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

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.

The importance of orangutans in small fragments for maintaining metapopulation dynamics

“…During these travels, orangutans may use any vantage point (such as isolated trees) to navigate within the plantation as suggested by the frequent report of nests built in scattered single trees, or small groups of trees, within plantations far away from any forest. Orangutans also appear to use forest corridors to move across the landscape, as recorded by Seaman et al (2019) and our surveys in commercially administrated lands. These corridors are often set aside as high conservation value forests to meet sustainability certification criteria, either as riparian buffers or other linkages between forest patches.…”

“…Orangutans persist and reproduce in forests logged for timber (Husson et al, 2009;Ancrenaz et al, 2010), in industrial timber plantations Spehar and Rayadin, 2017) and in agricultural landscapes, for example (Campbell-Smith et al, 2011). They are also found in isolated patches of forest within landscapes that have been extensively transformed by humans, albeit at lower densities than in more extensive natural forests (Ancrenaz and Lackman, 2014;Ancrenaz et al, 2015;Spehar et al, 2018;Seaman et al, 2019). Orangutan survival and population viability within these heavily managed landscapes is likely contingent on hunting and killing being minimized (Marshall et al, 2006;Husson et al, 2009).…”

Importance of Small Forest Fragments in Agricultural Landscapes for Maintaining Orangutan Metapopulations

“…Males are the most frequently observed sex in oil palm plantations, including numerous mentions of them walking on the ground, along rivers or streams and even on plantation roads. Orangutans use forest corridors to move across the landscape, as recorded by Seaman et al (2019) and during our investigations. For example, one estate included in our analysis allowed natural forest regeneration under unmanaged oil palms in a 40 m wide corridor 1.2 km long, to link two isolated forest patches.…”

“…Today they persist and reproduce in forests logged for timber (Husson et al, 2009;Ancrenaz et al, 2010), in industrial timber plantations (Meijaard et al, 2010;Spehar and Rayadin, 2017) and in agricultural landscapes (Campbell-Smith et al, 2011). They are also found in isolated patches of forest, albeit at lower densities than in more extensive natural forests (Ancrenaz et al, 2015;Seaman et al, 2019). In landscapes that have been extensively transformed by humans, orangutan survival is contingent on hunting and killing being minimized (Marshall et al, 2006;Husson et al, 2009;Spehar et al, 2018).…”

Grouping behavior of Sumatran orangutans (Pongo abelii) and Tapanuli orangutans (Pongo tapanuliensis) living in forest with low fruit abundance

“…While logging does not result in low orangutan densities per se (Borneo: Marshall et al, 2006; Seaman et al, 2019; Sumatra: Knop et al, 2004), behavioral changes that accompany logging suggest increased energetic costs (Hardus et al, 2012; Rao & van Schaik, 1997) or decreased nutrient intake (e.g., increased reliance on fallback foods: Russon, Kuncoro, & Ferisa, 2015) in logged or damaged forest. These behavioral changes may ultimately also influence costly activities, such as party formation.…”