Effects of a toxicant on population growth rates: sublethal and delayed responses in blowfly populations

Summary

This study demonstrates that effects of chemical stress on functional endpoints such as population growth rates may not be readily predictable from the known effects at the individual level. According to a previous study, cadmium exposure of laboratory blowflies results in increased individual growth (due to reduced larval competition); nevertheless, the population growth rate is decreased. We have investigated this discrepancy between the positive effect on individual growth and the negative effect on population growth, by analysing direct and delayed effects of cadmium in the adult stage. Blowfly populations (Lucilia sericata) were exposed to cadmium through the diet in four treatment combinations: larval stage, adult stage, both stages, or neither stage. We measured the effects on accumulation of cadmium, survival, development time, weight and reproductive rate. The flies accumulated cadmium from both life-history stages. Individuals exposed to cadmium in the larval stage obtained higher mean pupal and adult weight (due to reduced densities), but also reduced adult longevity and fecundity. Adult longevity and fecundity were also reduced by cadmium exposure in the adult stage. In stage-structured populations, hence, 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.