Estimating macrozoobenthic species richness along an environmental gradient: Sample size matters

Along monotonous environmental gradients (such as increasing temperatures, salinities, heights, sediment coarseness, etc), numbers of species per unit area can change in consistent ways, either in a monotonous or a more complex way (e.g. showing a peak somewhere along the gradient). We studied patterns of species density in bivalves in a Wadden Sea tidal flat area from a data set obtained by long-term monitoring at numerous sampling stations at various distances (0-10 km) from the shore line. At short distances from the shores, tidal flats are generally high and sheltered from strong winds and tidal currents and sediments are muddy. With increasing distance from the shore, both intertidal level as well as shelter tends to decline, resulting in declining trends in mud content and increasing trends in median grain size of the sediments. Species numbers in samples of 0.1 and 1 m(2) were found to decline monotonously with increasing distance from the shore. However, in large aggregated samples (38 m(2)), maximal species numbers (approaching real species richness) were not found close to the shore, but had shifted into an off-shore direction. As a consequence, differential multiplication factors had to be used to obtain an asymptotic estimate of total species richness from actually observed figures of species numbers in small samples that were taken in different areas: for samples of 0.1 m(2) (1 m(2)) these factors amounted to similar to 3 (similar to 2) in near-shore areas and to similar to 6 (similar to 2.5) in off-shore areas. Species accumulation curves were differentially shaped in near-shore and off-shore areas and intersected, allowing a reliable estimate of assemblage species richness in an area by extrapolation only when large aggregated samples (of well over 1 m(2)) were available. The base of these differences was a differential distribution of abundant and rare species. The few species with numerical densities frequently exceeding 100 individuals m(-2) occurred particularly in near-shore areas, whereas the more numerous low-density species (