Host-plant genotypic diversity mediates the distribution of an ecosystem engineer

Ecosystem engineers affect ecological communities by physically modifying the environment. Understanding the factors determining the distribution of engineers offers a powerful predictive tool for community ecology. In this study, we examine whether the goldenrod bunch gall midge (Rhopalomyia solidaginis) functions as an ecosystem engineer in an old-field ecosystem by altering the composition of arthropod species associated with a dominant host plant, Solidago altissima. We also examine the suite of factors that could affect the distribution and abundance of this ecosystem engineer. The presence of bunch galls increased species richness and altered the structure of associated arthropod communities. The best predictors of gall abundance were host-plant genotype and plot-level genotypic diversity. We found positive, nonadditive effects of genotypic diversity on gall abundance. Our results indicate that incorporating a genetic component in studies of ecosystem engineers can help predict their distribution and abundance, and ultimately their effects on biodiversity.     

Insects mediate the effects of propagule supply and resource availability on a plant invasion

Invasive species are a global threat to biodiversity and the functioning of natural ecosystems. Here, we report on a two-year experiment aimed at elucidating the combined and relative effects of three key controls on plant invasions: propagule supply, soil nitrogen (N) availability, and herbivory by native insects. We focus on the exotic species Lespedeza cuneata, a Rank 1 invasive species. Propagule supply and soil N-availability interacted to control the density and foliar cover of L. cuneata. In low N plots, density and foliar cover of L. cuneata were higher in the propagule addition plots than in the plots to which propagules were not added. Surprisingly, this interaction was significant only when the abundance of herbivores was experimentally reduced. This experiment provides evidence that native insect herbivores mediate the interactive effects of propagule supply and resources on invasion by a widespread invasive plant species.