Pairwise versus diffuse natural selection and the multiple herbivores of scarlet gilia, Ipomopsis aggregata

Recent theoretical studies have argued that plant-herbivore coevolution pro ceeds in a diffuse rather than a pairwise manner in multispecies interactio ns when at least one of two conditions are met: (1) genetic correlations ex ist between plant resistances to different herbivore species; and (2) ecolo gical interactions between herbivores sharing a host plant cause nonadditiv e impacts of herbivory on plant fitness. We present results from manipulati ve field experiments investigating the single and interactive fitness effec ts of three types of herbivory on scarlet gilia (Ipomopsis aggregata) over two years of study. We utilize these data to test whether selection imposed by herbivore attack on date of first flowering is pairwise (independent) o r diffuse (dependent) in nature. Our results reveal complex patterns of the fitness effect of herbivores. Simulated early season browsing had a strong negative fitness effect on plants and also reduced subsequent insect attac k. Surprisingly, this ecological interaction did not translate into signifi cant interactions between clipping and insect manipulations on plant fitnes s. However, we detected a significant interaction between seed fly and cate rpillar herbivory on plant fitness, with the negative effect of either inse ct being greatest when occurring alone. These results suggest that herbivor e-imposed selection may have pairwise and diffuse components. In our select ion analysis of flowering phenology, we discovered significant pairwise lin ear selection imposed by clipping, diffuse linear selection imposed by inse cts, and diffuse nonlinear selection imposed by clipping and insect attack acting simultaneously. Our results reveal that the evolution of flowering p henology in scarlet gilia may be in response to diffuse and pairwise natura l selection imposed by multiple herbivores. We discuss the evolution of res istance characters in light of diffuse versus pairwise forms of linear and nonlinear selection and stress the complexity of selection imposed by suite s of interacting species.