WEBVTT 00:00:06.966 --> 00:00:08.830 We all know about the dinosaurs 00:00:08.830 --> 00:00:10.714 that once roamed the planet. 00:00:10.714 --> 00:00:12.966 But long after they went extinct, 00:00:12.966 --> 00:00:15.380 great beasts we call the megafauna 00:00:15.380 --> 00:00:17.964 lived on every continent. 00:00:17.964 --> 00:00:21.843 In the Americas, ground sloths the size of elephants 00:00:21.843 --> 00:00:24.216 pulled down trees with their claws. 00:00:24.216 --> 00:00:26.799 Sabor-toothed cats the size of brown bears 00:00:26.799 --> 00:00:28.266 hunted in packs, 00:00:28.266 --> 00:00:30.300 but they were no match for short-faced bears, 00:00:30.300 --> 00:00:33.216 which stood thirteen feet on their hind legs, 00:00:33.216 --> 00:00:35.131 and are likely to have driven these cats 00:00:35.131 --> 00:00:36.690 away from their prey. 00:00:36.690 --> 00:00:39.610 There were armadillos as big as small cars, 00:00:39.610 --> 00:00:41.131 an eight foot beaver, 00:00:41.131 --> 00:00:44.324 and a bird with a 26 foot wingspan. 00:00:44.324 --> 00:00:46.909 Almost everywhere, the world's megafauna 00:00:46.909 --> 00:00:50.381 were driven to extinction, often by human hunters. 00:00:50.381 --> 00:00:54.298 Some species still survive in parts of Africa and Asia. 00:00:54.298 --> 00:00:59.005 In other places, you can still see the legacy of these great beasts. 00:00:59.005 --> 00:01:00.882 Most trees are able to resprout 00:01:00.882 --> 00:01:02.547 where their trunk is broken 00:01:02.547 --> 00:01:04.966 to withstand the loss of much of their bark 00:01:04.966 --> 00:01:07.798 and to survive splitting, twisting and trampling 00:01:07.798 --> 00:01:12.132 partly because they evolved to survive attacks by elephants. 00:01:12.132 --> 00:01:14.632 The american pronghorn can run so fast 00:01:14.632 --> 00:01:17.799 because it evolved to escape the american cheetah. 00:01:17.799 --> 00:01:21.029 The surviving animals live in ghost ecosystems 00:01:21.029 --> 00:01:25.215 adapted to threats from species that no longer exist. 00:01:25.215 --> 00:01:28.381 Today, it may be possible to resurrect those ghosts, 00:01:28.381 --> 00:01:31.163 to bring back lost species using genetic material. 00:01:31.163 --> 00:01:33.216 For instance, there's been reserach in to 00:01:33.216 --> 00:01:36.716 cloning woolly mammoths from frozen remains. 00:01:36.716 --> 00:01:38.466 But even if it's not possible, 00:01:38.466 --> 00:01:41.340 we can still restore many of the ecosystems 00:01:41.340 --> 00:01:43.217 the world has lost. 00:01:43.217 --> 00:01:47.095 How? By making use of abandoned farms. 00:01:47.095 --> 00:01:49.133 As the market for food is globalized, 00:01:49.133 --> 00:01:51.966 infertile land becomes uncompetitive. 00:01:51.966 --> 00:01:54.186 Farmers in baren places can't compete 00:01:54.186 --> 00:01:57.382 with people growing crops on better land elsewhere. 00:01:57.382 --> 00:02:01.360 As a result, farming has started to retreat from many regions, 00:02:01.360 --> 00:02:03.988 and trees have started to return. 00:02:03.988 --> 00:02:07.116 One estimate claims that two-thirds of land in the US 00:02:07.116 --> 00:02:09.827 that was once forested but was cleared for farming 00:02:09.827 --> 00:02:11.632 has become forested again. 00:02:11.632 --> 00:02:13.832 Another estimate suggests that by 2030, 00:02:13.832 --> 00:02:16.166 an area in Europe the size of Poland 00:02:16.166 --> 00:02:18.299 will be vaccated by farmers. 00:02:18.299 --> 00:02:20.670 So even if we can't use DNA to bring back 00:02:20.670 --> 00:02:23.173 ground sloths and giant armadillos, 00:02:23.173 --> 00:02:25.509 we can restore bears, wolves, pumas 00:02:25.509 --> 00:02:27.466 lynx, moose and bison 00:02:27.466 --> 00:02:29.382 to the places where they used to live. 00:02:29.382 --> 00:02:32.019 Some of these animals can reshape their surroundings, 00:02:32.019 --> 00:02:35.518 creating conditions that allow other species to thrive. 00:02:35.518 --> 00:02:37.049 When wolves were reintrocued to 00:02:37.049 --> 00:02:39.938 the Yellowstone National Park in 1995, 00:02:39.938 --> 00:02:42.382 they quickly transformed the ecosystem. 00:02:42.382 --> 00:02:44.987 Where they reduced the numbers of overpopulated deer, 00:02:44.987 --> 00:02:47.071 vegitation began to recover. 00:02:47.071 --> 00:02:51.299 The height of some trees quintupled in just six years. 00:02:51.299 --> 00:02:54.632 As forests returned, so did song birds. 00:02:54.632 --> 00:02:57.706 Beavers, which eat trees, multiplied in the rivers, 00:02:57.706 --> 00:02:59.582 and their dams provided homes 00:02:59.582 --> 00:03:04.049 for otters, muskrats, ducks, frogs and fish. 00:03:04.049 --> 00:03:06.630 The wolves killed coyotes, allowing rabbits 00:03:06.630 --> 00:03:08.426 and mice to increase, 00:03:08.426 --> 00:03:11.011 providing more food for hawks, weasels, 00:03:11.011 --> 00:03:12.930 foxes and badgers. 00:03:12.930 --> 00:03:15.766 Bald eagles and ravens fed on the carrion 00:03:15.766 --> 00:03:17.603 that the wolves abandoned. 00:03:17.603 --> 00:03:19.632 So did bears, which also ate the berries 00:03:19.632 --> 00:03:21.966 on the returning shrubs. 00:03:21.966 --> 00:03:23.966 Bison numbers rose as they browsed 00:03:23.966 --> 00:03:26.153 the revitalized forests. 00:03:26.153 --> 00:03:29.549 The wolves changed almost everything. 00:03:29.549 --> 00:03:32.382 This is an example of a trophic cascade, 00:03:32.382 --> 00:03:34.410 a change at the top of the food chain 00:03:34.410 --> 00:03:36.799 that tumbles all the way to the bottom, 00:03:36.799 --> 00:03:38.466 affecting every level. 00:03:38.466 --> 00:03:41.295 The discovery of widespread trophic cascades 00:03:41.295 --> 00:03:44.085 may be one of the exciting scientifc findings 00:03:44.085 --> 00:03:45.920 of the past half century. 00:03:45.920 --> 00:03:48.131 They tell us that ecosystems that have lost 00:03:48.131 --> 00:03:50.633 just one or species of large animals 00:03:50.633 --> 00:03:53.215 can behave in radically different way 00:03:53.215 --> 00:03:54.931 from those that retain them. 00:03:54.931 --> 00:03:56.851 All over the world, new movements are trying 00:03:56.851 --> 00:03:59.215 to catalyze the restoration of nature 00:03:59.215 --> 00:04:01.716 in a process called rewilding. 00:04:01.716 --> 00:04:04.523 This means undoing some of the damage we've caused, 00:04:04.523 --> 00:04:07.049 reestablisihing species which have been driven out, 00:04:07.049 --> 00:04:08.737 and then stepping back. 00:04:08.737 --> 00:04:11.882 There is no attempt to create an ideal ecosystem, 00:04:11.882 --> 00:04:15.243 to produce a heath, a rainforest or a coral reef. 00:04:15.243 --> 00:04:17.787 Rewilding is about bringing back the species 00:04:17.787 --> 00:04:19.966 that drive dynamic processes 00:04:19.966 --> 00:04:22.337 and then letting nature take its course. 00:04:22.337 --> 00:04:25.134 But it's essential that rewilding must never be used 00:04:25.134 --> 00:04:27.967 as an excuse to push people off the land. 00:04:27.967 --> 00:04:29.716 It should happen only with the consent 00:04:29.716 --> 00:04:32.548 and enthusiasm of the people who work there. 00:04:32.548 --> 00:04:35.015 Imagine standing on a cliff in England, 00:04:35.015 --> 00:04:38.049 watching sperm whales attacking shoals of herring 00:04:38.049 --> 00:04:40.216 as they did within sight of the shore 00:04:40.216 --> 00:04:41.881 until the 18th century. 00:04:41.881 --> 00:04:43.549 By creating marine reserves 00:04:43.549 --> 00:04:45.382 in which no commerical fishing takes place, 00:04:45.382 --> 00:04:46.900 that can happen again. 00:04:46.900 --> 00:04:48.984 Imagine a European Serengeti 00:04:48.984 --> 00:04:51.322 full of the animals that used to live there: 00:04:51.322 --> 00:04:54.947 hippos, rhinos, elephants, hyenas and lions. 00:04:54.947 --> 00:04:57.132 What rewilding reintroduces 00:04:57.132 --> 00:05:00.163 alongside the missing animals and plants 00:05:00.163 --> 00:05:02.882 is that rare species called hope. 00:05:02.882 --> 00:05:05.001 It tells us that ecological change 00:05:05.001 --> 00:05:07.921 need not always proceed in the same direction. 00:05:07.921 --> 00:05:11.633 The silent spring could be followed by a wild summer.