Single best species or natural enemy assemblages? a correlational approach to investigating ecosystem function

Though biodiversity can have an effect on biological control of pests, there is debate about whether a single species or a more complex assemblage of natural enemies will exert better control of the pest population. We explore the relationship between numbers of different taxa of natural enemies in an olive grove to identify cases of significant positive and negative correlations between enemy taxa. Integrating herbivore data we identified enemy taxa and assemblages that were associated with low numbers of olive pests. Overall, single species such as Anthocoris nemoralis, or relatively simple predator assemblages, such as that formed by the spider families Araneidae and Liniphiidae, and the green lacewing Chrysoperla carnea, were associated with better biological control than complex assemblages, where intraguild predation and other trophic interactions might hamper the effectiveness of enemies. For a Lepidopteran pest with a complex life cycle, the single best predator taxon was markedly poorer at suppression than the most effective assemblage. In contrast, a Hemipteran pest with a simple life cycle was controlled nearly as well by the single best predator taxon as by the most effective assemblage. Statistical approaches offer good scope to identify optimal aspects of biodiversity to maximise ecosystem services such as biological control.