Numbers and density of species as measures of biodiversity on rocky shores along the coast of New South Wales

Numbers of species at mid-shore levels on rocky shores were sampled across 415 km to test hypotheses about patterns along the coast of New South Wales, Australia. In two different years, sampling in winter revealed increased numbers of species sampled over shores with increasing distance from north to south. There was no such trend during summer. The latitudinal increase in species was due to sessile fauna and to a lesser extent, to mobile fauna. Encrusting and foliose algae did not contribute to the pattern. The seasonal difference was mostly due to changing numbers of species between seasons. The numbers of species per sample-unit (i.e. species-density) always increased with distance from north to south in both seasons and both years when grain-size of sampling was quadrats (scattered < 1 m apart) or sites on the shore (20 – 30 m apart). Species-density was unreliable as an estimate of diversity along the coast, because it revealed spurious trends in summer when there was no increased number of species from north to south. Analyses of densities, dispersions, frequencies of occurrences and multivariate dissimilarities of the organisms did not explain why species' densities showed a trend along the coast. Comparisons of diversity where species are not censused, but must be sampled, are made difficult by the dispersions of individual taxa across sample-units.