Introduction

All terrestrial ecosystems are constituted of a belowground and an aboveground subsystem. These subsystems depend on each other, since above the ground primary producers are the main source of organic carbon for the system, whereas below the ground soil organisms are responsible for the breakdown and recycling of organic matter and the mineralization of the nutrients therein (Scheu and Setälä 2002, Wardle 2002, Porazinska et al. 2003). To understand community and ecosystem-level processes it is necessary to study the interactions within and between these subsystems. However, most ecologists have investigated belowground and aboveground communities separately, leaving the “between subsystems” interactions unstudied (but see Wardle 1999, 2002, Bonkowski et al. 2001, Masters et al. 2001, van der Putten et al. 2001, Brown and Gange 2002). Moreover, studies of different types of plant–animal interactions, such as pollination, herbivory, and seed dispersal, have traditionally progressed separately, focusing on just one kind of interaction and ignoring the possible interference with others (Herrera et al. 2002). This leads to an oversimplification of our understanding of plant–animal interactions, as most plants interact simultaneously with a broad spectrum of animals. Aboveground communities are affected by both direct and indirect effects of soil organisms on plants. Soil biota exert direct effects on plants by feeding on roots and forming antagonistic or mutualistic relationships with their host plants (Wardle et al. 2004). Such direct interactions with plants influence not only the performance of the host plants themselves, but also that of the herbivores and their predators.