  Field and mesocosm studies repeatedly show that higher tier process reduce
the predictive accuracy of toxicity evaluation and consequently their value for
pesticide risk assessment. Therefore, understanding the influence of ecological
complexity on toxicant effects is crucial to improve realism of aquatic risk
assessment. Here we investigate the influence of repeated exposure to
ecologically realistic concentrations of esfenvalerate on the similarly
sensitive species Daphnia magna and Culex pipiens in a food limited and highly
competitive environment. We show that significant perturbations in population
development are only present close to the EC50. In contrast, interspecific
competition between species is already reduced at concentrations 3-4 orders of
magnitude below the acute EC50. We conclude that extremely low, environmentally
relevant concentrations can disrupt species interactions. This toxicant
mediated alteration of competitive balances in ecological communities may be
the underlying mechanism for shifts in species distribution at ultra-low
pesticide concentrations. A realistic risk assessment should therefore consider
these processes in order to predict potential pesticide effects on the
structure of communities.
