Stress is here defined as an environmental condition that reduces Darwinian fitrirss whcn first applied. Optimal stress responses (i.e. those that maximize Darwinian fitness) are ralrulated for different levels of growth and mortality stress, and are found to depend critically on the shape of the trade-off curve relating mortality to growth rate. If the trade-off does not rhange shape whcn stress is applied, then the optimal strategy is to spend less on personal defence for both mortality and growth stresses. However, if stress does change the shape of the trade-off thr predirtions may be modified, or reversed. This optimality analysis is rigorous and easy to apply. What is more difirult, is to establish the shapes and positions of trade-off curves in particular cases. This problem is discussed and some suggestions are made. The theory's predictions are applied speculatively t o hiogeographical data on marine animals and are found to he qualitatively surressful, although some of the needed data are lacking. The applications and testability of the theory in the study of ageing and a variety of other processes are considered.

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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.