The use of surrogate species is a tool commonly used to predict the effects of toxicants on endangered/threatened or economically important species. While use of surrogate species has been critized as being overly simplistic, a quantitative measure linking life history traits and population predictions has been sorely missing. We derive here a closed-form expression aimed at determining conditions under which sublethal effects of a toxicant on surrogate species population outcomes will reliably predict responses of species of concern. We derive a simple inequality that allows us to compare critical thresholds in fecundity reduction across species and thereby pinpoint the level below which surrogate species outcomes indicate a positive population growth, while the listed species actually is driven to extinction. We thus establish a means of determining conditions under which we might be prone to making a “type II” error in assessing ecological risk using surrogate species. Finally, we use the derived expression to illustrate two cases studies – one in which we are using several fish species as surrogates for endangered salmonids, and the second in which we are comparing the compatibility of a suite of parasitoid wasps with pesticide use. In both cases we highlight potential pitfalls associated with the use of a “one-size-fits-all” approach to protection of species. We discuss the ramifications of these findings on risk assessment and resource management.