Trophic cascades, ecological interactions in which predators suppress herbivores and indirectly benefit plants, can dramatically alter the abundance and composition of species in a community. However, these dramatic changes suggest that trophic cascades may also affect the evolution of community members. In previous research, we have examined how trophic cascades driven by wolf spiders on the common monkeyflower (Mimulus guttatus) affect natural selection on traits of this plant species. We found that removing predators can change the strength and direction on of natural selection on plant traits involved in defense against herbivory. Here we extend these results to explore how variation in the strength of trophic cascades across multiple sites over four years affects observed patterns of natural selection. We removed wolf spider predators and arthropod herbivores in a full factorial experiment replicated across sites and years. Biomass and seed production of Mimulus guttatus were measured in each treatment replicate, allowing us to measure the magnitude of the predator effect (i.e. the strength of the trophic cascade). We also measured selection on both resistance and tolerance to herbivory in each treatment replicate using genotypic selection analysis. Thus we can determine both the ecological and evolutionary effects of predators on Mimulus guttatus and compare the strength of these effects.

Results/Conclusions

We found significant variation in the strength of trophic cascades across both sites and years. The strength of natural selection on plant traits also displayed substantial spatial and temporal variation. Further, the variation in cascade strength appears to drive variation in the pattern of selection on resistance and tolerance to herbivory. When trophic cascades are strongest, predators also have large selective effects on plant traits. However, even weak cascades can produce significant natural selection. These results suggest that variation in communities that affects the strength of indirect ecological interactions may have important consequences for the evolution of community members.