Adjacent forest moderates insect pollination of oil palm

Natural habitat plays a role in many agroecosystems as a source of pollination services and other ecological spillover, but these effects are largely unquantified in oil palm (Elaeis guineensis), a globally important crop linked to deforestation. In a field experiment in Sumatra, Indonesia, we manipulated floral visitor access to female oil palm inflorescences over a 100 m distance gradient from forest into oil palm and sampled insects with sticky traps placed above male and female inflorescences. Full exclusion of floral visitors decreased mean oil palm fruit set to 12%, demonstrating that insect pollination was necessary to maintain favorable fruit set and yield. Treatment group means of fruit set under open pollination (62%) and when excluding large (>1.4 mm diameter) organisms (72%), did not differ significantly from open pollination augmented with hand pollination (61%), suggesting no difference in pollen limitation. In contrast, when we examined change in fruit set with distance from forest, we found a significant trend of higher fruit set in oil palms closer to the forest when large organisms were excluded, which increased estimated fruit set at the forest edge to 87%, compared to open-pollinated palms (70%). This trend with distance from forest was absent when we fully excluded floral visitors, showing that the effect of forest was not likely due to an abiotic gradient (e.g., changing soil nutrients). Of the arthropod taxa collected from sticky traps, Drosophilidae (Diptera) and Gelechiidae (Lepidoptera) decreased and increased with distance from forest, respectively. The taxa Elaeidobius kamerunicus (Coleoptera: Curculionidae), Gelechiidae, and the families Thripidae and Phlaeothripidae (Thysanoptera) were abundant on both male and female inflorescence sticky traps. Elaeidobius kamerunicus, an introduced oil palm pollinator, had the only significant relationship with fruit set. Our results confirm pollination by insects as a key ecosystem service for oil palm production. Although further work is needed to clarify the relationship between fruit set, biodiversity, and distance from forest, we speculate that excluding large organisms could have increased fruit set closer to forest by mediating interactions between pollinators, forest predators, and farm mesopredators. Understanding the relationships between nearby forest and pollination services could better connect oil palm production to its landscape context and associated biodiversity. This would be important for landscape-scale conservation planning that considers both the ecosystem service needs and ecological impacts of oil palm agriculture.

Automated summary

Natural habitats have a significant impact on pollination services and ecological spillover in oil palm farming. In this study, we found that insect pollination is necessary for favorable fruit set and yield in oil palm. Moreover, oil palms closer to the forest showed higher fruit set when large organisms were excluded, indicating the importance of interactions between pollinators, forest predators, and farm mesopredators.