Palm oil is the most produced vegetable oil in the world. The rapid expansion of oil palm plantations into millions of hectares of forest has resulted in severe losses of biodiversity and increased pollution. Management interventions to mitigate these impacts are clearly needed. Oil palm is capable of supporting a dense understory with vegetation that can reach several metres high. However, some plantation owners completely remove this by spraying herbicide liberally. This destructive practice is likely to have a negative impact on biodiversity and associated ecosystem services, as it reduces habitat complexity and the capacity of plantations to buffer microclimate.

This study is based at the Biodiversity and Ecosystem Function in Tropical Agriculture (BEFTA) Programme in Sumatra, Indonesia. Study sites are based across four habitat types, including three different levels of understory management in mature oil palm plantations and a recently-replanted oil palm site. We investigated the effect of these management treatments on several key groups, including large mammals, rats, ants, and termites, and associated ecosystem functions.

Large Mammals & Rats: Using camera traps to assess mammal populations we showed that Leopard cats, which are an important predator of pest rats, prefer plantations with more vegetation. However, by using live traps to quantify rat abundance and manual counts to measure rat damage to palm oil fruit bunches, we showed that this did not translate to a reduction in pest abundance or damage.

Termites: By surveying and manually searching termite mounds, we showed that altering understory vegetation management in mature plantations does not affect the abundance of termite mounds or the likelihood that they are inhabited by termites. We also found a great diversity of organisms nesting in the mounds, including four species of snake, making this the first scientific documentation of this symbiosis.

Ants: By conducting 18 month-long manipulative suppression experiments, we showed that there is little redundancy for the role of ants in maintaining ecosystem functioning in oil palm plantations. Where ant abundance was suppressed, herbivore predation and seed removal were significantly reduced. There was no difference between the habitat types, indicating that ants are important under all management scenarios.

This thesis clearly shows the value of biodiversity within the oil palm matrix, by directly linking it to important ecosystem services. It shows that the impact of understory vegetation management and replanting on biodiversity is variable, with some taxa being robust to highly destructive practices. It demonstrates the importance of investigating the impact of conservation interventions on a range of taxa, as complex interactions are at play even in these simplified landscapes.