ORGANIC PESTICIDE MODIFICATION OF SPECIES INTERACTIONS IN ANNUAL PLANT-COMMUNITIES

A method is proposed and tested for assessing multispecies responses t o three pesticides (atrazine, 2,4-D and malathion). Pesticides were ap plied at two concentrations, on model plant communities grown in raise d beds using soil containing a natural seed bank. Cover by species was monitored over time in nested 10 and 20 cm diameter neighbourhoods ar ound Poa annua and Calandrinia ciliata target plants. All tested compo unds modified relative species abundance, altered dominance and simpli fied the treated communities. Community biomass decreased with atrazin e and 2,4-D treatments, but not with malathion. Each chemical altered species interactions for all treated communities, including the identi ties of interacting species and the timing of interactions. Each targe t species had its own suite of interacting species that individually c hanged with chemical treatment. When cover was used as a predictor of neighbour influence, analysis of species interactions using 10 cm neig hbourhoods indicated more interactions than using 20 cm neighbourhoods . When biomass was used as the predictor, use of the 20 cm neighbourho ods indicated more interactions. This method of using model plant comm unities for field toxicity testing is simple, economical and effective . It uses naturally occurring plants while reducing the environmental heterogeneity common in most field studies.