactions were analysed, involving 16 species of cheilostomes (genera Callopora, Celleporella, Cribrilina, Electra, Escharella, Escharoides, Haplopoma, Membraniporella, Microporella, Phaeostachys, Schizomavella, Schizoporella and Umbonula) and two ctenostome genera (Alcyonidium and Flustrellidra). Neither absolutely transitive nor intransitive arrangements of competitive outcome occurred among the species. Instead, the high incidence of 'ties' and reversals of outcome produced a network-like arrangement. In terms of outcome alone Escharoides coccinea (Abildgaard), Membraniporella nitida (Johnston) and Schizoporella unicornis (Johnston in Wood) were apparently 'overgrowth dominants', Alcyonidium spp. and Cribrilina cryptooecium Norman were 'intermediate dominants' and Callopora lineata (L.), Callopora craticula (Alder), Celleporella hyalina (L.) and Electra pilosa (L.) were 'inferior competitors' for space. The effects of four variables on the outcome of specific competitive encounters were assessed in detail and included (i) the identity of the colony encountered, (ii) the angle of contact between the competing colonies, (iii) the location (two sites, on both the east and west coasts of Scotland) and (iv) the year (1983, 1984). Models incorporating these variables were developed, using the Generalized Linear Interactive Modelling System (GLIM), to examine their influence on the outcome of bryozoan encounters. All of these factors contributed to explaining the considerable variability in the outcome of competitive interactions. The species were not, however, affected equally by all the variables, although encounter angle and year were significant for dominants and encounter angle, site and year all were significant for intermediate species.