Environmental stressors can enhance the development of community tolerance to a toxicant.

Ecosystems are subject to a combination of recurring anthropogenic and natural disturbances, such as climate change and pesticide exposure. Biological communities are known to develop tolerance to recurring disturbances due to successive changes at both the community and organismal levels. However, information on how additional stressors may affect the development of such community tolerance is scarce to date. We studied the influence of hydrological disturbance on the reaction of zooplankton communities to repeated insecticide pulses in outdoor microcosms. The communities were exposed to three successive pulses of the insecticide esfenvalerate (0.03, 0.3, and 3 µg/L) and to the gradual removal of water and its subsequent replacement over three cycles or to a constant water level. Except at the highest esfenvalerate concentration, the communities developed tolerance to the toxicant, as indicated by their decreasing reaction to subsequent insecticide applications, and this development was enhanced by hydrological disturbance. The pronounced decline of the key taxa Daphnia spp. through the combined action of the two stressors was identified as the main mechanism responsible for the increase in community tolerance under a fluctuating water level. Under a constant water level, the abundance of Daphnia spp. did not decrease significantly without the insecticide treatment, indicating that other mechanisms were responsible for the observed community tolerance. The present study shows that additional stressors can facilitate the development of community tolerance and that such facilitation is propagated through community-level mechanisms.