In this paper we consider the relationships between effects of toxicants on population growth rate (r) and the individual-level traits (survival and reproduction) contributing to it by examining population dynamics theory and by reviewing the experimental work that has been carried out in this context. There was no consistent pattern in terms of which individual-level traits were most or least sensitive to toxicant exposure, and it is therefore impractical to select traits for ecotoxicological testing on this basis. Although percent changes in several of the individual-level traits showed significant overall correlations with percent changes in r, changes in any single trait could explain no more than about one-half of the percent change in r. Our conclusion is that r is a better measure of responses to toxicants than are individual-level effects, because it integrates potentially complex interactions among life-history traits and provides a more relevant measure of ecological impact.
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