Human modifications of natural ecosystems can expose organisms to novel stressors. For instance, the use of anthropogenic chemicals to control disease vectors, improve crop yields, and eliminate pests has led to the ubiquitous contamination of ecosystems and, in many cases, the evolution of increased tolerance. The evolution of increased pesticide tolerance has historically been attributed to natural selection on constitutive traits over many generations. Recently, however, researchers have discovered that pesticide tolerance can be induced in a single generation through phenotypic plasticity. Moreover, populations living far from agriculture possess inducible tolerance whereas populations living close to agriculture possess a high constitutive tolerance in a pattern that suggests genetic assimilation. We asked, “Why does the ancestral state (i.e., inducible pesticide tolerance) exist in populations with no history of pesticide use?” We hypothesized that the inducible pesticide tolerance represents a generalized stressor response shared among natural and anthropogenic stressors. To test this hypothesis, we exposed larval wood frogs (Lithobates sylvaticus) to seven sublethal stressor treatments (0.5 or 1.0 mg/L of the insecticide carbaryl; 200 or 1000 mg Cl-/L of road salt; predator cue; and two controls). We subsequently exposed tadpoles to no insecticide or lethal carbaryl treatment (18 mg/L) using time-to-death assays.
Results/Conclusions
We discovered increased pesticide tolerance when wood frog tadpoles had been previously exposed to either 0.5 mg/L carbaryl or predator cues, but not when previously exposed to the other treatments. Moreover, tadpoles exposed to 0.5 mg/L carbaryl induced a higher pesticide tolerance than those exposed to predator cues. In short, we discovered that the evolution of pesticide-induced tolerance may have its origins in the evolutionary responses to predators. Given the widespread modification of natural ecosystems caused by anthropogenic activities, it is vital to understand stressor-specific responses of organisms and if pre-existing responses to natural stressors can be co-opted to ameliorate negative effects of novel stressors.