How important are rare species in aquatic community ecology and bioassessment?

We examined the effects of excluding rare species on the comparison of spec ies richness. A river benthic data set and a randomization resampling proce dure were used to show the importance of this consideration in aquatic bioa ssessment in particular. The data set was manipulated by deleting species a t three levels of rarity as defined by occurrence frequency: once in all 24 replicates from each of three sites, no more than twice, and no more than five times. We focused on differences in species richness because many othe r bioassessment metrics are dependent on species richness and species compo sition. Species abundance patterns at the three sites were very different, with many more rare species at the least impacted site than at the more imp acted sites. As sample size increased, the differences in species richness among the three sites markedly increased in the original data set. However, the exclusion of rare species at the same level of rarity substantially re duced species richness at the least impacted site but had little effect on the most impacted site. This result led to a serious underestimation of the differences in species richness among the sites in terms of both absolute numbers and species loss percentages. Deleting rare species can damage the sensitivity of community-based methods to detect ecological changes; rare s pecies are critical for bioassessment.