Indirect interaction webs: An introduction

The modification of plant traits by species utilizing a plant can be of crucial importance in determining the ecological and evolutionary interactions among community members centered on the plant. The authors in this volume review studies that integrate direct and indirect effects of plant-mediated traits on the ecology and evolution of plant-herbivore and other plant-mediated interactions. They synthesize these data to provide a more complete understanding of plant-based ecological community structure, which will provide a new perspective on the organization of these communities. Also, they suggest directions for future study. The richness of plant-trait-mediated effects results in the production of far more linkages among species than food web interactions alone. This richness also strengthens the argument that insect species on plants constitute real communities rather than noninteracting assemblages of individual species. Herbivore modification of plant traits is very common and widespread in terrestrial plants, and this initiates indirect interactions between species utilizing the same host plant. These interactions can be mediated by herbivore-induced changes in plant defensive chemicals, nutritional status, and the subsequent growth of plants attacked. Modification of plants in ways other than the simple removal of tissue can have complex impacts on other herbivores, and produce effects that cascade upward to higher trophic levels including predators and parasitoids. Thus, the structure and biodiversity of plant-based terrestrial communities can be strongly influenced by interaction linkages initiated by herbivory. Indirect effects occur when the impacts of one species on another are influenced by one or more intermediate species.