The importance of orangutans in small fragments for maintaining metapopulation dynamics

Abstract

Orangutans (Pongo spp.) occur at low densities and therefore large areas are necessary to sustain viable metapopulations, defined here as sets of conspecific units of individuals linked by dispersal. Historically, orangutans lived in large contiguous areas of intact rainforest, but are now increasingly found in agricultural and other landscapes modified by people. Here we collate evidence of orangutans utilizing isolated forest fragments (< 500 ha) within multiple-use landscapes dominated by oil palm monoculture across Borneo. Orangutan signs (i.e. nests) were evident in 76 fragments surveyed by helicopter, and in 50 of 70 additional fragments surveyed on the ground; on average 63 ha in size. This includes presence of adult resident females with dependent young confirmed in 40% of the fragments assessed by ground survey. Our study revealed some resident females are raising offspring in isolated forest patches within mature oil palm stands. This not only confirms that some forest patches can sustain orangutans, but indicates migratory males are capable of reaching these fragments scattered throughout the multiple-use landscape. Therefore, orangutans that use or live in even small isolated forest patches are an essential part of the overall metapopulation by maintaining gene flow between, and genetic connectivity within, populations distributed across larger multiple-use landscapes. Orangutan survival is commonly thought to be low in small, isolated forest patches, and the customary management strategy is to remove (translocate) these individuals and release them in larger forests. In some cases, translocations may be necessary, i.e. in case of fire or when the animals are in eminent danger of being killed and have no other refuge. However, the small amount of data available indicates that mortality rates during and after translocations are high, while the impacts of removing animals from spatially dispersed metapopulations are unknown. Therefore, we argue the current policy of routine translocation rather than conserving the species within human-modified landscapes could inadvertently decrease critical metapopulation functionality necessary for long-term viability. It is clear that orangutans need natural forest to survive, but our findings show that fragmented agricultural landscapes can also serve as complementary conservation areas in addition to fully protected areas if they are well designed with ecological connections, and if orangutan killing can be prevented. To achieve this, we call for a paradigm shift from the traditional large single forest model to one that emphasizes metapopulation functionality in the fragmented foresthuman use matrix characteristic of the Anthropocene.

Effectiveness of 20 years of conservation investments in protecting orangutans

“…During the study period, the Sabah government increased the size of protected areas from 12% to nearly 30% of the state land area, 22,23 and by 2020, more than 70% of orangutans in Sabah were found inside protected areas. 16,22 This is quite different from the situation in Indonesia, where most terrestrial protected areas were established before 2005 (currently covering 23% of the total land area for Kalimantan and Sumatra) and the expansion of forest protection since 2005 was mainly through the establishment of community-based land tenure and acquisition of private land by conservation non-government organizations (NGOs). Consequently, a high level of investment specifically from the Sabah government has been allocated to resource-intensive patrolling activities for these protected areas, but lower investment had been allocated to habitat protection since all these new areas were gazetted by the government without incurring any high significant direct cost or land purchase for their creation.…”

“…Removing orangutans and translocating them to large forest blocks deemed more suitable for their survival may seem straightforward and is often presented as an efficient conservation tool, particularly when alternative conservation activities may require planning and extensive negotiation with multisectoral and multilevel stakeholders. 16 However, the relative success of this conservation approach is still not known and might be relatively low, and there is a potential negative impact of these exercises on the viability of metapopulations. 16 Furthermore, translocation and reintroduction can be costly and are associated with high mortality rates.…”

“…16 However, the relative success of this conservation approach is still not known and might be relatively low, and there is a potential negative impact of these exercises on the viability of metapopulations. 16 Furthermore, translocation and reintroduction can be costly and are associated with high mortality rates. 17 In Kalimantan, translocations were the most expensive conservation activities in terms of operational cost per km 2 , and the cost greatly exceeds those in other regions ($427 per km 2 versus $41 and $121 per km 2 for Sabah and Sumatra) (Figure 4B).…”

The historical range and drivers of decline of the Tapanuli orangutan

“…Behavioural traits of female site fidelity and male territoriality, and adaption issues P . pygmaeus released in unfamiliar habitats indicate that translocation risks are high and survival rates may be low [ 108 , 112 ].…”

The historical range and drivers of decline of the Tapanuli orangutan

“…While translocation has been used as a response to orangutans in conflict with humans, translocated animals are not monitored beyond a few days following release, sometimes not at all, and their long-term survival is not known. Behavioural traits of female site fidelity and male territoriality, and adaption issues P. pygmaeus released in unfamiliar habitats indicate that translocation risks are high and survival rates may be low [108, 112].…”