SURFACE TOPOGRAPHY INFLUENCES COMPETITIVE HIERARCHIES ON MARINE HARD SUBSTRATA: A FIELD EXPERIMENT

¹ Department of Biology, University of South Carolina, Columbia, South Carolina 29208

Among colonial encrusting organisms, competitive relations often are intransitive, with individuals of subordinate species occasionally winning encounters with higher ranking species. Many mechanisms, including allelopathy, growth form, and disturbance have been proposed to explain this phenomenon. We show experimentally that such reversals of rank can occur purely as a result of surface topography. The bryozoan Alcyonidium was superior to the bryozoan Electra when encounters occurred on level substrata (8 wins, 0 losses). When Electra was given a 1.6 mm height advantage in the zone of contact, it won in 15 out of 16 encounters with Alcyonidium. This mechanism may explain the higher incidence of intransitivity among epifauna on the topographically variable Fucus serratus in Wales, compared to the competitive hierarchy seen on the topographically more uniform Fucus distichus ssp. evanescens and Chondrus crispus in New England.