Overgrowth competition in encrusting bryozoan assemblages of the intertidal and infralittoral zones of Alaska

Almost 2000 cheilostomatid-cheilostomatid (bryozoan) interactions were reco rded from 110 rocks from intertidal and infralittoral zone locations at Kod iac island in the Alaskan Boreal-Arctic. Intraspecific interactions were un usually rare; this may have resulted from inhibition of con-specific settle ment by adult colonies. In one species, rank did vary as a function of dept h. The proportion of determinate-species encounter-pairings (a pairing in w hich one competitor/species wins all encounters) and tied outcomes (whereby neither competitor/species wins or loses but are involved in a "standoff" or mutual overgrowth) increased from the upper midlittoral to the lower inf ralittoral. The assemblage of bryozoans formed a clear hierarchy, with a sc ore of 0.83 on Tanaka and Nandakumar's transitivity index. Overgrowth domin ants did not monopolise the assemblage. The most abundant species were mid- or lower-ranked competitors. The identity of the competitive species had a major influence on the outcome of encounters. Zooidal height and colony gr owth morphology were important factors determining overall overgrowth rank. The greatest number of interaction types was recorded on medium-sized (sur face area > 300 and < 600 cm(2)) rocns, ana rne overall alverslry or Intert idal species was very high compared with studies conducted elsewhere. The h igh interaction and species diversity recorded may both have resulted from intermediate levels of disturbance.