Canopy openness drives insect-mediated ecosystem functions in oil palm biodiversity enrichment experiment in Indonesia

As the extent of oil palm (<em>Elaeis guineensis</em>) cultivation has expanded at the expense of tropical rainforests, enriching conventional large-scale oil palm plantations with native trees has been proposed as a strategy for restoring biodiversity and ecosystem function. However, how tree enrichment affects insect-mediated ecosystem functions is unknown. We investigated impacts on insect herbivory and pollination functions in the fourth year of a plantation-scale, long-term oil palm biodiversity enrichment experiment in Jambi, Sumatra, Indonesia. Within 48 plots systematically varying in size (25-1600 m2) and planted tree species richness (1-6 species), we collected data on vegetation structure, understory insect abundances, and pollinator and herbivore activity on chili peppers (<em>Capsicum annuum</em>), a widespread home garden crop, as phytometer plants to quantify insect-mediated ecosystem functions. We examined the independent effects of plot size, tree species richness, and tree identity using the linear model for random partition design. These experimental treatments had the most relationships with vegetation structure: the tree species <em>Peronema canescens</em> strongly decreased (by approximately one standard deviation) both canopy openness and understory vegetation cover; the smallest plots had the lowest understory flower density and richness; and total planted tree species richness decreased flower density. Enrichment influenced herbivorous insects and natural enemies in the understory to a lesser extent: both groups had higher abundances in plots with two enrichment species planted, while herbivores decreased with increasing tree species richness. We found no strong direct relationships between experimental treatments and pollinator activity or most phytometer plant variables, except for lower leaf damage in the smallest plot size. Linking relationships in structural equation models shows that the negative association between <em>P. canescens</em> and understory vegetation cover was mediated through canopy openness. Likewise, canopy openness mediated increases in herbivore and pollinator insect abundances. Higher pollinator visitation increased phytometer yield, while impacts of insect herbivores on yield were not apparent. Our results demonstrate that even at an early stage, ecological restoration influences insect-mediated ecosystem functions, mainly through canopy openness. These findings suggest that maintaining some canopy gaps while enrichment plots develop may be beneficial for increasing habitat heterogeneity and insect-mediated ecosystem functions.