Ecological impacts of biodiversity enrichment in oil palm plantations

Land-use change is one of the major drivers of biodiversity loss. To satisfy the demand for palm oil in food and biofuel, complex, species-rich rainforests are converted into large, simply-structured mono-culture oil palm plantations. This has dramatic consequences for biodiversity and ecosystem functioning, thereby also putting human well-being at risk. Facing the severe decline in biodiversity, the re-establishment of diverse habitats and their multi-functionality through restoration measures could help biodiversity and ecosystem functioning recover faster. However, knowledge about the underlying ecological and socio-economic mechanisms of restoration in oil palm plantations and clear instructions towards a wildlife-friendly management of oil palm are lacking. In this thesis, I provide initial insights into the relationship between ecology and economics when a wildlife-friendly farming strategy is applied in oil palm systems. Focusing on birds of smallholder oil palm-dominated landscapes in the Jambi province, Sumatra, Indonesia, I uncovered the ecological-economic relationship when having remnant or planted trees within oil palm plantations and estimated the costs for the conservation of bird diversity and abundance. The results suggest that bird diversity and abundance depends on the number of trees on the plot and that an increase in bird diversity and abundance results in revenue penalties, indicating that there is a win-lose relationship between ecological and economic outcomes. However, since the relationship was non-linear, an increase in bird diversity could be achieved at lower costs in highly intensified oil palm plantations as compared to extensively managed oil palm plantations. Furthermore, the costs for increased bird abundance were lower than for increased bird diversity. Overall, these findings illustrate that there is room for tree-based enrichment in intensively managed oil palm plantations as a measure to maintain a baseline level of biodiversity at relatively little costs. In order to address various open questions and to effectively be able to shed light on additional ecological and socio-economic mechanisms linked to enrichment plantings, I established a long-term, large-scale biodiversity enrichment in a mono-culture oil palm plantation in the Jambi province, Sumatra, Indonesia. The establishment of the experiment comprised planting tree islands of different sizes and with varying tree diversity and composition within gaps of an oil palm plantation. I assessed initial environmental and biotic characteristics of the plantation prior to the tree planting against which the longitudinal data from the tree islands will be compared to throughout the years following the establishment. The design allows for disentangling the effects of tree diversity and island size on the diversity and composition of different organism groups such as plants, birds and invertebrates. Herewith, conclusions can be drawn on changes in ecosystem functioning. I investigated early effects of the tree plantings on the bird and invertebrate communities. Interestingly, birds and invertebrates responded positively to the enrichment plantings already one year after the establishment of the tree islands. Overall bird species richness and abundance of herb-layer invertebrates was increased on plots with trees. Invertebrates were not only positively affected by enrichment plantings on a landscape scale but also on plot level. In summary, these findings illustrate the great potential of restoration plantings to benefit biodiversity and associated ecosystem functioning as birds and invertebrates play a key role in initiating succession processes, thereby enhancing biodiversity. Both, birds and invertebrates fulfil many tasks that are essential for the functioning and resilience of ecosystems. The biodiversity enrichment experiment provides lucrative ground for further research in various disciplines in order to develop ecologically improved and socio-economically viable management strategies for oil palm plantations. Overall, this thesis contributes substantially to make advances in BEF and restoration research in tropical agricultural landscapes. Scientific evidence on the costs and benefits of enrichment plantings provides the ground for future political decision-making towards increased ecological and socio-economic sustainability in oil palm management. Ultimately, the biodiversity enrichment experiment may contribute to increasing and conserving biodiversity in tropical agricultural landscapes without jeopardizing the food security of a growing human population.