Interactions among predators and the cascading effects of vertebrate insectivores on arthropod communities and plants

Abstract

Theory on trophic interactions predicts that predators increase plant biomass by feeding on herbivores, an indirect interaction called a trophic cascade. Theory also predicts that predators feeding on predators, or intraguild predation, will weaken trophic cascades. Although past syntheses have confirmed cascading effects of terrestrial arthropod predators, we lack a comprehensive analysis for vertebrate insectivores-which by virtue of their body size and feeding habits are often top predators in these systems-and of how intraguild predation mediates trophic cascade strength. We report here on a meta-analysis of 113 experiments documenting the effects of insectivorous birds, bats, or lizards on predaceous arthropods, herbivorous arthropods, and plants. Although vertebrate insectivores fed as intraguild predators, strongly reducing predaceous arthropods (38%), they nevertheless suppressed herbivores (39%), indirectly reduced plant damage (40%), and increased plant biomass (14%). Furthermore, effects of vertebrate insectivores on predatory and herbivorous arthropods were positively correlated. Effects were strongest on arthropods and plants in communities with abundant predaceous arthropods and strong intraguild predation, but weak in communities depauperate in arthropod predators and intraguild predation. The naturally occurring ratio of arthropod predators relative to herbivores varied tremendously among the studied communities, and the skew to predators increased with site primary productivity and in trees relative to shrubs. Although intraguild predation among arthropod predators has been shown to weaken herbivore suppression, we find this paradigm does not extend to vertebrate insectivores in these communities. Instead, vertebrate intraguild predation is associated with strengthened trophic cascades, and insectivores function as dominant predators in terrestrial plant-arthropod communities.bottom-up and top-down control | intraguild predation | meta-analysis | trophic cascade | vertebrate predator exclusion R esearch demonstrates that predators, by feeding on herbivores, can increase plant biomass via the indirect interaction commonly labeled a trophic cascade (1). In recent years, metaanalyses have quantified trophic cascades separately in terrestrial (2, 3) and aquatic systems (4, 5) and in multiple habitats together to compare the strength of trophic cascades among ecosystem types (6). Although the strengths of trophic cascades vary across ecosystem types, explanations for the significant residual variation within ecosystems remain enigmatic (6-8).Vertebrate insectivores such as birds, bats, and lizards often feed as top predators on terrestrial arthropod communities, but based upon current theory it is unclear whether their effects should cascade down to affect plant biomass. Because of their large body size relative to arthropod prey, vertebrate insectivores can consume both predatory and herbivorous arthropods (9, 10). As a consequence, vertebrate insectivores may feed as so-called intraguild predators ...

Natural enemy interactions constrain pest control in complex agricultural landscapes

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