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 e nvironmental gradients are common in ecology. Few studies have examined whe ther these patterns are consistent at a hierarchy of spatial scales, despit e increasing evidence indicating that spatial variability is not scale inde pendent. In this study I have investigated the general model that much of t he variation on rocky sea shores occurs across the vertical gradient, unles s major physical attributes of the habitat change. This model was contraste d 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 ch aracteristic scale at which vertical variation is larger than horizontal va riability. In order to distinguish among these alternatives, I compared var iability in abundance of organisms across heights on the shore on rocky coa sts in the north-west Mediterranean, with estimates of horizontal variabili ty 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 k m apart). Results indicated that vertical and horizontal variability were c omparable in magnitude at the smallest spatial scale, while horizontal vari ability 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 multiva riate analyses also revealed that much of the horizontal variation was amon g quadrats 10s of cm apart. These results indicated that any description of spatial pattern in abundance for organisms living on rocky sea shores in t he NW Mediterranean must contemplate both vertical and horizontal sources o f variation, regardless of the scale investigated. It is concluded that fai lure to recognise the importance of sources of variation other that those e xpressed by the most obvious gradients detracts from potentially important causal processes hindering progress in ecological understanding.