Eye contact. Through a tangle of vines and epiphytes. A face I'll not forget, looking at me looking at him with unearthly calm. His calm, not mine. He wore the grotesque face plate of a dominant male and his eyes were pure gold, but it was not the things that divided us that I sensed in that brief instant. It was the stuff we had in common. All great apes in the forest together.

This was an orang-utan, a wild animal that had come to socialise with the many half-tame, half-rehabilitated, more-than-half looked-after animals that lived in and around the place I was visiting, a centre for orang-utan victims: orphans and former illegal pets. It was hard to think clearly after this encounter, hard to work out if this was a tourist trap pure and simple, a good-hearted piece of animal welfare or a valuable contribution to conservation.

I made this trip to Borneo some years ago, but the spell of that brief moment of eye contact has never been broken. And my mind was filled with memories of that strange stare (what was he thinking? For thought was clearly something he was capable of,) this week when I attended the Great Ape Debate, set up by the World Land Trust at the Linnean Society in London. In fact, it took place in the same room that Charles Darwin and Alfred Russel Wallace presented a certain theory to the world 150 years ago.

These days it is not our origins, but our futures that are at issue. The place was packed with big names concerned with the future of great apes, all

discussing one big idea - what is the best thing that we can do for orang-utans? And it all comes down to an ancient dichotomy.

Twin priorities

It's one of those stories that you stumble across regularly enough: somebody finds a bird with a broken wing, and rings up the RSPB. “And do you know what? They didn't want to know!” That's because they don't. They will advise you to contact the RSPCA, which does want to know, and which will put you in touch with an accredited bird-mender.

That's because the RSPCA is an organisation concerned with animal welfare, and the RSPB is concerned with conservation - colleague organisations with different agendas. The RSPB doesn't mend birds, and the RSPCA doesn't run nature reserves. You could throw this back a little: welfare is concerned about individuals, while conservation operates at species level.

I have been in circles where these two notions are polarised: anyone interested in welfare is a bunny-hugger, a mere sentimentalist, while the tough-guy line on conservation, often accompanied by an unnatural enthusiasm for culling, can be equally alienating. The answer is not to take sides - indeed, the debate itself was not truly adversarial. The point is to work out what matters more: for orang-utans, and for that matter, for humans.

The greater crime

You can raise a lot of money quite quickly by showing pathetic pictures of caged orang-utans, or cute pictures of orang-utans in nappies. Save Ozzie the Orang! It's a lot sexier than Save a Frightfully Important Chunk of the Palaeo-Tropical Ecosystem. Ashley Leiman, founder of the Orangutan Foundation, produced a killing set of numbers. For $250,000 you can look after 350 captive orang-utans for a year. For the same sum, for the same period, you can look after a chunk of forest that contains 6,000 wild orang-utans.

Which makes it a no-brainer. Conservation of such tropical rainforest as we have left is as essential for humans as it is for orang-utans. The safeguarding of habitat has a million more benefits than the veterinary care of a single lost individual. But it's not as sexy.

This is a complex issue for conservation charities. It is far easier to raise money for something cuddly than for an abstract idea such as biodiversity. Charitable giving is frequently and very largely an emotional matter: and it is easier to engage emotionally with a cute baby covered in red fur than a load of trees.

It is important to understand that many, if not most people in conservation are not exactly opposed to welfare. People in conservation tend to be in it because they have powerful feelings for non-human life: it is impossible to harden your heart and say, damn welfare, let the orphan orang-utans go hang. It comes down to priorities. Where is our money best spent? Money raised for wildlife is always limited, no matter how sexy your pictures.

I'm as much in favour of welfare as anybody, for orang-utans or any suffering creature. But if the demands of welfare mean easing up on conservation, then welfare is a wrong priority. It's a crime to take an orang-utan from the forest. But the far greater crime is to take the forest from the orang-utan.

Times Recommends

• Copenhagen will fail – and quite right too
• They’re still wriggling to avoid the flak on Iraq
• Rule the waves? Not any more we don’t