The effects of induced defense-related phenotypic changes in plants on community dynamics are difficult to predict, because many aspects are involved and the variability of plant responses is enormous. While many bi- and tritrophic interactions are well studied, plants in nature are usually under the attack of a range of organisms at the same time. How this affects plant defense has only just begun to be addressed, and first results show that the effects may be an increase as well as a decrease in defense intensity (e.g., Dicke et al. 2003; Rodriguez-Saona et al. 2005; Cardoza and Tumlinson 2006).
Although most studies thus far have focused on abovegroundprocesses, the influence of the changes in plant phenotype is not limited to the aboveground community. Aboveground interactions can change belowground root exudates and influence the soil community, and belowground damage can influence aboveground indirect defense (Bezemer and van Dam 2005). The reverse, however, i.e. the effect of above-ground interactions on belowground indirect defense, remains as yet uninvestigated (Bezemer and van Dam 2005). Incorporating these interactions in future studies will greatly enhance our insight into the effects of induced indirect defense on the functioning of complex communities. In addition to plant-pathogen and plant-herbivore interactions, plants may also be under the attack of parasitic plants (Bouwmeester et al. 2003; Runyon et al. 2006), or interact with belowground symbiotic organisms such as mycorrhiza or symbiotic bacteria (Gange et al. 2002). Furthermore, above-ground endophytic organisms can influence the plant's defensive phenotype and consequently also the interactions with community members (Omacini et al. 2001). Incorporating these interactions in the investigations of indirect defense of plants in a community ecology approach will increase complexity, yet doing so is essential to gain a meaningful understanding of the effects of indirect plant defense on plant ecology.
Another area of research that has not received a lot of attention so far is the searching behavior of members of the higher trophic levels, such as hyperpara-sitoids. How they find their host and whether they use plant cues remains largely unknown (Buitenhuis et al. 2005). The same applies for pollinators. Though some effects of herbivory on pollination have been reported (Lehtila and Strauss 1997; Poveda et al. 2003), the underlying mechanisms remain to be unraveled and which signal-transduction pathways are important in this respect, waits to be investigated.
For a complete understanding of the ecology and evolution of communities, it is necessary to include all trophic levels in field studies. Manipulative studies are likely to provide the best way forward. They can be used in the laboratory to investigate individual interactions and are a valuable tool to investigate the effects of induced defenses on the community in the field (Kessler and Baldwin 2001; Kessler et al. 2004). Using an integrated approach with molecular, chemical, and behavioral methodology will significantly advance the research in this area (Baldwin et al. 2001).
Acknowledgments We thank Joop van Loon for valuable comments on the manuscript. This work has been financially supported by a VICI grant from the Netherlands Organisation for Scientific Research, NWO (865.03.002).
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