Abstract
The prediction of pest-control functioning by multi-predator communities is hindered by the non-additive nature of species functioning. Such non-additivity, commonly termed an emergent multi-predator effect, is known to be affected by elements of the ecological context, such as the structure and composition of vegetation, in addition to the traits of the predators themselves. Here we report mesocosm experiments designed to test the influence of plant density and species composition (wheat monoculture or wheat and faba bean polyculture) on the emergence of multi-predator effects between Adalia bipunctata and Chrysoperla carnea, in their suppression of populations of the aphid Metopolophium dirhodum. The mesocosm experiments were followed by a series of behavioural observations designed to identify how interactions among predators are modified by plant species composition and whether these effects are consistent with the observed influence of plant species composition on aphid population suppression. Although plant density was shown to have no influence on the multi-predator effect on aphid population growth, plant composition had a marked effect. In wheat monoculture, Adalia and Chrysoperla mixed treatments caused greater suppression of M. dirhodum populations than expected. However this positive emergent effect was reversed to a negative multi-predator effect in wheat and faba bean polyculture. The behavioural observations revealed that although dominant individuals did not respond to the presence of faba bean plants, the behaviour of sub-dominants was affected markedly, consistent with their foraging for extra-floral nectar produced by the faba bean. This interaction between plant composition and predator community composition on the foraging behaviour of sub-dominants is thought to underlie the observed effect of plant composition on the multi-predator effect. Thus, the emergence of multi-predator effects is shown to be strongly influenced by plant species composition, mediated, in this case, by the provision of extra-floral nectar by one of the plant species.
Highlights
In recent years, there has been considerable interest in the relationship between community attributes and the provision of ecosystem services, such as pest population suppression
Numerous studies have focussed on the role of natural enemy species richness or composition in determining prey population suppression, predicting the impact of natural enemy community change on ecosystem functioning is often hindered by emergent multi-predator effects; the functional impact of a species assemblage is not a straight-forward linear combination of the constituent species impacts [1]
There was no significant difference in aphid population suppression between the high and low density wheat treatments
Summary
There has been considerable interest in the relationship between community attributes and the provision of ecosystem services, such as pest population suppression. As multi-predator communities are the norm in real ecosystems, understanding the ecological mechanisms underlying multi-predator effects is fundamental to our understanding of prey population regulation. This understanding is required both for the effective management of biological control [2], and for the identification of conflicts between biological control and conservation of biodiversity in agricultural systems [3,4]. Predator species can facilitate each other by inducing behavioural changes in the prey or in the predators themselves [6,7], resulting in positive emergent effects on predation rate (prey risk enhancement). Interactions between species can result in negative multi-predator effects (prey risk reduction) if, for example, interference between heterospecific individuals reduces predation rate. Intraguild predation, which has been shown to be extremely widespread in natural communities [8], may result in reduced prey suppression [9]
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