Scale-dependent variation in competitive ability among encrusting Arctic species

Finding evidence for climate change in the sea has been less easy than on land. As ice-loading of nearshore waters (through ice sheet collapses) seems a most likely signal of climate warming, we looked at communities likely to be affected by changing disturbance, i.e. those encrusting boulders in shallow water. The structures of such assemblages at high latitude are highly hierarchical in which if uninterrupted by disturbance (ice-scour) succession would lead to domination by just a single species. However, unless pecking orders from place to place involve the same specific species, with the same competitive rankings, monopolisation will remain only local. To see how pecking orders varied, we examined variability in competitive performance of common species in a high Arctic lithophillic assemblage at several spatial scales. These were sampled 101, 103 and 105 m apart on the west coast of Spitsbergen, and 106 m apart, achieved by sampling southern Iceland and the Faeroe islands. We found that higher taxonomic membership was the major factor determining overgrowth performance of species. Overgrowth performance of each study species changed relatively little between samples, the same good competitors being top performers in any samples where they occurred. Overgrowth performance of each study species was also most similar in samples at the smallest spatial scale. Apart from this, the performance of each study species did not, however, become more dissimilar with increasing distance between samples. Most noteworthy was that susceptibility to variation in overgrowth performance at the regional 105 m scale altered with competitive ranking, i.e. pioneer and dominant species lost and won nearly all encounters respectively, wherever they were. The performance of mid-ranking species was much more variable. These results suggest that whilst the patterns of succession may differ from place to place, the end results will not. Our study provides data to support the theory that appreciation of scale is crucial to understanding community structure, diversity and potential for response to climate change. If ice-loading (disturbance) in polar waters does decrease, we suggest from our findings that a very small number of encrusting species may monopolise large areas of the shallows. Before this, though, we predict that different mid-ranked species will become more common from site to site, with limited reduction of ice (disturbance) increasing regional diversity.

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