Forgotten forest turns out to be the last paradise for endangered tigers

However, on the Indonesian island of Sumatra, an important population of the critically endangered Sumatran tiger may remain in relatively good health. This is evidenced by a new study published in the journal Frontiers in Conservation Science.

Using a network of infrared cameras, scientists assessed the tigers' sex and age structure and density, as well as their movements during three separate monitoring phases.

"We've documented a stable population of tigers, apparently one of the healthiest on the island," says the work's leader, conservation biologist Joe Figel, who is working with Indonesian conservation and forestry agencies. "Our challenge on the ground now is to redouble our efforts and ensure these animals are properly protected," he adds.

The Leuser ecosystem is in many ways an ideal habitat for Sumatran tigers. It is about three times the size of Yellowstone National Park and is considered the largest continuous tract of suitable forest for tigers remaining in Sumatra. Lowland, foothill and montane forests are present, with about 44 per cent of the area categorised as "intact forest landscape".

"In addition, this area is patrolled more closely by rangers than most other areas on the island," notes Figel.

Together with locals living along the fringes of the study area, the team set up camera traps in northern Leuser, in Aceh province. In the first series - from March to May 2023 - 34 cameras were put up, in the second - from June to December 2023 - 59 cameras, and in the third - from May to November 2024 - already 74 devices.

Fiegel emphasises that multi-year camera trap surveillance is critical to assessing key population demographics: survival, recruitment, 'tenure' of plots and growth rates.

"Only by relying on such data can we truly assess the effectiveness of conservation measures," says the researcher.

During the monitoring period, the cameras captured 282 images of Sumatran tigers sharp enough to be individually identifiable. Based on the unique banding pattern, the scientists identified 27 individuals: 14 females, 12 males and one tiger of undetermined sex.

The relatively high number of animals indicates a sufficient number of prey necessary for their existence. On average, each female was captured about 14 times during the study period, each male about 16 times. The high density of females indicates a "healthy" social structure and quality habitats where tigresses can raise about three litters in a decade.

During the six-month session in 2023, three different broods of tiger cubs were documented. Two brothers, initially captured together as cubs, were later documented separately as adults.

Within the Leuser ecosystem lies Gunung Leuser National Park, but the current study was conducted in forests under the provincial protection of the Aceh government. Such areas receive significantly fewer resources than centrally funded national parks.

The results are all the more telling: the cameras installed by Figel and his colleagues captured almost three times as many images of tigers during the observation period as previous 90-day surveys in other parts of Sumatra. The researchers were also able to identify significantly more individuals than in most earlier work.

Previously, only three studies, all within national parks, had recorded more than 10 tigers in a single survey. Higher density estimates than in the present study have only been obtained in an area of particularly intensive conservation in southern Sumatra.

The data collected on animal movements could form the basis of future monitoring programmes, including helping to optimise camera spacing and placement patterns.

According to the authors, the high frequency of tiger sightings is the result of a complex of factors.

"Thanks to the work of government agencies, the support of the local communities of Aceh and Gayo, donors and other researchers, it has been possible to preserve important areas of lowland and foothill forests in Leuser, where tiger prey species densities are at their highest in Sumatra," Figel emphasises.