Herbivores can influence plant community structure and the genetic composition of plant populations, which can in turn drive ecosystem-level effects. For example, herbivores that select for plant genotypes or species with increased concentrations of defensive compounds will indirectly alter the chemical composition of plant-derived nutrients entering the soil. This change in plant-derived soil nutrients is expected to select for soil microbes that are able to utilize inputs distinctive to the particular plant species or genotype, potentially altering the functional properties of the soil ecosystem.
Here we investigated the effects of herbivory on ecosystem-level processes of soil. We used plots from a long-term field experiment that were either exposed or protected from ambient insect herbivory. This manipulation caused rapid evolution in O. biennis populations and altered plant communities. Our research questions were: 1) What are the relative contributions of evolutionary and ecological processes to the ecosystem-level effects caused by herbivory? 2) Can these ecosystem-level effects feedback to alter plant performance? In each plot we quantified leaf decomposition rates, net N mineralization rates, and soil microbial activity under additions of various substrates. We measured performance of O. biennis genotypes grown in soil from all plots to assess the possibility of a feedback.
Results/Conclusions
Overall, we found evidence that divergent herbivore regimes led to differences in ecosystem-level processes. Leaf decomposition rates in the lab and in the field were dependent on evening primrose genotype and herbivore regime, where certain genotypes decomposed faster in either exposed or protected plots. Net N mineralization rates were greater in exposed plots. Seeds germinated faster on soil from exposed plots and accumulated more biomass than those sown in soil from protected plots. Soil microbial communities from exposed and protected plots differed in their ability to utilize complex C substrates, and substrates derived from different O. biennis genotypes, which correlated with O. biennis genotype frequency in the plots. Our results indicate that herbivory, by way of altered plant communities and evolution within plant populations, can result in ecosystem-level change.