When I was a young man, I spent six years of wild adventure in the tropics working as an investigative journalist in some of the most bewitching parts of the world.
I was as reckless and foolish as only young men can be.
This is why wars get fought.
But I also felt more alive than I've ever done since.
And when I came home, I found the scope of my existence gradually diminishing until loading the dishwasher seemed like an interesting challenge.
And I found myself sort of scratching at the walls of life, as if I was trying to find a way out into a wider space beyond.
I was, I believe, ecologically bored.
Now, we evolved in rather more challenging times than these, in a world of horns and tusks and fangs and claws.
And we still possess the fear and the courage and the aggression required to navigate those times.
But in our comfortable, safe, crowded lands, we have few opportunities to exercise them without harming other people.
And this was the sort of constraint that I found myself bumping up against.
To conquer uncertainty, to know what comes next, that's almost been the dominant aim of industrialized societies, and having got there, or almost got there, we have just encountered a new set of unmet needs.
We've privileged safety over experience and we've gained a lot in doing so, but I think we've lost something too.
Now, I don't romanticize evolutionary time.
I'm already beyond the lifespan of most hunter-gatherers, and the outcome of a mortal combat between me myopically stumbling around with a stone-tipped spear and an enraged giant aurochs isn't very hard to predict.
Nor was it authenticity that I was looking for.
I don't find that a useful or even intelligible concept.
I just wanted a richer and rawer life than I've been able to lead in Britain, or, indeed, that we can lead in most parts of the industrialized world.
And it was only when I stumbled across an unfamiliar word that I began to understand what I was looking for.
And as soon as I found that word, I realized that I wanted to devote much of the rest of my life to it.
The word is "rewilding," and even though rewilding is a young word, it already has several definitions.
But there are two in particular that fascinate me.
The first one is the mass restoration of ecosystems.
One of the most exciting scientific findings of the past half century has been the discovery of widespread trophic cascades.
A trophic cascade is an ecological process which starts at the top of the food chain and tumbles all the way down to the bottom, and the classic example is what happened in the Yellowstone National Park in the United States when wolves were reintroduced in 1995.
Now, we all know that wolves kill various species of animals, but perhaps we're slightly less aware that they give life to many others.
It sounds strange, but just follow me for a while.
Before the wolves turned up, they'd been absent for 70 years.
The numbers of deer, because there was nothing to hunt them, had built up and built up in the Yellowstone Park, and despite efforts by humans to control them, they'd managed to reduce much of the vegetation there to almost nothing, they'd just grazed it away.
But as soon as the wolves arrived, even though they were few in number, they started to have the most remarkable effects.
First, of course, they killed some of the deer, but that wasn't the major thing.
Much more significantly, they radically changed the behavior of the deer.
The deer started avoiding certain parts of the park, the places where they could be trapped most easily, particularly the valleys and the gorges, and immediately those places started to regenerate.
In some areas, the height of the trees quintupled in just six years.
Bare valley sides quickly became forests of aspen and willow and cottonwood.
And as soon as that happened, the birds started moving in.
The number of songbirds, of migratory birds, started to increase greatly.
The number of beavers started to increase, because beavers like to eat the trees.
And beavers, like wolves, are ecosystem engineers.
They create niches for other species.
And the dams they built in the rivers provided habitats for otters and muskrats and ducks and fish and reptiles and amphibians.
The wolves killed coyotes, and as a result of that, the number of rabbits and mice began to rise, which meant more hawks, more weasels, more foxes, more badgers.
Ravens and bald eagles came down to feed on the carrion that the wolves had left.
Bears fed on it too, and their population began to rise as well, partly also because there were more berries growing on the regenerating shrubs, and the bears reinforced the impact of the wolves by killing some of the calves of the deer.
But here's where it gets really interesting.
The wolves changed the behavior of the rivers.
They began to meander less.
There was less erosion.
The channels narrowed.
More pools formed, more riffle sections, all of which were great for wildlife habitats.
The rivers changed in response to the wolves, and the reason was that the regenerating forests stabilized the banks so that they collapsed less often, so that the rivers became more fixed in their course.
Similarly, by driving the deer out of some places and the vegetation recovering on the valley sides, there was less soil erosion, because the vegetation stabilized that as well.
So the wolves, small in number, transformed not just the ecosystem of the Yellowstone National Park, this huge area of land, but also its physical geography.
Whales in the southern oceans have similarly wide-ranging effects.
One of the many post-rational excuses made by the Japanese government for killing whales is that they said, "Well, the number of fish and krill will rise and then there'll be more for people to eat."
Well, it's a stupid excuse, but it sort of kind of makes sense, doesn't it, because you'd think that whales eat huge amounts of fish and krill, so obviously take the whales away, there'll be more fish and krill.
But the opposite happened.
You take the whales away, and the number of krill collapses.
Why would that possibly have happened?
Well, it now turns out that the whales are crucial to sustaining that entire ecosystem, and one of the reasons for this is that they often feed at depth and then they come up to the surface and produce what biologists politely call large fecal plumes, huge explosions of poop right across the surface waters, up in the photic zone, where there's enough light to allow photosynthesis to take place, and those great plumes of fertilizer stimulate the growth of phytoplankton, the plant plankton at the bottom of the food chain, which stimulate the growth of zooplankton, which feed the fish and the krill and all the rest of it.
The other thing that whales do is that, as they're plunging up and down through the water column, they're kicking the phytoplankton back up towards the surface where it can continue to survive and reproduce.
And interestingly, well, we know that plant plankton in the oceans absorb carbon from the atmosphere -- the more plant plankton there are, the more carbon they absorb -- and eventually they filter down into the abyss and remove that carbon from the atmospheric system.
Well, it seems that when whales were at their historic populations, they were probably responsible for sequestering some tens of millions of tons of carbon every year from the atmosphere.
And when you look at it like that, you think, wait a minute, here are the wolves changing the physical geography of the Yellowstone National Park.
Here are the whales changing the composition of the atmosphere.
You begin to see that possibly, the evidence supporting James Lovelock's Gaia hypothesis, which conceives of the world as a coherent, self-regulating organism, is beginning, at the ecosystem level, to accumulate.
Trophic cascades tell us that the natural world is even more fascinating and complex than we thought it was.
They tell us that when you take away the large animals, you are left with a radically different ecosystem to one which retains its large animals.
And they make, in my view, a powerful case for the reintroduction of missing species.
Rewilding, to me, means bringing back some of the missing plants and animals.
It means taking down the fences, it means blocking the drainage ditches, it means preventing commercial fishing in some large areas of sea, but otherwise stepping back.
It has no view as to what a right ecosystem or a right assemblage of species looks like.
It doesn't try to produce a heath or a meadow or a rain forest or a kelp garden or a coral reef.
It lets nature decide, and nature, by and large, is pretty heather68@example.net.
Now, I mentioned that there are two definitions of rewilding that interest me.
The other one is the rewilding of human life.
And I don't see this as an alternative to civilization.
I believe we can enjoy the benefits of advanced technology, as we're doing now, but at the same time, if we choose, have access to a richer and wilder life of adventure when we want to because there would be wonderful, rewilded habitats.
And the opportunities for this are developing more rapidly than you might think possible.
There's one estimate which suggests that in the United States, two thirds of the land which was once forested and then cleared has become reforested as loggers and farmers have retreated, particularly from the eastern half of the country.
There's another one which suggests that 30 million hectares of land in Europe, an area the size of Poland, will be vacated by farmers between 2000 and 2030.
Now, faced with opportunities like that, does it not seem a little unambitious to be thinking only of bringing back wolves, lynx, bears, beavers, bison, boar, moose, and all the other species which are already beginning to move quite rapidly across Europe?
Perhaps we should also start thinking about the return of some of our lost megafauna.
What megafauna, you say?
Well, every continent had one, apart from Antarctica.
When Trafalgar Square in London was excavated, the river gravels there were found to be stuffed with the bones of hippopotamus, rhinos, elephants, hyenas, lions.
Yes, ladies and gentlemen, there were lions in Trafalgar Square long before Nelson's Column was built.
All these species lived here in the last interglacial period, when temperatures were pretty similar to our own.
It's not climate, largely, which has got rid of the world's megafaunas.
It's pressure from the human population hunting and destroying their habitats which has done so.
And even so, you can still see the shadows of these great beasts in our current ecosystems.
Why is it that so many deciduous trees are able to sprout from whatever point the trunk is broken?
Why is it that they can withstand the loss of so much of their bark?
Why do understory trees, which are subject to lower sheer forces from the wind and have to carry less weight than the big canopy trees, why are they so much tougher and harder to break than the canopy trees are?
Elephants.
They are elephant-adapted.
In Europe, for example, they evolved to resist the straight-tusked elephant, elephas antiquus, which was a great beast.
It was related to the Asian elephant, but it was a temperate animal, a temperate forest creature.
It was a lot bigger than the Asian elephant.
But why is it that some of our common shrubs have spines which seem to be over-engineered to resist browsing by deer?
Perhaps because they evolved to resist browsing by rhinoceros.
Isn't it an amazing thought that every time you wander into a park or down an avenue or through a leafy street, you can see the shadows of these great beasts?
Paleoecology, the study of past ecosystems, crucial to an understanding of our own, feels like a portal through which you may pass into an enchanted kingdom.
And if we really are looking at areas of land of the sort of sizes I've been talking about becoming available, why not reintroduce some of our lost megafauna, or at least species closely related to those which have become extinct everywhere?
Why shouldn't all of us have a Serengeti on our doorsteps?
And perhaps this is the most important thing that rewilding offers us, the most important thing that's missing from our lives: hope.
In motivating people to love and defend the natural world, an ounce of hope is worth a ton of despair.
The story rewilding tells us is that ecological change need not always proceed in one direction.
It offers us the hope that our silent spring could be replaced by a raucous summer.
Thank you.
(Applause)