Indirect effects of pesticides

Posted on March 26, 2010

Most studies of the effects of contaminants on aquatic organisms have been performed in highly controlled conditions with small numbers of species.  This approach provides information on the toxic effects of chemicals on individual species, but it doesn’t reveal their indirect, often complex impacts on the food web as a whole.  Researchers inPittsburgh,U.S.A.seeded a series of large tanks with organic matter and pond water, and added different concentrations of malathion, a widely used pesticide that is toxic to aquatic animals.  They sampled plant plankton, periphyton (small attached algae) and 27 animal species over a three week period.  There was a dramatic reduction in animal plankton, especially water fleas, and the loss of these herbivores led to a strong bloom of plant plankton.  This in turn increased the amount of shade, which reduced the biomass of periphyton and the growth rates of grazers (frog tadpoles).  The tadpoles were also affected by an interaction between the effects of malathion and predators (especially  dragonflies): while the pesticide didn’t affect predator survival it did seem to affect their ability to capture prey.  The threatened status of frogs and other aquatic species  means that understanding the indirect impacts of chemical contamination has immediate relevance for conservation.

Reference:  Relyea, R.A. & Hoverman, J.T.  2008.  Interactive effects of predators and a pesticide on aquatic communities.  Oikos 117, 1647-1658.