Demographic responses of a tropical cladoceran to cadmium: Effects of food supply and density

Life table response experiments (LTRE) were per formed to investigate effects of food supply on density-dependent effect,, on life history and population responses of the tropical cladoceran Moinodaphnia macleayi to cadmium. Its short life cycle and relatively constant age-specific birth and mortality rate, allowed population growth rates (X) to be accurately estimated from short life table experiments and by using a simplified two-stage demographic model. Decomposition and regression analyses of X showed that density-dependent effects on Population responses to cadmium were modified by food availability through density-dependent effects on mortality and reproductive rate. At moderate food levels (1.8 mug C/mL of Chorella vulgaris) and low densities (<125 animals/L), the effects of cadmium on lambda decreased with increasing population density, due to antagonistic effects between the contributions of age at first reproduction and daily reproduction on X. At high densities (> 125 animals[L), the negative effects or cadmium on daily reproduction rates increased with density hence density promoted the negative effects of cadmium on X. At low, food levels (0.4 mug C/mL of Chorella vulgaris), increasing population density reduced juvenile survival, increasing food per head. Thus adverse effects of cadmium on reproduction, which had the greatest contribution to lambda, were buffered by increasing population density. Regression analysis performed on population responses across increasing population density levels and cadmium concentrations showed that at high densities and low food levels ecological compensation will prevent populations at the steady-state equilibrium size from being driven to extinction by toxicity effects at the individual level. Alternatively, at low densities, when food availability is not limiting, exposure to toxic substances can increase extinction probability. These results indicate that risk assessment procedures based on demographic analysis performed at low densities and high food levels may Overestimate the ecological risk posed by toxic substances.