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We all know about the dinosaurs

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that once roamed the planet,

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but long after they went extinct,

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great beasts we call the megafauna

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lived on every continent.

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In the Americas, ground sloths the size of elephants

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pulled down trees with their claws.

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Saber-toothed cats the size of brown bears

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hunted in packs,

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but they were no match for short-faced bears,

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which stood thirteen feet on their hind legs,

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and are likely to have driven these cats

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away from their prey.

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There were armadillos as big as small cars,

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an eight foot beaver,

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and a bird with a 26 foot wingspan.

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Almost everywhere, the world's megafauna

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were driven to extinction, often by human hunters.

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Some species still survive in parts of Africa and Asia.

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In other places, you can still see the legacy of these great beasts.

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Most trees are able to resprout

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where their trunk is broken

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to withstand the loss of much of their bark

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and to survive splitting, twisting and trampling,

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partly because they evolved to survive attacks by elephants.

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The American pronghorn can run so fast

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because it evolved to escape the American cheetah.

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The surviving animals live in ghost ecosystems

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adapted to threats from species that no longer exist.

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Today, it may be possible to resurrect those ghosts,

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to bring back lost species using genetic material.

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For instance, there's been research in to

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cloning woolly mammoths from frozen remains.

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But even if it's not possible,

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we can still restore many of the ecosystems

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the world has lost.

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How? By making use of abandoned farms.

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As the market for food is globalized,

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infertile land becomes uncompetitive.

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Farmers in barren places can't compete

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with people growing crops on better land elsewhere.

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As a result, farming has started to retreat from many regions,

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and trees have started to return.

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One estimate claims that two-thirds of land in the US

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that was once forested but was cleared for farming

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has become forested again.

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Another estimate suggests that by 2030,

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an area in Europe the size of Poland

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will be vaccated by farmers.

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So even if we can't use DNA to bring back

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ground sloths and giant armadillos,

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we can restore bears, wolves, pumas

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lynx, moose and bison

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to the places where they used to live.

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Some of these animals can reshape their surroundings,

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creating conditions that allow other species to thrive.

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When wolves were reintroduced to

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the Yellowstone National Park in 1995,

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they quickly transformed the ecosystem.

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Where they reduced the numbers of overpopulated deer,

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vegetation began to recover.

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The height of some trees quintupled in just six years.

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As forests returned, so did songbirds.

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Beavers, which eat trees, multiplied in the rivers,

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and their dams provided homes

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for otters, muskrats, ducks, frogs and fish.

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The wolves killed coyotes, allowing rabbits

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and mice to increase,

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providing more food for hawks, weasels,

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foxes and badgers.

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Bald eagles and ravens fed on the carrion

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that the wolves abandoned.

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So did bears, which also ate the berries

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on the returning shrubs.

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Bison numbers rose as they browsed

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the revitalized forests.

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The wolves changed almost everything.

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This is an example of a trophic cascade,

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a change at the top of the food chain

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that tumbles all the way to the bottom,

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affecting every level.

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The discovery of widespread trophic cascades

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may be one of the most exciting scientific findings

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of the past half century.

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They tell us that ecosystems that have lost

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just one or two species of large animals

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can behave in radically different ways

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from those that retain them.

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All over the world, new movements are trying

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to catalyze the restoration of nature

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in a process called rewilding.

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This means undoing some of the damage we've caused,

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reestablishing species which have been driven out,

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and then stepping back.

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There is no attempt to create an ideal ecosystem,

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to produce a heath, a rainforest or a coral reef.

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Rewilding is about bringing back the species

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that drive dynamic processes

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and then letting nature take its course.

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But it's essential that rewilding must never be used

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as an excuse to push people off the land.

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It should happen only with the consent

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and enthusiasm of the people who work there.

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Imagine standing on a cliff in England,

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watching sperm whales attacking shoals of herring

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as they did within sight of the shore

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until the 18th century.

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By creating marine reserves

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in which no commerical fishing takes place,

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that can happen again.

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Imagine a European Serengeti

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full of the animals that used to live there:

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hippos, rhinos, elephants, hyenas and lions.

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What rewilding reintroduces,

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alongside the missing animals and plants,

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is that rare species called hope.

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It tells us that ecological change

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need not always proceed in the same direction.

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The silent spring could be followed by a wild summer.