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 nonsignificantly 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.

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