While some African elephants parade across the savanna and thrill tourists on safari, others are more discreet. They stay hidden in the forests, eating fruit.

"You feel pretty lucky when you catch sight of them," said Kathleen Gobush, a Seattle-based conservation biologist and member of the African Elephant Specialist Group within the International Union for Conservation of Nature, or IUCN.

The threat of extinction has diminished the odds of spotting one of these wood-dwelling elephants, said a new IUCN Red List assessment of African elephants. The Red List categorizes species by their risk of forever vanishing. The new assessment is the first in which the conservation union treats Africa's forest and savanna elephants as two species instead of one.

The last time the group assessed African elephants, in 2008, it listed them as vulnerable. Now it says savanna elephants are endangered, one category worse.

The shy forest elephants have lost nearly nine-tenths of their number in a generation and are now critically endangered — just one step from extinction in the wild.

Led by Gobush, the assessment team gathered data from 495 sites across Africa.

"We essentially looked at data from as far back as possible," Gobush said. But for the long-lived elephants, that's a challenge. The average savanna elephant mother gives birth at 25 years; forest elephant moms are 31 on average. Because the earliest surveys researchers could find were from the 1960s and 1970s, they could peer back only two generations for savanna elephants, and a single generation for forest elephants.

Even during those few decades, the changes were drastic. The population of savanna elephants has fallen at least 60%, the team found. Forest elephants have declined by more than 86%.

"That is alarming," said Ben Okita, a conservation biologist with Save the Elephants. Grouping the two elephants probably masked just how bad things were for the forest elephant, he said.

Alfred Roca, a geneticist at the University of Illinois at Urbana-Champaign, said the recognition of two African elephant species was a little tardy. More than two decades ago, a study of 295 skulls in museums found "enormous differences" between the two types of elephants, he said. Genetically, "The separation between them is probably greater than the separation between lions and tigers."

It will be especially hard for forest elephants to bounce back, Roca said, because of how long they wait to reproduce. The IUCN found that 70% might live outside protected areas, leaving them vulnerable to poachers.

Where elephants disappear, they leave a big gap. Some tree species depend entirely on forest elephants to eat their fruits, swallow their seeds and deposit them elsewhere. As they knock down trees and chew up plant material, forest and savanna elephants change their environments in ways that create new habitat for other species.

"Both of them really could be considered gardeners tending to the vegetation, more than probably any other animal," Gobush said. "We just can't afford to lose them."