Introduction

Arthropod predators and parasitoids play an important role in reducing herbivore damage to plants. Although most of these arthropods are mainly carnivorous, they also use plant-provided food (PPF) as a source of nutrients during at least part of their life cycle. These foods affect longevity, fecundity, and the distribution of carnivores (Olson et al., Chapter 5 and Eubanks and Styrsky, Chapter 6), and thus also the population dynamics of herbivore–carnivore systems.

Despite the importance of this type of omnivory for herbivore–carnivore interactions in general and biological control in particular, its population-dynamical consequences are not fully understood. Relatively few population studies have addressed the topic (Bakker and Klein 1992; Alomar and Wiedemann 1996; Stapel et al. 1997; Eubanks and Denno 2000; Van Rijn et al. 2002; Wäckers 2003), and even fewer theoretical studies (Krivan and Sirot 1997; Van Baalen et al. 2001; Van Rijn et al. 2002; Kean et al. 2003). Omnivory in general, however, has gained much attention since Polis' seminal papers (Polis et al. 1989; Polis and Holt 1992), both among empirical (Diehl 1995; Holyoak and Sachdev 1998; Pringle and Hamazaki 1998; Gillespie and McGregor 2000; Coll and Guershon 2002) and theoretical ecologists (Pimm and Lawton 1978; Holt and Polis 1997; McCann and Hastings 1997; Polis 1998; Mylius et al. 2001). These studies have mainly focussed on the consequences of omnivory for population persistence and community stability, and much less on the consequences for herbivory.