Variability in abundance of algae and invertebrates at different spatial scales on rocky sea shores

Analyses of spatial pattern in populations and assemblages along distinct environmental gradients are common in ecology. Few studies have examined whether these patterns are consistent at a hierarchy of spatial scales, despite increasing evidence indicating that spatial variability is not scale independent. In this study I have investigated the general model that much of the variation on rocky sea shores occurs across the vertical gradient, unless major physical attributes of the habitat change. This model was contrasted with 2 alternative models that incorporate patchiness at different scales: either vertical variation is larger than horizontal variability at small but not at large scales of horizontal spatial variability or there is no characteristic scale at which vertical variation is larger than horizontal variability. In order to distinguish among these alternatives, I compared variability in abundance of organisms across heights on the shore on rocky coasts in the north-west Mediterranean, with estimates of horizontal variability obtained at different scales, ranging from the scale of the patch (among quadrats 10s of cm apart) up to the regional scale (among shores 100s of km apart). Results indicated that vertical and horizontal variability were comparable in magnitude at the smallest spatial scale, while horizontal variability was generally larger when measured at scales of 100s to 1000s of m. When multivariate patterns in the structure of assemblages were examined, there was more vertical than along-shore variation at small (10s to 100s of cm) but not at large (> 1000s of m) spatial scales. Univariate and multivariate analyses also revealed that much of the horizontal variation was among quadrats 10s of cm apart. These results indicated that any description of spatial pattern in abundance for organisms living on rocky sea shores in the NW Mediterranean must contemplate both vertical and horizontal sources of variation, regardless of the scale investigated. It is concluded that failure to recognise the importance of sources of variation other that those expressed by the most obvious gradients detracts from potentially important causal processes hindering progress in ecological understanding.