The enemy of my enemy is not always my friend: Negative effects of carnivorous arthropods on plants

Plants are members of complex communities of which arthropods are the most speciose members. The role of carnivores in shaping the outcome of multi-trophic interactions by top-down control of herbivores has been well studied. Particularly, the positive impacts of natural enemies of herbivores on plants through direct (consumptive) and indirect (non-consumptive) effects on their prey and hosts have received considerable interest, while multi-trophic interactions that result in negative effects on plants have received little attention. Negative impacts of carnivorous arthropods have been documented and arise when carnivores directly affect plants and/or their interactions with beneficial arthropods. In general, negative effects may be compensated by positive effects of other carnivorous arthropods, but their presence and significance is likely to be underestimated in tri-trophic interactions. Recent studies have revealed that the composition and dynamics of the plant and arthropod community have a significant effect on plant fitness. Therefore, we encourage an approach that accounts for a larger community of species and interactions associated with plants, including interaction types in which carnivores may negatively affect plants. This review highlights specific interaction types that ultimately lead to negative effects of carnivores on plants. This synthesis presents alternative hypotheses to those that predict that carnivores invariably benefit plants. Testing potential costs and benefits of carnivores to plants will advance our understanding of indirect plant defence. A free Plain Language Summary can be found within the Supporting Information of this article.