Body size and spatial variation of community structure in subarctic intertidal boulder fields

Abstract

The influence of body size of organisms examined on the spatial variation in the structure of mussel communities (evaluated as Shannon-Wiener diversity, taxonomic richness, and density) in boulder fields was evaluated for 4 spatial scales (<1 m, 10 m, 10 km, and 50 km). A number of factors (orientation of mussel communities on boulders, water flux and intertidal height) that potentially influence community structure at the smallest scale examined were also investigated. Overall, although body size usually influenced the significance of the different spatial scales examined in parametric analyses, the proportion of the total variance explained by each spatial scale was relatively constant for each size class and index of community structure calculated. Community structure was typically most variable at the smallest spatial scale examined, with progressively larger spatial scales explaining less and less of the total variation among samples. The multivariate ANOSIM tests, which did not examine variation at the smallest spatial scale, always found significant differences among communities at the 10 m and 50 km scales but, depending on the body size examined, did not always detect significant differences among communities at the 10 km scale. The mean Bray-Curtis similarity among samples decreased with decreasing body size and with increasing spatial scale. As the absolute difference in mean similarities among the different spatial scales examined decreased with increasing body size, we suggest that the ability to distinguish groups using ANOSIM should be greatest for the smallest macro-organisms studied. Variation at the smallest scale was not due to the compass orientation of the communities on boulders. Small-scale variation in water flux and intertidal height had a variable influence on the indices of community structure, and they were also a function of body size. Potential mechanisms responsible for the observed spatial variation are discussed.