Plant neighbours mediate bird predation effects on arthropod abundance and herbivory

1. Tritrophic interactions among plants, herbivores and predators are expected to be influenced by the surrounding vegetation. Neighbouring plants can influence focal plant colonisation by herbivorous insects and the foraging behaviour of natural enemies, such as insectivorous birds. 2. The aim of the experiment was to disentangle the interactive effects of neighbouring plants and avian predation on arthropod abundance and insect leaf damage in oak tree seedlings, using exclusion cages and vegetation removal. 3. The presence or removal of surrounding herbaceous vegetation differentially mediated top-down effects of insectivorous birds on distinct arthropod guilds and herbivore damage in seedlings. Avian predation reduced sawfly larval abundance regardless of the presence of plant neighbours; lepidopteran larval abundance only when plant neighbours were removed; and spider abundance only when plant neighbours were left intact. The removal of plant neighbours increased prey accessibility for foraging insectivorous birds and decreased chewer damage on seedlings. The density of concealed-feeder insects (leaf miners) increased with plant neighbour removal and when seedlings were less damaged by chewer guild, suggesting intraguild competition. 4. These results highlight the strong indirect effects of neighbouring vegetation on tritrophic interactions involving a focal plant species, its associated herbivores and the upper trophic level of predators.