Trophic effects of vertebrate insectivores and carnivorous arthropods in a subtropical forest: the roles of functional redundancy and intraguild predation

Insectivorous predators play a crucial role in suppressing herbivore populations and mitigating herbivory in terrestrial ecosystems. However, the outcomes of ‘insectivore—herbivore—plant’ interactions are context dependent. This study examines the effects of vertebrate insectivores (birds and bats) and carnivorous arthropods (spiders) on herbivorous insects (caterpillars and sap suckers) and their host plant, Machilus thunbergii (Lauraceae), in a subtropical evergreen forest. We employed a factorial field experiment with four treatments: control (no exclusion), bird/bat exclusion, spider removal, and bird/bat exclusion plus spider removal. Forty trees (10 per treatment) were surveyed for herbivore abundance, degree of herbivory, and herbivore community composition. The results indicate that the predator treatments had no effects on sap suckers or sap-sucking damage. However, compared to the control, the leaf-chewing damage was higher in the bird/bat exclusion plus spider removal treatment, accompanied by shifts in leaf-chewer community composition and a trend toward increased leaf-chewer abundance. Spider abundance was lower in the control than in the bird/bat exclusion treatment, suggesting intraguild predation of spiders by birds and bats. Nevertheless, the leaf-chewing damage remained similar between these treatments, indicating that intraguild predation did not weaken the trophic cascade. This study is one of the few experiments that manipulate both vertebrate insectivores and carnivorous arthropods, and provides rare evidence that these two predator groups can perform complementary roles in suppressing herbivory. Our findings suggest that the strength of trophic interactions in the ‘insectivore—leaf chewer—plant’ system of this subtropical evergreen forest are likely to be relatively stable.