In conservation biological control (CBC), we attempt to reduce pest problems by increasing the abundance and diversity of the natural enemy community. However, rather than consistently strengthening herbivore suppression, studies show that the conservation of natural enemy species richness sometimes weakens, or has no affect, on biological control. Evidence is mounting that this idiosyncratic mix of positive, negative, and neutral effects of enemy diversity is caused by niche complementarity, intraguild predation, and functional redundancy, respectively. While the balance of evidence suggests that the conservation of natural enemy diversity and biological control are compatible goals, CBC practitioners cannot ignore the fact that conserving intraguild predators can sometimes disrupt biological control. Recent studies have made important progress toward identifying the traits of enemies and their prey that promote intraguild predation, functional redundancy, and niche complementarity. However, intraguild predation has received more attention than niche complementarity, and more theoretical and empirical work is needed rectify this asymmetry. We suggest that a continued focus on natural enemy functional traits, particularly those that are expressed at larger spatiotemporal scales, will increase our ability to identify the "right" kind of diversity and may ultimately improve the practice of conservation biological control. © 2007 Elsevier Inc. All rights reserved.
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