The influence of pattern and scale on rocky-shore macrofaunal communities along the Mediterranean/Atlantic interface through the Straits of Gibraltar

The rocky-shore macrofauna at a total of twenty sites along the lower Iberian peninsula were studied using continuous belt-transect sampling methods. A number of sites were also sampled within the smaller Bay of Gibraltar. Overall, species richness and number of individuals per site decreased into the Mediterranean, probably due to decreased tidal amplitude and wave impact, although there were localized drops in richness in the area of the Bay of Cádiz. This was ascribed to high pollution levels in the area. In contrast, index diversity was constant in the Atlantic but fluctuated dramatically in the Mediterranean, where equitability appears to have a more influential role than species richness. Bray-Curtis cluster and multi-dimensional scaling analyses were used to identify site subgroups based on species compositions. The possible influence of a range of abiotic parameters on overall and within-site distributions was investigated at various levels of spatial aggregation using a variety of univariate and multivariate techniques. In all cases, parameters associated with available energy, habitat complexity and exposure were consistently selected, except in cases where localized throughputs had an overriding influence, e.g. the Bay of Gibraltar. Range size was also found to be closely related to abundance, although as for previous analyses, the largest abundances and distributions were found at intermediate, rather than at the smallest, size classes. Variance in range size was also found to decrease from species-rich to species-poor areas with the largest ranges occurring in the latter. This finding was explored in the light of ‘Rapoport’s rule’ (Stevens, 1989), an inverse relationship between species richness and range size that has been proposed as a possible mechanism to explain the latitudinal diversity gradient.