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It's not often you get the chance to walk 30 metres up in the treetops. This canopy walkway certainly gives a unique perspective on the diversity of the rainforest. It's been built in Sabah in Borneo. There's no doubt that there is a huge range of species here. But the flipside is that in any given area, there aren't that many individuals of each of those species

Forest Canopy Walkway

This tree is just coming into flower. But the thing is, if I look around the forest here, I can't see another of these trees. And this is bound to present a challenge when it comes to sex, to pollination. But it's a challenge that nature has risen to.

The simple problem is, how do you cross-pollinate to fertilise your seeds if you're all alone? The methods British trees make use won't work here. Below the canopy, there's virtually no wind. And relying on a general pollinator, such as a honeybee, is a bit like posting a letter without an address.

It might have collected YOUR pollen, but it's just as likely to deposit it on the wrong flowers. Now, what's required in the rainforest is special delivery.

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The nectar in these long flowers is out of the reach of most animals. Because these petals have evolved in harmony with a very specific pollinator… A Sunbird. With his long bill, he can reach the nectar deep within the flower. And he needs to get this nectar because he's evolved to feed on little else. And critically, when he's had his fill here, he'll have to find another tree with the same type of flowers. If you're a tree in the rainforest, this relationship is ideal.

For plants, specialist pollinators act as a form of insurance policy. You see, these animals have evolved to feed on your flowers and only your flowers. So once they visited you, no matter how far away your nearest neighbour is, they'll get there and effect pollination. And this is typical. Many of the trees and plants out here will have specialist pollinators. And because there are thousands of species, this in turn means that there are a vast array of these types of animal.

The final leg of my journey takes me to Amazonian Peru. Now we know why jungles are so productive, so diverse, I want to see how the whole ecosystem fits together.

There's one series of remarkable relationships that even in this distant rainforest involves me and you now, you may not know what this is, but I'm certain you know what this is. This is a brazil nut. And this is a brazil nut pod. Inside here are about 20 of these. Come down here. Because quite naturally, this has come from a brazil nut tree. And here is that tree. It's magnificent.

The story of why the Brazil nut needs the orchid is extraordinary. Each huge seed pod takes 12 months to grow. And this mighty tree owes its entire existence to connections that have taken science years to uncover.

One of the most intricate relationships takes place up there. Well, after a bit of a claim he is a plan that I was looking for, and saw all the way down there on the ground. It's an orchid. This little orchid is absolutely essential to the Brazil nut. In fact, if you've ever eaten one yourself, then you all the debt of gratitude to these flowers.

The story of why the Brazil nut needs the orchid is extraordinary. It's so wonderful, it's almost beyond belief. And through unravelling that story, we can understand some astonishing things.

It starts with those amazing-looking seed pods. How on Earth do they get dispersed? They're not only very heavy, they're unbelievably tough. I want to try a little experiment to find out. All I need is some fishing line and some little pink flags. I know this might seem a bit odd, but trust me, this is cutting-edge ecology.