The effects of replanting and restoration of riparian buffers on arthropods in oil palm systems
Oil palm is a widely grown tropical crop, and its product – palm oil – has international economic importance. The expansion of oil palm plantations has caused substantial declines in biodiversity and changes in ecosystem processes. Protecting natural habitats is a conservation priority. However, once oil palm plantations are established, it is important to investigate how changes in management can affect oil palm systems, which can be relatively ecologically complex. Despite this, there is limited understanding of how most management strategies – such as those used to replant oil palms when they have reached the end of their commercial life cycle, and to maintain and restore areas around rivers in plantations (“riparian buffers”) – can affect oil palm systems.
This PhD was based primarily at the Biodiversity and Ecosystem Function in Tropical Agriculture (BEFTA) Programme in Sumatra, Indonesia, within oil palm plantations that received different levels of management. Across these plantations, we sampled arthropods in the canopy, understory, and ground microhabitats using insecticide fogging; sticky traps and hand collections; and pitfall traps, respectively. We sampled in different ages of oil palm, and in sites that were within, near, and far from riparian buffers undergoing different restoration management, in order to investigate the effects of replanting and restoration of riparian buffers on arthropod biodiversity. This thesis has four data chapters:
Chapter 2: By surveying arthropods across an oil palm chronosequence that spanned a replanting event, we showed that replanting using recommended strategies did not affect total arthropod abundance, but had effects on order-level community composition, and the biodiversity of functionally important groups, including spiders.
Chapter 3: Sampling the same chronosequence as in Chapter 2, we found that riparian buffers that were made of mature oil palms and being passively restored did not have consistent impacts on environmental conditions and arthropod biodiversity across the oil palm commercial life cycle.
Chapter 4: Working in the Riparian Ecosystem Restoration in Tropical Agriculture (RERTA) Project, we used an experimental approach to show that passive and active restoration of riparian buffers did not benefit arthropod biodiversity within two years of restoration treatments being implemented. Longer-term observations are needed before the full impacts of this experiment can be determined.
Chapter 5: Using systematic review and meta-analysis, we showed that anthropogenic disturbance reduced spider abundance and species richness, and restoration initiatives did not consistently benefit spiders in the tropics. We also demonstrated that spider responses to disturbance and restoration varied substantially, and were therefore context-dependent.
This thesis shows that replanting has substantial impacts on environmental conditions and biodiversity in oil palm plantations, but the effects of replanting are variable across microhabitats and taxonomic groups. It also shows that restoration of riparian buffers in oil palm systems is possible through tractable changes in management, but longer-term observations are needed to demonstrate whether there are consistent benefits of restored buffers to biodiversity, and to determine the comparative benefits of passive and active approaches to riparian restoration. Further, this thesis demonstrates that there is no “silver bullet” to restoring degraded tropical landscapes, and highlights the importance of conducting large-scale, long-term experiments to improve knowledge of how restoration can benefit degraded tropical systems.