Plans are under way for a last-chance effort to save the world's rarest ape, known as the Hainan gibbon, with the species down to just 25 individuals living in a forest on a Chinese island.

Critically endangered in its small seven-square-mile habitat on Hainan Island, the gibbon is one of the rarest animals living anywhere in the world, conservationists say.

Just three population groups make up the 25 survivors, and if it goes extinct — a real possibility given its small population size and isolation — it could be the first ape species to go extinct because of human activity, as the apes have been heavily hunted and are also losing habitat to the spread of logging and rubber plantations.

The population has been at its precariously low level for about 30 years now, experts say, a mere fraction of the more than 2,000 Hainan gibbons living on the island at the southern tip of China in the 1950s.

"It's the world's rarest primate and the rarest mammal," said Samuel Turvey of the Zoological Society of London.

As the earliest of four gibbon lineages to split off from earlier ancestors, it's also evolutionarily unique, he said. "Its genetic distinctiveness makes saving it a global priority."

One of the proposals for saving the gibbons is building artificial bridges in the forest canopy to help them extend their range. Although the creatures are fast and agile when moving through the treetops, they are much slower, and more vulnerable, if they have to travel on the ground from one forest patch to another.

Fragmentation of the forest landscape by new roads, power lines and clearance for agriculture is one of the main reasons the gibbon population has been unable to increase, Turvey said.

"If they [the gaps] are tens of meters across rather than hundreds of meters, there are different ways of bridging those," he explained. "Long-term, it's reconnections through forest corridors over 20 years or so. Shorter-term, you can build canopy bridges."

To encourage the remaining gibbons to move into new habitats, some trial bridges built to look like natural forest canopy will be constructed later this year, the researchers said.

Although both the gibbons and their remaining habitats are protected under Chinese law, their continued existence is still in doubt, making programs to save them an urgent matter, Turvey said.

Of the 25 remaining gibbons, only about five are healthy females who could bear young, the researchers pointed out.

"One major priority is to make sure the existing population doesn't go extinct," Turvey said. "The other, more important, objective is to make sure it increases and expands geographically — that's the key."