Effect of Copper on the interaction between the PredatorDidinium nasutumand its PreyParamecium caudatum

Abstract

Tests to determine acute copper toxicity levels demonstrated that the protozoan predator Didinium nasutum were more susceptible to copper stress than its prey Paramecium caudatum Thus we predicted that Paramecium and Didinium densities, at the equilibrium point of their interaction, would be higher at sublethal copper levels due to a decrease in the predator's efficiency. This situation is likely to produce a decrease in the stability of the system. However, isocline analysis did not support the predictions based on the acute lethality tests. Equilibrium densities of both predator and prey did not change at copper levels between 30 and 180 μg/L. The mathematical analysis suggested that the interaction became less stable with increasing copper concentrations. However, stability decreased due to hormesis in Didinium at sublethal copper levels and not due to a reduction in predator efficiency as expected. At 300 μg Cu/L, densities of both species at equilibrium were higher and the stability of the system decreased. This decrease in stability resulted from a reduction in predator efficiency, as 300 μg Cu/L is not sublethal for Didinium.