Quantifying biodiversity: procedures and pitfalls in the measurement and comparison of species richness

Species richness is a fundamental measurement of community and regional div ersity, and it underlies many ecological models and conservation strategies . In spite of its importance, ecologists have not always appreciated the ef fects of abundance and sampling effort on richness measures and comparisons . We survey a series of common pitfalls in quantifying and comparing taxon richness. These pitfalls can be largely avoided by using accumulation and r arefaction curves, which may be based on either individuals or samples. The se taxon sampling curves contain the basic information for valid richness c omparisons, including category-subcategory ratios (species-to-genus and spe cies-to-individual ratios). Rarefaction methods - both sample-based and ind ividual-based - allow for meaningful standardization and comparison of data sets. Standardizing data sets by area or sampling effort may produce very d ifferent results compared to standardizing by number of individuals collect ed, and it is not always clear which measure of diversity is more appropria te. Asymptotic richness estimators provide lower-bound estimates for taxon- rich groups such as tropical arthropods, in which observed richness rarely reaches an asymptote, despite intensive sampling. Recent examples of divers ity studies of tropical trees, stream invertebrates, and herbaceous plants emphasize the importance of carefully quantifying species richness using ta xon sampling curves.