Temporal-spatial stability of competition in marine boulder fields

Demonstrable examples of marine interference-competition on ecological, and particularly evolutionary, time-scales have been highly confined in space. However, studies of such competition across large-scale space are conversely mere 'snapshots' in time. The current study aimed at measuring interference-competition at multiple scales in space and time. Different aspects of interference-competition were measured in a model community (encrusters of boulder communities, e.g. ascidians, bryozoans, polychaetes and sponges) at 3 tropical, 3 temperate and 2 polar sites at intervals of days, weeks, months and years. The 3 aspects of competition measured-transitivity (hierarchicalness), number of clades involved, prevalence of interspecific encounters-varied non-significantly with time (from days to years) but significantly in space (from the tropics to the poles). Thus the strong differences observed in space are robust along ecological time-scales; boulder communities may be highly dynamic at local scales, but overall measures of interference-competition amongst their encrusters seem to vary little within a region. Why low- and high-latitude encrusting communities should differ may be linked to past selection pressures-to survive spatial competition in the more stable warm seas and to be able to recolonise from local ice-scour and regional ice sheets in the high temperate and polar realms. The results of the current study suggest that, despite being 'snapshots', the many point-in-time studies of competition in the literature are likely to be valid on ecological time-scales and thus useful for meta-analyses.