Applying species-sensitivity distributions in ecological risk assessment: Assumptions of distribution type and sufficient numbers of species

Species-sensitivity distribution methods assemble single-species toxicity d ata to predict hazardous concentrations (HCps) affecting a certain percenta ge (p) of species in a community. The fit of the lognormal model and requir ed number of individual species values were evaluated with 30 published dat a sets. The increasingly common assumption that a lognormal model best fits these data was not supported. Fifteen data sets failed a formal test of co nformity to a lognormal distribution; other distributions often provided be tter fit to the data than the lognormal distribution. An alternate bootstra p method provided accurate estimates of HCp without the assumption of a spe cific distribution. Approximate sample sizes producing HC5 estimates with m inimal variance ranged from 15 to 55, and had a median of 30 species-sensit ivity values. These sample sizes are higher than those suggested in recent regulatory documents. A bootstrap method is recommended that predicts with 95% confidence the concentration affecting 5% or fewer species.