Variability of competition at scales of 101, 103, 105, and 106 m: encrusting arctic community patterns
Variability in interference competition was studied in benthic marine communities of the arctic and subarctic Atlantic intertidal and shallow subtidal zones. We sampled multiple square-metre quadrats at distances of 101, 103, and 105 m apart around the high polar island of Spitsbergen (Svalbard Archipelago). We also took some similar samples in Iceland and in the Faeroe Islands (106 m apart from Spitsbergen samples). Encrusting fauna were present on high arctic intertidal rocks but we only found competitive interactions on subtidal substrata. On subarctic Icelandic and Faeroese shores, in contrast, spatial competition was common even in the intertidal zone. Analysis of variance of competition intensity data (numbers of interactions per area) revealed multiple factors to be significant influences explaining variability. Amongst the 101-, 103-, and 105-m spatial scales, only the largest emerged as a significant term. Whether intra- or interspecific competition dominated the types of interactions varied greatly between sites: 21–97% of competition was intraspecific. The proportion of competitive encounters resulting in a decided outcome (i.e. a win for one competitor and a loss for the other, rather than a tie or standoff between them) showed little variability at any scale. All the values of competition transitivity (how hierarchical a pecking order is) were very high compared to values reported in the literature from any other (polar or non-polar) locality. Variability in this measure was generally <10% across scales. We conclude from our data that great care must be taken in interpreting patterns of competition between similar taxa in large-scale space or time. Not only did most aspects of competition in our study communities vary significantly at the 105-m scale but different aspects of competition varied at different scales and by hugely different amounts.