Risk assessment of episodic exposures to chemicals should consider both the physiological and the ecological sensitivities of species.

In flowing water pollution regularly occurs in short pulses (hours to days). Populations of species affected by pulses have the potential to recover in the absence of further disturbance but recovery rates will vary between species due to resilience (e.g. generation time and dispersal ability) and avoidance traits. Current assessments of the risks of chemicals to community structure--predominantly based on species sensitivity distributions (SSDs)--only consider physiological sensitivity and do not give any consideration as to the rate at which populations will recover. We constructed SSDs of ecologically sensitive and tolerant stream invertebrate assemblages (based on 3 traits previously shown to be important in determining how species relative abundances respond to pesticide toxicity) from south-east Australia and in regions of Finland, Germany and France. There were differences in SSDs of a generic measure of physiological sensitivity to organic chemicals between ecologically sensitive and tolerant species, though these differences were not consistent between the regions studied. We conclude that it is important for community level risk assessments of pulses of chemicals that the ecological sensitivity of the regional species assemblage is considered and discuss several options as to how this could be achieved.

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