Introduction

Herbivory by arthropods induces a wealth of changes in the primary and secondary chemistry of plants (Karban and Baldwin 1997, Constabel 1999, Agrawal et al. 1999, Kessler and Baldwin 2002). These chemical changes in turn do not only affect the inducer, but also other herbivore species attacking the induced plant (Denno et al. 1995, Denno and Kaplan Chapter 2 this volume). This effect of one herbivore species on other herbivores is called “indirect,” because it can only arise via the plant as an intermediate organism (Wootton 1994). Moreover, it is called trait-mediated, because the immediate effect of herbivory is an induced change in plant quality, not in plant quantity (Werner and Peacor 2003, Schmitz et al. 2004).

The herbivore-induced state of plants may influence the community of arthopods that live on them. When the induced plant allocates much of its energy in compensatory growth or defense specifically aimed at the inducer, other herbivore species may profit from the increased nutritional quality or weakened defense of the plant, thereby giving rise to interspecific aggregations of herbivores on individual plants (Denno et al. 1995). If, however, the induced plant mounts a sufficiently generalized defense, the plant becomes “vaccinated” against attack by other herbivores, leading to species-poor communities of herbivorous arthropods on the plant (Karban and Baldwin 1997). Much the same reasoning applies to herbivore genotypes within a single species.