High polar spatial competition: extreme hierarchies at extreme latitude
On polar shores, as at lower latitude, intense battles for space ensue on boulders but the high wind, wave and ice disturbance make the colonisation race a short one in time. Here we test multiple hypotheses on the nature of competition at a high polar latitude (77degrees N, Arctic Spitsbergen): that interference competitive encounters would principally be (1) between colonial animals and (2) intraspecific in nature involving very few species; (3) intraspecific interactions would mostly result in ties and (4) interspecific interactions would conversely result in overgrowth, involve mainly 1 poor competitor species interacting with many others; and that (5) competition structure would be highly hierarchical. This is the highest latitude of any competition study and its structure was as extreme as hypothesised in many respects. Interference competition on boulders was dominated by encounters between colonial animals (almost entirely between cheilostome bryozoans). Nearly 80 % of all competitor interactions were intraspecific. Most intraspecific encounters involved just a single (bryozoan) species, Harmeria scutulata. In no other equivalent assemblage is there such an extreme balance of intra- to interspecific competition. Nearly 97 % of intraspecific fights for space ended in tied outcomes (standoffs). We can also find no described assemblage in which tied outcomes are so frequent in intraspecific encounters or so rare in interspecific meetings. Of the competitors present in more than 10 interactions H. scutulata was the poorest, winning just 24 % of encounters with other species. No other study, known to us, has shown an assemblage to be so dominated by a single species, which is such a poor spatial competitor. Compared to other similar assemblages the diversity (Shannon-Wiener H') and evenness (Pielou's J) of competitive encounters was lowest. This Spitsbergen assemblage is also the most severe hierarchy measured in a marine encrusting community. These findings have major implications for the response of the community to disturbance and climate change.