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

TLDR

Species richness is a key metric for community and regional diversity, yet its estimation is often distorted by abundance and sampling effort. The paper surveys common pitfalls in measuring and comparing taxon richness. The authors recommend using accumulation and rarefaction curves—based on individuals or samples—to standardize and compare richness, noting that area or effort standardization can yield different results. Asymptotic estimators give lower‑bound richness for highly diverse groups, and recent studies confirm the necessity of careful richness quantification using sampling curves.

Abstract

Species richness is a fundamental measurement of community and regional diversity, and it underlies many ecological models and conservation strategies. In spite of its importance, ecologists have not always appreciated the effects 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 rarefaction curves, which may be based on either individuals or samples. These taxon sampling curves contain the basic information for valid richness comparisons, including category–subcategory ratios (species‐to‐genus and species‐to‐individual ratios). Rarefaction methods – both sample‐based and individual‐based – allow for meaningful standardization and comparison of datasets. Standardizing data sets by area or sampling effort may produce very different results compared to standardizing by number of individuals collected, and it is not always clear which measure of diversity is more appropriate. 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 diversity studies of tropical trees, stream invertebrates, and herbaceous plants emphasize the importance of carefully quantifying species richness using taxon sampling curves.