Short- and long-term effects of the pyrethroid insecticide fenvalerate on an invertebrate pond community

Direct and secondary effects of fenvalerate on the structure of pond ecosystems were studied in six freshwater mesocosms simulating natural fish-free eutrophic ponds. Exposed mesocosms were compared with nonexposed ones and the effects of the added compound on the macroinvertebrate community were followed during three vegetation seasons (years) in two mesocosms. Exposure to fenvalerate at 1.3 and 0.54 microg liter-1 resulted in structural changes in the macroinvertebrate community. The insecticide was directly lethal to insects and other arthropods, but indirect community changes were also observed. For example, after exposure there was a remarkable (> 10-fold) increase in oligochaetes (Stylaria lacustris), probably caused by reduced predation and interspecific competition for food. When predators (insects) recolonized the system, the oligochaetes decreased in abundance and were replaced by ostracods (Herpetocypris reptans), which use similar food resources but are less susceptible to predation. The marked increase in these two taxa is probably explained by the mass death of arthropods, resulting in increased food availability. More than 2 years after treatment, the most exposed system was still different compared with nonexposed ones, suggesting that nonpersistent pesticides may produce detrimental effects resulting in long-term changes at the ecosystem level of organization.