Introduction

Ecologists have traditionally recognized the consequences that direct interactions between species have on the functioning of ecological communities and on the flow of energy through food webs (Pimm 2002). However, ecological communities are among the most complex natural systems, and thus the interactions between species are far from simple. In this respect, the importance of indirect effects (those effects transmitted from one species to another through one or more intermediate species) as a determinant of the structure and dynamics of ecological communities has been clearly acknowledged only in recent years (Wootton 1994, Abrams 1995, Abrams et al. 1996). Indirect effects are those transmitted from one species to another through one or more intermediate species. These indirect effects can be propagated through food webs as a consequence of changes in the density of the intervening species, a mechanism known as density-mediated indirect interaction (DMII) (Abrams 1995, Werner and Peacor 2003). Nevertheless, indirect effects can also occur through changes in the phenotypes of the interacting organisms, a mechanism known as trait-mediated indirect interaction (TMII) (Werner and Peacor 2003, Schmitz et al. 2004). Although DMIIs have been traditionally considered to be the main source of any variation in ecological communities, ecologists are progressively more conscious of the essential role played by TMIIs (Bolker et al. 2003, Dill et al. 2003, Luttbeg et al. 2003, Trussell et al. 2003, Werner and Peacor 2003; and chapters throughout this volume).