Print PageHerbivores are a potent agent of natural selection and are credited with driving the evolution and diversification of plant defensive traits. To date, however, no manipulative experiment has tested if the presence or absence of herbivores causes evolution in defensive traits. We tested this hypothesis using a field experiment where 22 local plant populations of the herbaceous plant Rumex acetosa were protected from natural rabbit herbivory for <1 to 26 years. With R. acetosa seeds collected in the field and grown in a common environment, we compared each population’s individual growth rate, tolerance to herbivory, competitive ability, and the concentration of two secondary metabolites.
Results/Conclusions
In 26 years without rabbit herbivory, plant growth rate decreased by 30%. Surprisingly, we found no evolution in tolerance to herbivory, competitive ability, or concentrations of secondary metabolites (tannins and oxalate). Increased growth rate likely facilitates persistence under intense and frequent grazing and thus serves as form of defense against rabbits. However, decreased growth rate is presumably adaptive in the environmental condition created in absence of herbivores leading to the observed evolution of decreased growth following removal of rabbits. This suggests that rabbits can influence the evolution of plants directly through consumption and indirectly by altering the environment. This study shows that herbivores can impose selection and drive rapid evolution in growth rate without causing significant changes in conventional defensive traits.
