Species Diversity Hinders Adaptation to Toxicants

Environmental toxicants such as pesticides
exert strong selection
pressure on many species. While the resulting development of pesticide
resistance in agricultural pest species is well-known, reports on
the extent of adaptation in exposed nontarget species are contradictory.
These contradictory reports highlight a continuing challenge in understanding
the relevant ecological mechanisms that facilitate or hinder adaptation
to toxicants in the field. Here we show that species diversity hinders
the adaptation to toxicants. In agricultural streams with low diversity,
we observed an up to 8-fold increase in insecticide tolerance in a
total of 17 macroinvertebrate species that was not observed in more
diverse communities under similar contamination. High species diversity
occurred independently from adjacent nonpolluted refuge areas. Therefore,
the low level of adaptation in diverse streams cannot be explained
by an increased recolonization of sensitive individuals from refuge
areas. Instead, high intraspecific competition may facilitate the
selection for increased tolerance in low-diverse communities. In diverse
communities, by contrast, species interactions may reduce intraspecific
competition and, thus, the potential for developing toxicant resistance.
We suggest that this mechanism may be the general case in adaptation
to environmental stressors. Additionally, we conclude that the current
framework for risk assessment of pesticides is not protective against
selection for tolerant organisms and the associated risk of genetic
erosion.