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  • We all know about the dinosaurs

  • that once roamed the planet

  • but long after they went extinct,

  • great beasts we call the megafauna lived on every continent.

  • In the Americas, ground sloths the size of elephants

  • pulled down trees with their claws.

  • Saber-toothed cats the size of brown bears

  • hunted in packs,

  • but they were no match for short-faced bears,

  • which stood thirteen feet on their hind legs,

  • and are likely to have driven these cats away from their prey.

  • There were armadillos as big as small cars,

  • an eight foot beaver,

  • and a bird with a 26 foot wingspan.

  • Almost everywhere, the world's megafauna were driven to extinction,

  • often by human hunters.

  • Some species still survive in parts of Africa and Asia.

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

  • Most trees are able to resprout where their trunk is broken

  • to withstand the loss of much of their bark

  • and to survive splitting, twisting and trampling,

  • partly because they evolved to survive attacks by elephants.

  • The American pronghorn can run so fast

  • because it evolved to escape the American cheetah.

  • The surviving animals live in ghost ecosystems

  • adapted to threats from species that no longer exist.

  • Today, it may be possible to resurrect those ghosts,

  • to bring back lost species using genetic material.

  • For instance, there's been research in to

  • cloning woolly mammoths from frozen remains.

  • But even if it's not possible,

  • we can still restore many of the ecosystems the world has lost.

  • How? By making use of abandoned farms.

  • As the market for food is globalized,

  • infertile land becomes uncompetitive.

  • Farmers in barren places can't compete with people growing crops on better land elsewhere.

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

  • and trees have started to return.

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

  • that was once forested but was cleared for farming

  • has become forested again.

  • Another estimate suggests that by 2030,

  • an area in Europe the size of Poland will be vacated by farmers.

  • So even if we can't use DNA to bring back ground sloths and giant armadillos,

  • we can restore bears, wolves, pumas

  • lynx, moose and bison

  • to the places where they used to live.

  • Some of these animals can reshape their surroundings,

  • creating conditions that allow other species to thrive.

  • When wolves were reintroduced to the Yellowstone National Park in 1995,

  • they quickly transformed the ecosystem.

  • Where they reduced the numbers of overpopulated deer,

  • vegetation began to recover.

  • The height of some trees quintupled in just six years.

  • As forests returned, so did songbirds.

  • Beavers, which eat trees, multiplied in the rivers,

  • and their dams provided homes for otters, muskrats, ducks, frogs and fish.

  • The wolves killed coyotes, allowing rabbits and mice to increase,

  • providing more food for hawks, weasels, foxes and badgers.

  • Bald eagles and ravens fed on the carrion that the wolves abandoned.

  • So did bears, which also ate the berries on the returning shrubs.

  • Bison numbers rose as they browsed the revitalized forests.

  • The wolves changed almost everything.

  • This is an example of a trophic cascade,

  • a change at the top of the food chain

  • that tumbles all the way to the bottom,

  • affecting every level.

  • The discovery of widespread trophic cascades

  • may be one of the most exciting scientific findings of the past half century.

  • They tell us that ecosystems

  • have lost just one or two species of large animals

  • can behave in radically different ways from those that retain them.

  • All over the world, new movements are trying

  • to catalyze the restoration of nature in a process called rewilding.

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

  • reestablishing species which have been driven out,

  • and then stepping back.

  • There is no attempt to create an ideal ecosystem,

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

  • Rewilding is about bringing back the species

  • that drive dynamic processes

  • and then letting nature take its course.

  • But it's essential that rewilding must never be used

  • as an excuse to push people off the land.

  • It should happen only

  • with the consent and enthusiasm of the people who work there.

  • Imagine standing on a cliff in England,

  • watching sperm whales attacking shoals of herring

  • as they did within sight of the shore until the 18th century.

  • By creating marine reserves in which no commerical fishing takes place,

  • that can happen again.

  • Imagine a European Serengeti

  • full of the animals that used to live there:

  • hippos, rhinos, elephants, hyenas and lions.

  • What rewilding reintroduces,

  • alongside the missing animals and plants,

  • is that rare species called hope.

  • It tells us that ecological change

  • need not always proceed in the same direction.

  • The silent spring could be followed by a wild summer.

We all know about the dinosaurs

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B2 TED-Ed bison land driven survive food chain

【TED-Ed】From the top of the food chain down: Rewilding our world - George Monbiot

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    李掌櫃 posted on 2014/03/16
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