COMPARING DIFFERENCES IN SPECIES SENSITIVITY TO TOXICANTS - PHENOTYPIC PLASTICITY VERSUS CONCENTRATION-RESPONSE RELATIONSHIPS

The comparison of species sensitivity to toxicants is classically deri ved from differences in the concentration-response relationships of a sensitive trait such as reproduction. We tested this general concept b y conceiving the concentration-response relationship as a plastic resp onse to a range of discrete environments. Using a demographic model ba sed on life-cycle experiments for two nematode species (Plectus acumin atus and Heterocephalobus pauciannulatus) we related copper-induced pl asticity in reproduction to changes in fitness, which was defined as t he population growth rate. Daily reproduction (n(t)) in P. acuminatus was more sensitive to copper (EC20 = 48 mu M) than in H. pauciannulatu s (EC20 = 138 mu M). However, the relationship between plasticity in n (t) and fitness showed that for both species, fitness was reduced with 5%. These findings imply that P. acuminatus and H. pauciannulatus are equally susceptible to toxicant-induced changes in n(t) despite their differences in EC20 values for reproduction. It may be concluded that differences in susceptibility of species to toxicants are not only de termined by the effect on sensitive traits but also by the relationshi p between plasticity in this trait and fitness.