Effects of a Toxicant on Population Growth Rates: Sublethal and Delayed Responses in Blowfly Populations

1. Previous studies have shown that cadmium exposure of blowfly populations (Lucilia sericata [Meigen 1826]) results in reduced population growth rate, but also in higher individual mass, because of reduced competition for food. In this study, the discrepancy between the positive effect on individual growth and the negative effect on population growth is investigated, by measuring direct and delayed effects of cadmium in the adult stage. 2. Blowfly populations were exposed to cadmium through the diet in four treatment combinations: larval stage, adult stage, both stages or neither stage. The effects on accumulation of cadmium, survival, development time, mass and reproductive rate were measured. 3. Cadmium was accumulated from both stages. 4. Individuals exposed to cadmium in the larval stage had higher mean pupal and adult mass (because of reduced densities), but also reduced adult longevity and fecundity. 5. Adult longevity and fecundity were also reduced by cadmium exposure in the adult stage. 6. In stage-structured populations, the link between individual-level and population-level responses to a toxicant may be complicated by stage-specific sensitivities to the toxicant, by delayed responses in the adult stage to sublethal effects in the juvenile stage, and by density-dependent compensatory responses to toxicant-induced mortality.