Anthropogenic chemicals, such as pesticides, are widespread contaminants of natural ecosystems. However, natural populations can evolve increased tolerance to pesticides through selection for constitutive tolerance or plasticity (i.e. inducible tolerance). While both mechanisms confer pesticide tolerance, theory predicts they may be associated with differential costs. These costs may subsequently lead to direct effects on fitness (i.e. growth or fecundity), yet no studies have compared the costs of these different mechanisms of pesticide tolerance. Therefore, using larval wood frogs (Lithobates sylvaticus) and Daphnia pulex from populations that exhibit either constitutive or inducible tolerance to the common insecticide carbaryl, we investigated whether the mechanisms for achieving tolerance are associated with differential costs. We reared populations of wood frogs (2 constitutive, 2 inducible) and Daphnia (2 constitutive, 1 inducible) on a fixed diet in either a sublethal concentration of carbaryl or a pesticide-free control. To assess costs of tolerance, we measured wood frog mass after 30 days and total offspring produced by Daphnia over 7 days as metrics of fitness. We characterized costs of tolerance as a reduction in growth (for wood frogs) or reproduction (for Daphnia) in sublethal pesticide environments compared to pesticide-free treatments.
Results/Conclusions:
In the absence of pesticides, tadpoles from constitutive populations were on average 28% smaller than tadpoles from inducible populations. However, exposure to sublethal concentrations of pesticides resulted in differential costs for tadpoles with constitutive vs. inducible tolerance. When exposed to sublethal concentrations of pesticides, tadpoles from the two populations with inducible tolerance had reduced growth compared to pesticide-free controls (by 27% and 30% respectively), while tadpoles from constitutive populations had similar growth in the pesticide and control treatments. Similar to the patterns observed in tadpoles, Daphnia from constitutive populations had, on average, 95% fewer offspring than Daphnia from inducible populations in the absence of pesticides. Further, Daphnia populations with inducible tolerance had 34% fewer offspring when reared in sublethal carbaryl concentrations compared to those in controls, while Daphnia populations with constitutive tolerance showed no difference in number of offspring produced by parents reared in pesticide vs. control. While both inducible and constitutive tolerance may allow populations to persist when faced with pesticide contamination, this study demonstrates that the different mechanisms for achieving tolerance are associated with different costs and that these costs may be generalizable across species.