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The number of Bornean orangutans is dwindling, and there is little evidence that efforts to relocate them from risky areas or rehabilitate those once held captive actually works to bolster their population.

Between 2007 and 2017, about 1200 Bornean orangutans (Pongo pygmaeus) were released into natural forests in Kalimantan, the Indonesian part of the island of Borneo. Nearly 500 of those were formerly captive individuals nursed back to health before being released into the wild. But how many of these animals are still alive remains unclear.

鈥淩escue centres do an important service by providing specialised care for a difficult-to-care-for species, but there is little publicly available evidence on the long-term survival of the reintroduced animals,鈥� says Julie Sherman at Wildlife Impact in Oregon.

Sherman and her team reviewed studies, news stories and publicly available data on conservation efforts to make these estimates. They also collected data from rescue centres, government agencies and zoos to determine the outcomes of relocation or rehabilitation for these great apes.

The handful of cases where these animals were tracked for more than three years suggest that fewer than 30 per cent of the released animals may have survived.

During the study period, at least 620 wild orangutans were also picked up outside protected areas in Kalimantan and released into a different wild site, mainly to prevent potential conflict with people.

鈥淭he assumption translocation practitioners make is that since these are wild orangutans, they will survive anywhere in the wild,鈥� says Sherman.

Again, the fate of such animals is usually not monitored. The few studies that Sherman鈥檚 team found in which relocated orangutans were tracked suggest that most animals probably disappeared after release and may not have survived beyond a few years.

Sherman and her team found that, in most cases, people weren鈥檛 tracking the impacts of conservation actions.

This is despite a number of action plans and millions of dollars being pumped into the great ape鈥檚 conservation in the past decade. The team鈥檚 analysis of 145 organisations鈥� spending showed that, in 2016 alone, rescue, reintroduction and translocation was the predominant conservation strategy for Bornean orangutans, with more than 拢3.8 million allocated to it.

But instead of improving the status of the species, it went from being endangered to critically endangered in 2016.

鈥淭his species is declining very rapidly, so whatever we鈥檙e trying isn鈥檛 working well enough,鈥� says Erik Meijaard, director of Borneo Futures in Brunei, Borneo.

What works

Generally, actively protecting orangutan habitats seems to be a reasonably successful strategy. In the Malaysian part of Borneo, where most such habitat is protected, orangutan populations are becoming relatively stable.

鈥淭his expansion of conservation areas has occurred as there is political will to protect orangutans and habitats,鈥� says Melvin Gumal of the Wildlife Conservation Society, who is based in Borneo.

Sherman鈥檚 team is still analysing data on how land management practices affect orangutan conservation.

鈥淭he evidence we鈥檙e collecting now suggests that where people have land ownership or management rights, whether it鈥檚 communities or organisations, and where they鈥檙e actively managing that land by patrolling, suppressing fires and preventing orangutan killing, that appears to be keeping habitats intact and orangutans in those habitats,鈥� says Sherman.