Sublethal effects and predator-prey interactions: implications for ecological risk assessment

Ecological risk assessments tend to focus on contaminant effects on single species in isolation. However, additional effects from interactions between species (e.g. predator-prey interactions) may also occur in natural systems. This study investigated the consequences of sublethal contaminant effects in prey on predator-prey interactions, particularly the interaction between prey behavioural changes and predation by predators with different hunting strategies. Ambush (Ischnura elegans Vander Linden (Insecta, Odonata)) and active (Notonecta glauca Linnaeus (Insecta, Heteroptera)) predator species were used in conjunction with three prey species (Asellus aquaticus Linnaeus (Crustacea, Isopoda), Cloëon dipterum Linnaeus (Insecta, Ephemeroptera), and Chironomus riparius Meigen (Insecta, Diptera)). Immobilised prey demonstrated the importance of prey behaviour for determining predation rates for both single and multiple prey species. Chironomus riparius was less responsive following exposure to cadmium, becoming more vulnerable to attack by the active but not the ambush predator. There was also some evidence for reduced general activity in C. dipterum following cadmium exposure. Sublethal exposure of prey did not affect the prey choice of active predators, possibly due to prey behavioural changes being insufficient to influence their relative availabilities. However, cadmium exposure of prey did alter their susceptibility to ambush predators. There was a reduction in the proportion of C. dipterum and an increased proportion of A. aquaticus in the diet of ambush predators, possibly due to reduced activity in C. dipterum affecting their relative encounter rates with predators. Sublethal exposures can therefore result in reduced prey survival that would not be predicted by single species toxicity tests.