No abstract

1. Urban trees provide important ecological services to urban ecosystems, including cooling, pollution filtering or improvement of human well-being. Urban trees can be more at risk to be damaged by herbivores than their rural counterparts. But not all urban trees suffer from the same amount of insect damage and intra-urban gradients of herbivory have been observed, although underlying drivers are still debated. 2. We monitored 48 urban trees from five speciesthree native and two exoticin three parks of Montreal (Canada) for leaf insect herbivory and predator activity on artificial larvae, and linked herbivory and predation with tree density and tree diversity in the vicinity of focal trees. 3. Leaf insect herbivory decreased with increasing tree diversity, and predator attack rate on artificial larvae increased with tree density around focal trees. Insect herbivory was negatively correlated with predator attack rate. 4. Our findings indicate that tree diversity and density are determinants of multitrophic interactions among trees, herbivores and predators in urban environments and that managing tree diversity and density could contribute to pest regulation in cities.

The natural enemies hypothesis predicts that the abundance and diversity of antagonists such as predators and parasitoids of herbivores increases with the diversity of plants, which can lead to more effective top-down control of insect herbivores. However, although the hypothesis has received large support in agricultural systems, fewer studies have been conducted in forest ecosystems and a comprehensive synthesis of previous research is still lacking. We conducted a meta-analysis of 65 publications comparing the diversity, abundance or activity of various groups of natural enemies (including birds, bats, spiders and insect parasitoids) in pure vs. mixed forest stands. We tested the effects of forest biome, natural enemy taxon and type of study (managed vs experimental forest). We found a significant positive effect of forest tree diversity on natural enemy abundance and diversity but not on their activity. The effect of tree diversity on natural enemies was stronger towards lower latitudes but was not contingent on the natural enemy taxon. Overall, our study contributes substantially toward a better understanding of the natural enemies hypothesis in forest systems and provides new insights about the mechanisms involved. Furthermore, we outline potential avenues for strengthening forest resistance to the growing threat of herbivorous insects.