Destroyed habitats, poaching, and prey depletion have dramatically reduced tiger habitats around the world. Today, tigers occupy just 5 to 10% of their historical habitats. But on the Indonesian island of Sumatra, an important population of critically endangered Sumatran tigers may persevere, a new Frontiers in Conservation Science study showed.
Using infrared cameras, researchers working on the island, have set out to estimate sex-specific population densities and tigers' movements during three surveys.
"We documented a robust tiger population, apparently among the healthiest on the island," said Dr Joe Figel, a conservation biologist, who works with Indonesian wildlife and forestry agencies. "For those on the ground, the onus now falls on us to double down and adequately protect them."
Long-time tenants
In many ways, the Leuser ecosystem is ideal habitat for Sumatran tigers. Three times the size of Yellowstone National Park, it is the largest contiguous tiger habitat remaining in Sumatra. It's made up of lowland, hill, and montane forests, of which 44% are classified as intact forest landscape. "It's also more thoroughly patrolled by rangers than nearly any other place on the island," Figel said.
Working with local collaborators from communities at the edges of the study area, the team put up cameras in the northern stretches of Leuser, located in Aceh province, and kept them there for three monitoring periods: 34 cameras were installed during March to May 2023, 59 cameras between June and December 2023, and 74 cameras between May and November of 2024.
"Multi-year camera trap monitoring is critically important for estimating key tiger demographic parameters such as survival, recruitment, tenure, and population growth rate," explained Figel. "With these data – and only with these data – can we even begin to evaluate conservation efforts."
During the monitoring periods, the team captured a total of 282 sufficiently clear images of Sumatran tigers to allow for the identification of individuals. Analyzing stripe patterns, the team identified 27 individuals from camera-trap images, including 14 females, 12 males, and one tiger of unknown sex. The relatively high number of tigers suggests there is adequate prey in the area to support tiger presence. Over the study period, female and male individuals were photographed an average of 14 and 16 times, respectively. High densities of female tigers indicate a healthy tiger social system and high-quality habitats, where they can raise about three litters of cubs over a decade. During the six-month session in 2023, three different sets of cubs were documented. Two tiger brothers photographed together as cubs were later spotted individually as adults.
Thriving tigers
Inside the Leuser ecosystem lies Gunung Leuser Nation Park, however, the present study was conducted in forests provincially protected by the Aceh government. In Indonesia, provincially protected forests receive far fewer resources than national parks, which are supported and managed by the central government.
The camera traps placed by Figel and colleagues snapped nearly three times as many tiger images as during previous 90-day surveys at other sites in Sumatra, and the team was able to identify many more individuals than reported in earlier studies. Only three previous surveys – all carried out in protected national parks – documented more than 10 tigers in a single survey. Higher tiger density estimates than reported in the present study were only documented in an intensive protection zone in southern Sumatra.
The current study also provides valuable insights for future monitoring of tigers, the team said. The data on tiger movement collected here could, for example, inform survey protocols and optimal camera spacing.
The high numbers of tiger sightings reported here highlights a success story that is due to a multitude of factors, said the team. "Thanks to the work, activities, and support of government agencies, local Acehnese and Gayo communities, donors, and other researchers, Leuser has maintained important patches of lowland and hill forests where, in Sumatra, tiger prey densities reach their highest levels," concluded Figel. "The persistence of these habitats and prey populations are the main reasons for our findings."
