Densities of Bornean orang-utans (Pongo pygmaeus morio) in heavily degraded forest and oil palm plantations in Sabah, Borneo.

Dave J.I Seaman,Thomas Swinfield,David Coomes,Henry Bernard,Matthew J Struebig,Tatyana Humle,David T Milodowski,Marc Ancrenaz

doi:10.1002/ajp.23030

Abstract

The conversion of forest to agriculture continues to contribute to the loss and fragmentation of remaining orang‐utan habitat. There are still few published estimates of orang‐utan densities in these heavily modified agricultural areas to inform range‐wide population assessments and conservation strategies. In addition, little is known about what landscape features promote orang‐utan habitat use. Using indirect nest count methods, we implemented surveys and estimated population densities of the Northeast Bornean orang‐utan (Pongo pygmaeus morio) across the continuous logged forest and forest remnants in a recently salvage‐logged area and oil palm plantations in Sabah, Malaysian Borneo. We then assessed the influence of landscape features and forest structural metrics obtained from LiDAR data on estimates of orang‐utan density. Recent salvage logging appeared to have a little short‐term effect on orang‐utan density (2.35 ind/km 2), which remained similar to recovering logged forest nearby (2.32 ind/km 2). Orang‐utans were also present in remnant forest patches in oil palm plantations, but at significantly lower numbers (0.82 ind/km 2) than nearby logged forest and salvage‐logged areas. Densities were strongly influenced by variation in canopy height but were not associated with other potential covariates. Our findings suggest that orang‐utans currently exist, at least in the short‐term, within human‐modified landscapes, providing that remnant forest patches remain. We urge greater recognition of the role that these degraded habitats can have in supporting orang‐utan populations, and that future range‐wide analyses and conservation strategies better incorporate data from human‐modified landscapes.

Highlights

Agriculture is a leading cause of deforestation globally and, with increasing demands for food and commodities, this trend is likely to continue (Sandker, Finegold, D’annunzio, & Lindquist, 2017)
Across all possible iterations of parameter values, the upper limits of our density estimates for remnant forest sites in oil palm were lower than half the upper limits for the logged forest, and density estimates were on average close to a third that of logged forest. These results provide strong evidence that, despite using parameters acquired from other sites in our density calculations, oil palm estates support
Despite pledges by the Indonesian and Malaysian government to stabilize orang‐utan populations, they have continued to decline by 25% over the past 10 years (Santika et al, 2017)

Summary

| INTRODUCTION

Agriculture is a leading cause of deforestation globally and, with increasing demands for food and commodities, this trend is likely to continue (Sandker, Finegold, D’annunzio, & Lindquist, 2017). Fragmentation, and hunting continue to be leading contributors of population decline (Meijaard et al, 2011; Voigt et al, 2018; Wich et al, 2016), and could have catastrophic consequences in combination with range contractions expected under climate change (Struebig et al, 2015). The highest densities of orang‐utans are in forests lower than 500 m above sea level (ASL; Voigt et al, 2018) These low‐lying areas are often the most suitable for agriculture, leading to high levels of deforestation and forest degradation within the orang‐utan range (Santika et al, 2017). We employ orang‐utan nest surveys to determine orang‐ utan population densities in the continuous logged forest and forest remnants in a recently salvage‐logged area and oil palm plantations in Sabah. We explore the role of forest structural data and landscape features in predicting orang‐utan density

| METHODS

| Ethical statement

| RESULTS

| DISCUSSION

| CONCLUSION