Sumatra is the only place on earth where orang-utans, tigers, elephants and rhinoceros are found together. But it may not be so for much longer. Now, only remnant populations of each survive as their habitat is cleared for yet more plantations, and by illegal loggers.
Driving through North Sumatra, on rutted roads from dawn to dusk, you see little else but oil palm trees in ordered rows and dozens of trucks, their precious kernels piled high.
Arriving at Tangkahan comes as a relief. This tiny village sits across the river from the Gunung Leuser National Park, where the landscape is how it used to be - a jungle so tangled even walking through it would seem impossible.
But even the national park is not entirely safe and, in search of food, animals regularly stray into the plantations and gardens of villagers. At the jungle's edge, elephants are in danger of being poisoned - five have died in the neighbouring province of Aceh in the past six weeks, their young taken as pets or left to die.
In Tangkahan a desperate rearguard action is taking place. It's home to the seven Sumatran elephants housed by a non-government organisation, the Conservation Response Unit. The animals act as the main attraction of an ecotourism venture. Tourists spend a few days in the jungle, watching the elephants up close, washing them, feeding them and riding them into remote waterfalls and hot springs at the jungle's edge.
With its concrete huts and open-sided cafe, cold-water bucket showers and limited electricity, Tangkahan is at the purist end of ecotourism.
Half a day's drive away, also in North Sumatra, Bukit Lawang is at the opposite end of the spectrum. Since the 1970s, it has sold tourists on the idea of watching orang-utans swinging through their jungle home.
Thousands turn up every month to fully appointed hotels, and the riverside town has grown so popular that pollution, development and noise are growing. Now, some of the orang-utans are exhibiting signs of dangerous aggression.
Between these two extremes lies a series of questions. Can ecotourism ever out-compete the $30,000 per hectare that palm oil earns the Indonesian economy? And can it help save the island's forests and their unique animal inhabitants?
Since 1974, John Purba has been patrolling the area now called the Gunung Leuser park, which straddles North Sumatra and Aceh in Indonesia's far west. He's a park ranger but so poorly paid that he is forced to subcontract himself out as a tour guide "to feed my family, pay for school for my children".
Purba is well trained and experienced, and he's been working for years to convince the local people that they should help conserve this last redoubt of rainforest and the animals who live here.
"I tell the local people that money from tourism is good money. I say that is why I always need help from the local people to protect the forest," he says.
Most villagers here - landowners or not - make a living by harvesting oil kernels or tending the remains of the rubber plantations which predated them. Others grow fruit and vegetables, or cut trees down illegally and sell them by the piece. Stray too far from Bukit Lawang itself, and the idea that wild animals are anything more than a pest or a danger is a relatively new one, Purba says.
"I tell them European people like to come to Indonesia because of the orang-utans. Some people are surprised about that."
Dinan lives in Tangkahan, which started out as a logging town. As a schoolboy, he felled trees and floated them down the river to market. He also recalls being paid by entrepreneurs to take cattle into the jungle to feed.
Now he works as a guide. He has learnt English and earned enough money from tourism to build a house overlooking the river and the jungle. He feels guilty about his earlier life and, with the passion of a convert, is trying to sell others on the idea.
In a nearby village where "they still have illegal logging", though, he recently ran into trouble. "We said to them, 'You must try, you must keep the views good in your place - good for trekking, good for sleeping in the jungle'," Dinan says. "But they were aggressive. It's hard to convince Indonesian people that tourism can bring money."
Agung Kacaribu and his young crew take little convincing. They run the catering at the little, riverside restaurant in Tangkahan, shyly speaking to tourists, practising their English, and extolling the virtues of a healthy forest.
Agung wants to be a diplomat - a job he would no doubt never have contemplated without the input of foreigners. Meanwhile, he and local girls Lisa and Dewi Pusfitasari educate Westerners in traditional cooking styles.
But it's a tiny operation. The elephants, all refugees from the jungle, continue to require donor funding as part of a complex economic model that's not entirely settled.
Bukit Lawang, by contrast, is big business. It's billed as ecotourism, but the hotels are airconditioned and have hot, running water. There are dozens of restaurants, backpacker joints and even a shopping centre of sorts. The rubbish runs slightly less freely in the rivers than in other parts of Indonesia, but during my visit, nothing stopped the ear-splitting Indonesian pop music that played half the night.
Sonya Prosser, the marketing manager of Australian ecotourism outfit Raw Wildlife Encounters, says Tangkahan has not yet benefited the community enough, and Bukit Lawang is so big it's in danger of damaging the environment.
Here, unregistered and untrained local "guides" hand-feed orang-utans, wanting to guarantee their customers an encounter, but also risking the transmission of deadly diseases such as tuberculosis. Orang-utans are now showing behavioural problems and it has been suggested that one animal, Mina, be moved further into the forest after becoming aggressive - though she could easily return.
"This kind of thing is going to increase until somebody gets injured," Prosser says. "There are 200 registered rangers and guides, some of whom are not well trained, then you've got illegal ones as well. There are perhaps 300 all up."
Prosser says Bukit Lawang long since made the transition from ecotourism to mass tourism, and the animals are paying the price. Indonesia is still full of wild places and astonishing beauty, despite deforestation and environmental degradation. But, apart from Bali, it makes little effort to invite foreigners to see it, or to train Indonesians to show it.
At an official level it's all about managing natural resources; ecotourism is barely a blip on the radar. The tourism ministry in Jakarta referred us through five different offices on the hunt for some expertise. Last stop was the office for market development, which had figures on culinary tourism, golf tourism, diving tourism, even religious tourism, but nothing at all on ecotourism.
Non-government organisation the Indonesia Eco-Tourism Network also has no detailed figures, but says the sector is growing - the Tanjung Puting National Park in Borneo grew by 60 per cent last year. But spokeswoman Wita Simatupang says the industry cannot grow too fast, and the biggest handbrake is poor training and bad infrastructure.
About 90 minutes by motorbike down hopelessly rugged roads from Bukit Lawang is the untouched village of Batukatak, perched on the banks of the stunning Berkail river. Maybe 70 families live here - about 210 people - surviving on the earnings from rubber extraction and palm oil. The village doesn't even have a nasi goreng stall and the locals still gape openly when a "bule" (white person) rides in. But the nearby jungle does have truly wild orang-utans and elephants. Even a tiger comes to a spectacular nearby cave - full of stalactites and stalagmites - to give birth to her cubs. Purba hopes that this village will be the location of a new ecotourism venture.
"At the moment it's a stable village," Purba says. Asked to explain he says: "It means not all have enough to eat."
Local leader Ngalemi Sinuraya says his neighbours "know already that Western people like animals and like to save them".
"It means if we save the forest completely, the animals will be healthy," he adds. And ecotourism? "We'd like it. We'd really like it," the men gathered around him nodding in furious agreement over their cups of gritty Sumatran coffee. "They'd like to be more advanced in the village," says Purba. "If ecotourism comes here maybe all the people can have an income; maybe they can fry the bananas that visitors can buy."
But according to Prosser, if Raw Wildlife or any other developer were to operate in Batukatak, it would need to be a product of high value - the chance to see and help preserve forest and animals that can be seen nowhere else. What's needed, she says, is "fewer and more high-quality punters, who are prepared to pay a lot for a unique experience".
In a developing country like Indonesia, with a social and physical infrastructure that is naive at best, it's a tough ask. "But in the end there's only one way ecotourism can work to save the forests: the animals have to outvalue the palm oil."