Even without poachers, Central Africa's forest elephants would need almost a century to get their numbers back up to 2002 levels, said a study Wednesday that pried into the elusive creatures' slow-breeding ways.
The population had been decimated by illegal hunting, with an estimated 65 percent decline between 2002 and 2013, said researchers.
Roaming the tropical forests of Cameroon, Central African Republic, Congo, Gabon and Democratic Republic of Congo, the tusker sub-species is thought to have numbered about one to two million at its peak, study co-author George Wittemyer of Colorado State University told AFP.
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In 1993, the rough estimate was 500,000, and in 2013 some 100,000.
"The forest populations are reproducing now, though at a very slow rate," Wittemyer said by email.
"The problem is that poaching is removing individuals at a rate that either drives the population to decline or negates any increases due to births."
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Forest elephants are smaller than savanna elephants -- the other, much better studied, African sub-species.
Their ears are more oval-shaped, while their tusks are straighter and point downward, according to environmental group WWF.
Targeted by poachers for their meat and ivory-bearing tusks, the forest elephant is categorized as "vulnerable", which means "facing a high risk of extinction in the wild," the WWF website says.
Wittemyer and a team analysed data obtained from decades-long, on-sight monitoring of the births and deaths of elephants at Dzanga Bai, a park in Central African Republic.
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In what is claimed to be the first-ever study of forest elephant demography, they concluded the creature was a much slower breeder than its open-air cousin.
Female forest elephants only start reproducing after the age of 20, and give birth once every five to six years, the team observed.
Their cousins from the savanna, by comparison, typically start breeding at 12 and produce a calf every three to four years.
"Their reported low birth rates mean that it will take forest elephants at least 90 years to recover" from poaching losses, the researchers said in a statement.
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The data suggested that what are considered sustainable levels of trade in forest elephant ivory, were calculated on the basis of overestimated population growth rates, they added.
This should be kept in mind when ivory trade limits are next debated, said the team -- crucially at a meeting of the Convention on International Trade in Endangered Species which opens in Johannesburg on September 24.
Forest elephants are crucial for their environment, and many tree species rely on the giants to disperse their seeds. The trees, in turn, absorb climate-altering greenhouse gases.