My Ph.D. thesis tested the hypothesis that intraspecific genetic variation and evolution in a native plant (Oenothera biennis, Onagraceae) is a major factor shaping the structure of a diverse arthropod community. In a series of field experiments, I showed that genotypic variation among plants is often as important as environmental variation (Johnson and Agrawal 2005 Ecology), competition among herbivores (McGuire and Johnson 2006 Ecol Entomol), predation, and mutualisms (Johnson In Press Ecology), in shaping the structure of multitrophic communities. I also found that increased genetic diversity within plant populations led to a greater diversity and abundance of arthropods and higher plant fitness (Johnson et al. 2006 Ecol Lett). These community-level effects of plant genetic variation were accompanied by natural selection on plant traits (Johnson 2007 J Evol Biol), which are predicted to cause an ecological change in the structure of arthropod communities over time (Johnson and Stinchcombe 2007 Trends Ecol Evol). As a whole, these experiments show that intraspecific genetic variation and evolution within a native plant can be one of the most important ecological factors affecting a diverse consumer community.

The results of my research suggest that a dynamic interplay exists between the community ecology and evolution of O. biennis and its arthropod fauna. I am now extending on this work by testing the predictions that herbivores influence the ecological dynamics and drive rapid evolution of plant defenses within populations of O. biennis, and this evolution feeds back to shape the diversity, composition, and abundance of arthropods on plants.

In collaboration with researchers from several institutions (Anurag Agrawal, Cornell; John Maron, Montana; J-P Salminen (Turku), we are currently testing these predictions in a multigenerational selection experiment in the field. We first used common gardens to measure genetic variation in plant resistance traits that genetically correlated with herbivory. At the same time, we developed microsattelite markers for the clonal plant genotypes used in the common garden experiment (Larson et al. In Press Mol Ecol Notes), which unambiguously allows us to identify individual clonal genotypes because O. biennis is functionally asexual. We have used these preliminary data to create multiple experimental populations in the field that exhibit a range in plant defense traits, yet are all identical in genotypic composition. Using insecticide applications we are now continually removing herbivores from half of the populations and following the long-term demography and evolution of these plant populations. As the experiment progresses we will also determine whether the structure of the arthropod communities change as a function of evolution within O. biennis populations.