Competitor- and herbivore-mediated indirect effects of elevated CO2 on plant evolution

Global changes, such as elevated CO₂ (eCO₂), have strong ecological effects on communities.  Because of these strong ecological impacts, selective regimes may be altered, and populations also may evolve in response to these changing environmental conditions.  Previous work demonstrated that eCO₂ only slightly influenced the evolution of an experimental Arabidopsis thaliana population and that patterns of natural selection, the expression of genetic variation, and the genetic covariances between traits corresponded closely in ambient and eCO₂ treatments.  Here we examined potential evolutionary effects of eCO₂ in more complex environments.  First, we investigated how eCO₂ alters the fitness effects of competition and herbivory.  Then, we explored how the expression of genetic variation and/or patterns of natural selection depend on herbivory or the presence and identity of competitors in both ambient and eCO₂ treatments.  Our hypothesis was that while eCO₂ may have minimal direct impacts on plant evolution, it may have strong indirect effects by altering interactions with competitors and herbivores.  We found that eCO₂ affected components of plant evolution when additional interacting species were present and that these effects likely resulted because eCO₂ altered the intensity of the interaction.  The magnitude of this indirect eCO₂ effect, however, was small relative to the direct effects of competition or herbivory.  Our results, therefore, suggest that while eCO₂ will have large effects on plant ecology, impacts on the evolutionary trajectories of plant populations will be weak and primarily mediated by biotic interactions.