Abundance variability of benthic intertidal species: effects of changing scale on patterns perception

For the last years scale issues have contributed to the understanding of the complex organization of communities. In practice however the choice of observational scales remains one of the major issues of discussion among ecologists. Case studies showing the implications of tuning up the observational scales are still needed to illustrate the crucial role played by sampling designs on the perception of population and community variability. On the ground of simulations on real data sets, this paper presents empirical evidence showing how the abundance patterns of benthic intertidal species change according to the scale (grain size and extent) of observation. A field experimental study was carried out in which abundance of rocky shore intertidal benthic species and physical factors associated to the substratum were recorded in contiguous quadrats (5x10 cm) along three 100-m transect lines. Subsampling routines were developed to generate different scale scenarios by monotonically increasing the grain size and extent of the data. Results showed that: 1) changes in the observational grain size and extent convey different information about the spatial patterning of the recorded factors, and 2) the strength of the relationship between species abundance and physical factors tends to decrease as extent increase. Under the continuous simulations of scale scenarios along transects, variance estimation was the criterion adopted to make explicit the degree of precision in patterns description obtained by each scale scenario. The experimental variogram demonstrated to be a useful statistic for estimating the variance associated with discrete levels along the continuous varying function of scale.