Responses of invertebrate natural enemies to complex-structured habitats: a meta-analytical synthesis.

The structural complexity of habitats has been espoused as an important factor influencing natural-enemy abundance and food-web dynamics in invertebrate-based communities, but a rigorous synthesis of published studies has not heretofore been conducted. We performed a meta-analytical synthesis of the density response of natural enemies (invertebrate predators and parasitoids) to experimental increases and decreases in the structural complexity of their habitats using data from 43 published studies, reporting 62 independent taxa. Studies varied in structural complexity at two spatial scales (habitat and within-plant architecture) and comprised a diverse array of natural-enemy taxa (natural-enemy assemblage at large, the entire spider assemblage, hunting spiders, web-building spiders, mites, hemipterans, coccinellid beetles, carabid beetles, ants, and parasitoids). For all taxa combined, increasing habitat structure resulted in a large and significant increase in natural enemy abundance. Similarly, decreasing habitat structure significantly diminished natural enemy abundance. Separate meta-analyses at two spatial scales (habitat and within-plant architecture) found that increasing habitat complexity resulted in significant increases in abundance. In particular, manipulating levels of detritus at the habitat spatial scale had the strongest effect on natural enemy abundance. In general, most guilds of natural enemies were significantly affected when the structural complexity of the habitat was altered. Seven of nine natural enemy guilds were more abundant under conditions of increased habitat complexity, with hunting spiders and web-building spiders showing the strongest response followed by hemipterans, mites, and parasitoids. Spiders in particular were negatively affected when habitat structure was simplified. The mechanisms underlying the accumulation of natural enemies in complex-structured habitats are poorly known. However, refuge from intraguild predation, more effective prey capture, and access to alternative resources (alternative prey, pollen, or nectar), are possible candidates. Our analysis was unable to confirm that predators aggregate in complex-structured habitats because prey (mostly herbivores) are more abundant there. The results of this meta-analysis support the view that basal resources mediate top-down impacts on herbivores, and provide encouragement that manipulations of habitat complexity can be made in agroecosystems that will enhance the effectiveness of the natural enemy complex for more effective pest suppression.