Forget the illegal loggers. Think the long-tailed macaque in particular, the prevost's squirrel, even the endangered orang utan or any other mammal for that matter.
They are the new bad actors in Indonesian forestry - and across Borneo as well.
The trees under threat in Sumatra are not of the pristine, triple-canopy kind.
They are in the 1.5 million ha of acacia plantation from where the island's thriving pulp and paper industry now gets most of its wood.
But, as The Straits Times has learnt exclusively, the animals are only part of a much wider problem that forestry experts say threatens the sustainability of the plantations and could ultimately drive the multibilliondollar industry to the wall.
They say that by planting a species that is non-indigenous to the island through what is now three seven-year rotations, companies are facing the same ecological disaster that the Germans experienced with imported spruce in the late 1900s.
"They should have a natural forest with mixed species and a longer rotation with sound socio-economic principles, less corruption and better law enforcement," says one expert.
"It is simply not sustainable if they go on as they are." Nature, it appears, is now taking revenge on Asia Pulp and Paper (APP) and Asia Pacific Resources International (April), the two biggest companies that for years were reviled for the destruction of the Sumatran rainforest.
Since the turn of the current decade, local mammals have developed a taste for maturing Acacia mangium, the monoculture species that delivers commercial wood crops in as little as five years on degraded or infertile soil.
The animals gnaw off the bark to get at the sap and its sweet inner layers of phloem and cambium, either killing the tree or exposing it to ceratocystis - a fungal disease even more difficult to control than root rot.
The animals are causing so much havoc that companies are now busy substituting their acacia for the less tasty Eucalyptus pelita. It grows more slowly, and there is another significant drawback: It does not grow on Sumatra's peatlands.
The monkeys and squirrels have laid waste to half of the Acacia mangium, while root rot is destroying a similar percentage of Acacia crassicarpa, the species grown exclusively on peatland, which makes up about 60 per cent of the overall plantation area.
Dr Chris Beadle, a plant scientist with Australia's Commonwealth Scientific and Industrial Research Organisation, says root rot kills a tree in a year, rendering it commercially useless. Ceratocystis does the same damage in weeks.
Mill owners are already having to cut trees after only four years, which means a sharp reduction in resources.
As in the more valuable oil palm plantations, chemicals could be used to treat the acacia. But this would cost more than the wood itself.
From 1988 to 2003, an estimated 80 per cent of the 185 million cubic m of wood fibre processed by paper mills came from tropical hardwoods.
Under mounting pressure from environmentalists, most companies were expanding capacity at a faster rate than their plantations were reaching maturity. Its own hardwood resources close to exhaustion, APP finally signed a landmark agreement with The Forest Trust (TFT) and Greenpeace in February last year to end all natural forest-clearing in its supply chain. TFT says the company is sticking to the deal.