Fan Peng-Fei of China's Dali University was worried the first time he entered the forest habitat of the critically endangered cao vit gibbon (Nomascus nasutus). The isolated forest, skirting the China–Vietnam border, had been heavily degraded by years of agricultural development, firewood collection and charcoal production. What little forest remained provided poor habitat for the arboreal gibbons, which are not adapted for life on the ground.

The cao vit gibbons (also known as eastern black-crested gibbons) are probably lucky to have as much habitat as they do. Not so long ago scientists feared that the species had gone extinct. But in 2002—about 40 years after they were last seen—the conservation organization Fauna & Flora International (FFI) discovered a small population of 26 gibbons in northeastern Vietnam. In 2006 another 10 were located across the border in China. The two governments established conservation areas in 2007 and 2009, and FFI has since helped protect the habitat from further degradation and the gibbons themselves from being hunted. Today, after a decade of conservation work, the gibbons are bouncing back. Several births last year raised the population of cao vit gibbons to 129. Although the species is still considered one of the world's 25 most endangered primates, its situation is far less precarious than it once was.

But the gibbons' mini population rebound has also created a bit of a problem. Their protected forest is not only still suffering from the effects of degradation, with the lowest level of canopy height in any known gibbon habitat, it is also woefully small: just 1,600 hectares in Vietnam and another 400 in China. According to a new paper co-authored by Fan and published April 12 in Biological Conservation, this will not be enough room in the long run. Fan says the gibbons are within a few generations of reaching the carrying capacity of their current habitat, meaning their forest home can't support many more animals. And because the forest has become isolated and fragmented by roads and other development, there is no easy way for young gibbons to disperse to new territories.

Fan and his colleagues have spent a lot of time over the past few years walking in the gibbons' habitat to study their diet, how they use their time and how far each subgroup travels. Evaluating the entire habitat, though, would not have been possible on foot, he says. For one thing, it's just too large. For another, he was allowed to spend only two weeks at a time in Vietnam, where most of the gibbons live. To speed things up the scientists turned to the sky for their new study, tapping high-resolution satellite images and remote-sensing data to fully evaluate the gibbons' existing habitat and the regions surrounding it.

The results show that the gibbons could have room to grow if the governments of China and Vietnam allow it. The researchers found another 3,400 total hectares of high-quality potential habitat and 1,125 hectares of moderate-quality habitat, although most of it was in fragmented smaller patches rather than completely continuous forest. They are now calling for a ban on agriculture and grazing in both countries within the three most suitable zones as well as the expansion of Vietnam's Cao Vit Gibbon Conservation Area to include new territory. They also suggest planting new food trees to help feed the gibbons and the establishment of forest corridors to connect the habitats.

Fan says it can be difficult to preserve a species such as the cao vit gibbon, which lives across international boundaries, "but our project has very good experiences in trans-boundary cooperation." Meetings were held in 2011 and 2012 to involve local governments and forestry departments, and an international conservation agreement was signed in 2011. "I think both governments will follow my suggestions," he predicts.

Even if both governments get onboard, there will still be struggles. Villages would need to be relocated, which is never a popular decision, and the habitat could take decades to recover. Even then it probably won't exactly thrive. The forest is in a rocky limestone landscape known as karst, with poor soil that Fan says may keep the trees from growing very tall. "It is very difficult to recover forest in karst areas," he says, "but I believe some trees will grow tall and the mean canopy height will increase in the future. I guess it will need around 20 years for the trees to grow to a size that can provide locomotion support for the gibbons." He says it may take even longer for the trees to start producing enough fruit to support the population.

Meanwhile, conservation work continues on a number of different fronts. Brian Crudge, FFI's Vietnam Primate Program Technical Advisor reports that their focus over the next few years includes "continuing livelihood development activities in the local communities, increasing scientific understanding and local awareness of the species, and improving conservation area management and law enforcement to ensure the continued protection and growth of the gibbon population and its habitat."

The two governments and FFI have done a remarkable job of protecting the cao vit gibbon since it was rediscovered a few years ago. We can only hope that the next few years continue to bring good news and further development toward the conservation of this rare species.

Previously in Extinction Countdown:

Photo: A male cao vit gibbon © Zhao Chao, courtesy of Fauna & Flora International