Making the best of a bad situation: The effect of agriculture on the life history traits of amphibian populations

Background/Question/Methods Agriculture can have profound impacts on the environmental conditions experienced by populations (e.g., contaminants, nutrients, and light levels) and thus can be a potentially strong source of selection. Individuals may cope with such adverse environmental conditions by increasing their tolerance or decreasing their exposure time. For example, several studies have documented that pesticides can cause the evolution of resistance in agricultural pests with concomitant fitness costs. However, responses to pesticides and other agricultural-induced changes to the environment by non-target species have received little attention. Our goal was to assess whether amphibian populations living in areas with greater amounts of surrounding agricultural land differed in their life history traits compared to populations living in areas with lesser amounts of agricultural land. We collected newly oviposited eggs from 10 populations of wood frogs (Rana sylvatica) across a land use gradient (ranging from 18.6 % - 61.3 % agriculture). We then used a common-garden mesocosm experiment to assess the life history traits of populations when raised in low-stress, predator-stress, and competition-stress environments.

Results/Conclusions Wood frog life history traits (growth rate, time to metamorphosis, and size at metamorphosis) exhibited significant effects of populations and stress environments, but not their interaction. Responses to the stress environments mirrored those previously reported in the literature (e.g., individuals in the competition treatment were smaller at metamorphosis). Interestingly, the population differences in time to and size at metamorphosis were correlated with land use. Populations with greater amounts of surrounding agriculture had shorter times to metamorphosis and produced smaller metamorphs. This may be because historically they have been exposed to stressful conditions (e.g., eutrophic conditions due to nutrient runoff and pesticides) that have favored shorter larval periods. The shorter larval periods resulted in smaller metamorphs and smaller metamorphs generally experience lower fitness including post-metamorphic survival and size at reproduction. Therefore, from a meta-population standpoint, individuals emerging from ponds close to agriculture may incur substantial fitness costs compared to individuals emerging from more pristine ponds. Future studies should address the fitness consequences of employing other mechanisms of coping with adverse effects of agriculture, including evolved tolerance to pesticides.