Biologically generated habitat provision and diversity of rocky shore organisms at a hierarchy of spatial scales

The diversity and abundance of intertidal organisms found on moderately exposed shores were compared between biologically generated complex and less complex habitats at four different spatial scales. For macrobiota, comparisons were made between Fucus clumps and open areas (barnacle covered or bare rock) and then between barnacle covered rock and 'bare' rock. Microbiota were compared on barnacle plates and on the rock surface and at a finer resolution on the plates themselves and pits in the plates. Macrobiota were quantified at two levels of resolution by direct observation and under low power magnification. Microbiota were examined at two levels of resolution with a scanning electron microscope. For macrobiota the average number of species was significantly greater in complex habitats at both levels of resolution. The average number of individuals associated with the Fucus clumps was significantly lower than that on adjacent areas of barnacle covered and bare rock. However, the average number of individuals associated with the barnacle matrix was significantly greater than that on surrounding rock. The differences were entirely caused by a greater abundance of barnacles and species associated with the barnacle matrix on exposed rock outside the Fucus clumps. For microbiota there were large differences in species composition; cyanobacteria were predominant on the rock surface whilst diatoms dominated the barnacle plates. The diversity of this assemblage was low and no differences in species numbers were apparent between the two habitats. At a finer scale, abundance of individuals was much greater in the pits than on flat areas of the surrounding plates and there was a slightly elevated average number of species per sampling unit. This paper illustrates the importance of using a variety of resolutions to measure biodiversity and the importance of biological habitat provision in maintaining biodiversity.