Density-dependent interactions give indirectly positive effects of a toxicant

Summary

Effects of cadmium acetate were studied in laboratory blowfly populations, to test whether this toxicant could give indirectly positive effects on demographic rates by interacting with density-dependent processes. A previous study has demonstrated that cadmium exposure of blowfly populations results in higher mean pupal weight and biomass production. Moreover, statistical modelling (generalised additive modelling) of demographic rates has indicated that cadmium-exposed populations have higher larva-to-adult survival. The aim of the present study was to test this model prediction, and to describe underlying density-dependent mechanisms. Blowflies populations in both high and low densities were exposed to cadmium through their diet; in low densities, all demographic rates were reduced, whereas in high densities, survival of old larvae and pupae was increased by the cadmium exposure. The increased survival was found to be due to reduced larval competition, which resulted from lower reproduction and lower survival of young larvae. Hence, the indirectly positive effects of the toxicant were caused by compensatory density-dependent reactions.