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

Abstract

Historically, orangutans (Pongo spp.) lived in large contiguous areas of intact rainforest. Today, they are also found in highly modified and fragmented landscapes dominated by oil palm or industrial timber plantations; a situation that calls for new conservation approaches. Here we report signs of orangutan presence in more than 120 small forest fragments of <500 ha in size and isolated in extensive oil palm plantations across Borneo. We confirmed the long-term presence of adult resident females with dependent young in 42% of the fragments assessed by ground survey (n = 50), and the regular sightings of males traveling across the landscape. We argue that orangutans using and living in small isolated forest patches play an essential part in the metapopulation by maintaining gene flow among larger sub-populations distributed across multiple-use landscapes. In some cases, translocations may be necessary when the animals are in imminent danger of being killed and have no other refuge. However, the impacts of removing animals from spatially dispersed metapopulations could inadvertently decrease critical metapopulation functionality necessary for long-term viability. It is clear that orangutans need natural forest to survive. However, our findings show that forest fragments within agricultural landscapes can also complement conservation areas if they are well-distributed, properly connected and managed, and if orangutan killing is prevented. Efforts to better understand the dynamics and the functionality of an orangutan metapopulation in forest-farmland landscape mosaics characteristic of the Anthropocene are urgently needed to design more efficient conservation strategies for the species across its range.

Orangutan movement and population dynamics across human-modified landscapes: implications of policy and management

“…A phenomenon we captured in our simulations were female orangutans occupying and reproducing in remnant forest patches in oil palm. This is consistent with field observations, where practitioners have identified female orangutans residing in remnant forest in oil palm dominated landscapes for a sufficient amount of time for an offspring to reach maturity and leave to establish its own home range (Ancrenaz et al 2021).…”

“…However, most orangutans translocated from agricultural landscapes appear to be in good health (Sherman et al 2020b), indicating that these individuals have been surviving in these landscapes. In addition, our models demonstrate the potential importance of these individuals in maintaining movement and connectivity across the landscape (Ancrenaz et al 2021). Although orangutan translocation can be justified when the life of an individual is threatened, such as during fire events or direct conflict with people, the blanket deployment of translocation may in fact be detrimental to the species (Sherman et al 2020b).…”

“…In large forest areas we reduced density estimates by half to represent the female density. However, for small remnant forest patches that are likely to be occupied by resident females (Ancrenaz et al 2021) we maintained density estimates to reflect this. Female orangutan home ranges are difficult to determine, and on Borneo estimates vary from 40 to 600 ha (Singleton et al 2009).…”

Deforestation projections imply range-wide population decline for critically endangered Bornean orangutan

“…1,800 individuals, 2.6% of all Bornean orangutans, unpublished analysis) potentially surviving in forest fragments within agricultural landscapes or other marginal habitat. Although orangutans depend on forest habitat, individuals have occasionally been found to persist in mosaics of forests and plantations (Ancrenaz et al, 2021;Seaman et al, 2019). Populations thus seem to persist in human-modified landscapes where there is connectivity with larger forest areas and no mortality.…”

Disease Risk and Conservation Implications of Orangutan Translocations

“…This is due to the risks that translocation poses to wild populations and its use of funds and political will, which might otherwise be available for habitat conservation and anti-poaching efforts. Specific risks of translocation for orangutans include: disease transmission, competition and social stress for released and wild orangutans; suspected high incidences of mortality following release; and negative impacts on genetic and socioecological functioning of the source populations if individuals are removed through wild-to-wild translocation (4,5,(15)(16)(17)(18). While translocations are never entirely without risk, infectious disease transmission from released animals to wild populations can pose particularly consequential risks to wild individuals and to population health (19).…”