Trophic cascades alter natural selection on plant populations: Resistance and tolerance to herbivory on Mimulus guttatus

The effects of predators can be intense, driving strong direct and indirect interactions in ecological communities. In particular predators often reduce herbivore populations and benefit plants in a trophic cascade. The evolutionary consequences of these indirect interactions, however, have rarely been explored. In this study, we manipulated the presence of both herbivores and predators in a factorial field experiment and then measured selection on several traits of Mimulus guttatus. Using a genotypic selection design we compared selection on plant traits in treatments with either one, two, or three trophic levels present. We found that removal of herbivores reduced the strength of selection for both tolerance and resistance to herbivory and the observed patterns of selection indicate that resistance is costly. Conversely, predator removal increased the strength of selection for both tolerance and resistance, suggesting that in the absence of predators M. guttatus would evolve increased defense against herbivory. These results suggest that the strong indirect effects that predators often produce can also alter patterns of natural selection throughout ecological communities.