Overgrowth competition between clades: Implications for interpretation of the fossil record and overgrowth indices

Overgrowth interactions (2693 in total) were observed among three major gro ups (arguably clades) of bryozoans-cheilostomatids (57 species), ctenostoma tids (3 species), and cyclostomatids (14 species). The bryozoans studied he re occur in shallow water at high-temperate polar latitudes where they encr ust hard substrata such as rock piles. The main study site was the intertid al and infralittoral zones of Kodiak Island, Alaska, but observations were also made in similar zones of South Georgia Island and the Falkland Islands in the South Atlantic Ocean. Cheilostomatids dominated the number of speci es, individuals, and interactions at all depths. Intraclade interactions fo rmed 73.7% of the encounters for cheilostomatids, 1.6% for ctenostomatids, and 5.7% for cyclostomatids. The competitive ranking of the three clades wa s broadly ctenostomatids > cyclostomatids > cheilostomatids. Significantly, these results contradict all previous quantitative studies of bryozoan ove rgrowth, in which cheilostomatids are reported to overgrow cyclostomatids a t a higher rate. From these studies and the literature, we calculated win i ndices to vary from 0 to 0.42 for living cyclostomatids, from 0.08 to 0.9 f or living cheilostomatids, and from 0.25 to 0.75 for living ctenostomatids. The win indices of cyclostomatid and cheilostomatid clades show significan tly more variation in living assemblages than in fossil assemblages. This d isparity may be due to differential preservation (polar and subpolar assemb lages last less than 4 years). The diversity was very high in terms of both species richness and interaction types (outcomes between competitor pairs) . Comparison with the literature suggests the possibility that nearshore di versity of bryozoans may be bimodal (have two peaks) between high arctic an d antarctic latitudes. Indices of success in overgrowth competition have be en constructed in various ways. For cheilostomatids, the method of calculat ion had little influence on the ranking of representatives. In contrast, th e apparent success of ctenostomatids and cyclostomatids varied hugely with how the index was calculated. This inconsistency is due to the use of very different strategies in overgrowth competition; among the two latter groups , many interactions involve tied outcomes.