Genetic diversity in plant populations can enhance plant fitness by altering abundance and impact of herbivores, yet the mechanisms driving these patterns and their evolutionary implications are unclear.  Diversity effects can arise from two non-exclusive processes.  First, diverse communities may be dominated by members with particular traits that confer success, i.e., selection or sampling effects.  In contrast, complementarity effects arise when community members perform better than expected due to positive interactions among diverse community members.  In the parlance of plant-herbivore interactions, selection effects are analogous to herbivore feeding selectivity, whereas complementarity effects are analogous to associational effects.  To test the mechanisms generating genetic diversity effects, we conducted a quantitative genetics field experiment manipulating genotypic diversity of the native plant Oenothera biennis and the presence of its major vertebrate herbivores, white-tailed deer Odocoileus virginianus and meadow voles Microtus pennsylvanicus.  To examine which traits were correlated with plant lifetime fitness and resistance to herbivores, we also measured a suite of 14 plant traits related to growth and defense.  Results/Conclusions

There were strong effects of genotypic diversity on plant fitness.  Plants grown in genotypically diverse patches were more likely to survive vole attack, and survivors from diverse patches produced 45% to 83% more fruits than survivors from genetically homogenous patches in the presence and absence of deer, respectively.  Voles selectively killed early bolting genotypes, thus some of the positive effects of diversity could be attributed to herbivore selectivity among susceptible genotypes.  However, plant genetic diversity moderated vole attack rates, and early bolters performed better than expected in diverse patches, indicating that the associational effects of a plant’s genetic neighborhood also influenced its fitness.  Genetic selection analyses also revealed that vertebrate herbivores imposed directional selection for several plant traits, but these effects differed by genetic neighborhood.  This study documents natural selection for plant traits by vertebrate herbivores and indicates that a prey’s genetic neighborhood can influence the strength and direction of selection by its major consumers.