The Dao Tien Endangered Primate Species Centre, on an island a few minutes downriver, can show visitors several species, including gloriously exuberant gibbons that careen through the high tree canopy. At times they commence a sudden, nearly deafening concert. It sounds like a symphony of demented slide-whistles, sirens and the world’s loudest theremins. Gibbon song betrays the locations of these, our kindred species, to hunters in the national park across the river. They are sometimes shot out of the trees over there, and sold to be eaten in the cities.

Park rangers and others I spoke with at Cat Tien affirmed that its animal populations are declining; that some rangers have been caught colluding with hunters to bring down high-value animals (though they are said to be severely dealt with), and that the rangers earn something like $200 a month to start, which makes this a less-lucrative career option — and poaching an attractive one.

I stayed at the edge of the park in the well-appointed Cat Tien Jungle Lodge. Its proprietors, Duong Thi Ngoc Phuong and Gary Leong, work to help protect Cat Tien from mass tourism and to build economic ties with impoverished local communities to dissuade them from poaching. “Without the animals, there is little reason for the park’s existence,” Mr. Leong said. “We have to give everyone a stake in protecting them.”

That means, at least in part, creating economic incentives for local people to preserve native species in their natural habitats. And it needs to start soon, wildlife advocates say.

“Every day we all wake up and say, ‘do we have time? Do any of these species have time? Are we just fighting a war that we’ve already lost?’” said Quyen Vu, the executive director of Education for Nature-Vietnam. “But if we don’t fight, then we definitely have lost.”

Stephen Nash is the author of “Grand Canyon for Sale — Public Lands versus Private Interests in the Era of Climate Change.”