Evaluating the effectiveness of human–orangutan conflict mitigation strategies in Sumatra

1. Crop raiding by great apes is an emerging conservation issue across their range. It is important because it involves highly threatened species that can cause significant economic damage and be killed in retribution. Yet, to date, no quantitative study has sought to test possible solutions for preventing this form of human–wildlife conflict. 2. From February 2007 to August 2009, we monitored crop‐raiding patterns across a Sumatran agroforest landscape to determine background levels of human–orangutan conflict. We also intensively monitored a subset of 50 farms to assess changes in farmer attitudes towards orangutan management; differences between farmer reported and independently enumerated monetary loss from crop raiding; and the effectiveness of mitigation techniques in reducing orangutan crop raiding on 35 treatment farms (25 trialling noise deterrents and 10 trialling tree nets) in comparison with 15 control farms over a pre‐trial (12 month) and a trial (18 month) phase. Five months after the trials had ended, the ongoing use or uptake of the techniques were assessed. 3. Across the wider landscape, background levels of mean daily orangutan crop‐raiding incidents per month (±SE) farms did not significantly differ between the pre‐trial (9·1 ± 3·7) and trial (7·1 ± 4·3) phases, whilst on the 35 treatment farms it reduced significantly. Furthermore, crop yield increased (+60·8%) on the netted trial trees, but reduced (−27·4%) on the control farm trees. Despite this, there was no subsequent use of this technique, unlike those farmers (40%) who continued using the less‐effective noise deterrents. 4. Farmer participation in the project yielded unexpected and positive attitude changes, from preferring orangutan removal (pre‐trial) to in situ management with crop protection measures (post‐trial). However, project participation may have increased farmer expectations of receiving compensation because the treatment farmers consistently overestimated their crop losses, unlike the control farmers who did not. 5. Synthesis and applications. Whilst human–orangutan conflicts caused substantial losses to local livelihoods, the identification of an effective mitigation method (nets) neither guaranteed its continued use nor uptake. Developing easy to install nets for valuable tree crops is therefore recommended. Nevertheless, the project intervention efforts did create benign farmer attitudes towards orangutan management, an essential prerequisite for managing large‐bodied mammals in conflict with people.