Do riparian forest fragments provide ecosystem services or disservices in surrounding oil palm plantations?

Abstract

Agricultural expansion across tropical regions is causing declines in biodiversity and altering ecological processes. However, in some tropical agricultural systems, conserving natural habitat can simultaneously protect threatened species and support important ecosystem services. Oil palm cultivation is expanding rapidly throughout the tropics but the extent to which non-crop habitat supports biodiversity and ecosystem services in these landscapes is poorly documented. We investigated whether riparian forest fragments (riparian reserves) provide a pest control service or increase pest activity (disservice) within oil palm dominated landscapes in Sabah, Malaysian Borneo. We assessed the activity of potential predators of pest herbivores using plasticine caterpillar mimics and quantified herbivory rates on oil palm fronds in areas with and without riparian reserves. We also manipulated the shape and colour of the mimics to assess the extent to which artificial pest mimics reflect a predatory response. The presence of riparian reserves increased the attack rate on mimics by arthropods, but not by birds. Our methodological study suggested attacks on artificial pest mimics provide a better indication of predatory activity for birds than for arthropod predators. Herbivory rates were also not significantly affected by the presence of a riparian reserve, but we found some evidence that herbivory rates may decrease as the size of riparian reserves increases. Overall, we conclude that riparian forest fragments of 30 – 50m width on each side of the river are unlikely to provide a pest control service. Nevertheless, our results provide evidence that these riparian buffer strips do not increase the density of defoliating pests, which should reassure managers concerned about possible negative consequences of preserving riparian buffers.