Sammendrag
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.
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