Introduction

Community dynamics are determined by demographic processes such as reproduction and predation. These demographic processes are influenced in turn by individual traits. While earlier community models assumed that individual traits were static (e.g., Rosenzweig and MacArthur 1963), in the past decade community studies have begun to focus on flexible traits (see Abrams 2010, for review). For example, the Holling type II functional response model describes the predation rate as aR/(1 + ahR) where a is the attack rate, h is the handling time, and R is the prey (resource) density. The original derivation of the model assumed that predators’ search behaviour and handling behaviour were static (Holling 1959), and thus could be represented by the static parameters. However, because real individuals’ behaviours are flexible, their traits may vary with changing ecological conditions. For example, the antipredator behaviour of a consumer may interfere with its searching behaviour, thus reducing its foraging (attack) rate a as it responds to increasing predator density (Lima and Dill 1990; Persons and Rypstra 2001; Werner and Peacor 2003; Caro 2005). Similarly, handling time may also change with resource density (Cook and Cockrell 1978; Giller 1980; Samu 1993). It is now well accepted that considering flexible traits in ecological studies is both justified by the results of many experimental studies (Werner and Peacor 2003) and important for understanding community dynamics (Fryxell and Lundberg 1998; Bolker et al. 2003).

Flexible traits generate a variety of indirect interactions. (This is not their only important effect: they can also modify the dynamics of simple communities consisting of only two species without indirect interactions, e.g., Křivan (2007).) Indirect interactions that are driven by changes in the traits of intermediate species are called trait-mediated indirect interactions (TMIIs), while those that are instead driven by changes in the density of the intermediate species are called density-mediated indirect interactions (DMIIs) (Abrams 1995; Abrams et al. 1996). DMIIs were traditionally considered to drive such classic examples of indirect interactions as trophic cascades and keystone predation, but the importance of TMIIs in these and other large-scale phenomena is increasingly acknowledged (Schmitz et al. 2004). Despite their importance, however, the specific roles of each indirect interaction are relatively poorly understood.