行星地球第七集：遼闊平原 Great Plains

[00:27.84]Vast open plains. [00:31.16] [00:31.79]Immense spaces. [00:34.39] [00:35.05]Eerie silence [00:37.25] [00:39.71]but any feeling of emptiness is an illusion. [00:43.20] [00:46.64]The plains of our planet [00:48.12] [00:48.22]support the greatest gatherings of wildlife on Earth. [00:52.50] [00:56.92]At the heart of all that happens here [00:59.19] [00:59.31]is a single living thing. [01:02.24] [01:03.38]Grass. [01:04.80] [01:08.30]This miraculous plant covers a quarter of all the lands of the Earth. [01:14.54] [01:28.63]Grasslands exist [01:30.39] [01:30.54]where ever there is a little rain, but not enough to sustain the forests. [01:35.69] [01:36.06]Some are huge. [01:38.15] [01:38.42]The Central Asian Steppe alone [01:40.38] [01:40.52]extends one third of the way around our planet. [01:45.03] [01:49.96]It's summer [01:50.98] [01:51.08]and eagles effortlessly cruise the thermals [01:54.20] [01:54.32]scanning the ground beneath for signs of prey. [01:57.98] [02:00.21]In the distant reaches of Outer Mongolia [02:02.79] [02:02.99]one of the planet's great migrations is underway. [02:07.35] [02:12.98]Few people ever see this extraordinary annual event. [02:17.81] [02:25.28]Mongolian gazelle. [02:27.34] [02:27.65]Two million are thought to live here but no one really knows. [02:32.93] [02:34.83]For much of the time they're scattered through this vast landscape [02:38.68] [02:38.80]but once a year they come together to have their young. [02:42.77] [02:44.02]Nearly all will give birth within the next ten days. [02:48.87] [02:53.05]Out in the open, communal calving is the safest way to have young. [02:57.84] [02:58.10]With so many pairs of eyes keeping watch [03:00.29] [03:00.37]it's almost impossible for predators to sneak up. [03:04.16] [03:10.01]There are no bushes, no trees, there's only one thing to hide behind, [03:14.84] [03:15.43]grass [03:17.44] [03:18.79]and it's not very effective. [03:21.61] [03:28.69]Predators also have a hard time raising their young on open grassland. [03:33.58] [03:33.79]Without trees eagles have to nest directly on the ground. [03:38.61] [03:41.14]All inhabitants of the Great Plains are exposed to the elements. [03:46.80] [04:03.44]Fire sparks panic in the herd. [04:07.16] [04:13.08]Gazelles are born to run, and even young calves easily outpace the flames [04:18.31] [04:18.65]if they can avoid being trampled. [04:21.29] [04:54.57]With nothing to stand in it's way the blaze consumes anything that can't flee. [05:00.77] [05:01.34]Huge quantities of grass, valuable food have been lost [05:06.16] [05:10.26]and with it the old and the weak. [05:13.98] [05:18.50]The gazelles move on to new pastures and leave the desolation behind them. [05:24.91] [05:27.47]From the ashes rises the phoenix. [05:30.99] [05:31.37]Grass the incredible survivor. [05:35.63] [05:37.50]Because it grows from a protected part at the base of its stems, [05:41.53] [05:41.71]grass is almost indestructible. [05:45.48] [05:55.56]Able to repair and reproduce itself rapidly [05:58.69] [05:58.77]it covers more of the Earth's land than any other plant [06:02.20] [06:02.56]and feeds more wildlife than any other. [06:06.28] [06:25.83]Red billed quelea. [06:27.80] [06:28.11]One and a half billion swarm across the savannas of Africa. [06:32.06] [06:32.37]These are the most numerous birds on Earth. [06:37.34] [06:40.52]Some flocks are so vast, that they could take five hours to pass overhead. [06:47.06] [07:18.95]Only grass can feed plagues of these proportions. [07:23.56] [07:23.74]The ravenous hordes devour the seeds [07:27.36] [07:27.87]and the leaves and stems are cropped by great herds of antelope. [07:33.31] [07:43.34]The East African savannas alone sustain nearly two million wildebeest. [07:49.68] [08:01.17]They trim the grass down to it's roots leaving little in their wake [08:05.38] [08:05.81]but within days the plant will recover and continue to sustain the biggest herds on Earth. [08:13.28] [08:23.79]Grass is not confined to the tropics. [08:28.00] [08:31.54]It manages to grow even in the bitter conditions of the Arctic. [08:36.57] [08:44.00]Beyond the limits of the last tree the planet is barren and ice locked. [08:49.80] [08:50.11]The frozen no man's land at the end of the Earth. [08:53.93] [08:54.55]But, for a short time each year, the long dark winter releases it's grip. [09:01.68] [09:12.09]Temperatures rise, and grass that has lain dormant and frozen throughout the winter [09:17.40] [09:17.53]sprouts once more. [09:20.25] [09:24.45]Green returns to the Arctic. [09:27.84] [09:28.71]The receding ice reveals an immense flat plain, the size of Australia. [09:34.17] [09:34.35]This is the Arctic tundra. [09:37.71] [09:43.59]It's a desolate silent wilderness [09:46.77] [09:46.97]but it's about to change. [09:50.15] [10:05.78]Snow geese. [10:08.24] [10:09.65]They winter along the Gulf of Mexico [10:12.21] [10:12.21]and in spring they fly the entire length of North America to reach the Arctic tundra. [10:18.78] [10:19.63]Five million birds make this journey every year. [10:24.55] [10:39.86]Their marathon migration is almost three thousand miles long [10:44.22] [10:44.27]and has taken them three months. [10:47.51] [10:53.25]Exhausted and starving, they touch down inside the Arctic Circle [10:58.30] [10:58.38]back at their traditional breeding grounds, at last. [11:02.54] [11:04.02]Snow geese pair for life. [11:06.18] [11:06.46]As soon as couples arrive, they must stake a claim to a nesting patch. [11:12.26] [11:15.13]Ideal sites are in short supply and quarrelsome neighbors are all around. [11:21.11] [11:24.24]Disputes can be vicious. [11:26.73] [11:59.79]Point taken. [12:01.82] [12:03.66]It's a long way to travel, but for a short period the tundra is the ideal place for a grazer. [12:10.18] [12:10.41]The grass grows vigorously during the short intense summer [12:13.82] [12:13.95]and the are fewer predators than farther south. [12:17.62] [12:21.21]Here, geese can nest on the ground in relative safety [12:25.00] [12:25.41]nonetheless this female must incubate her eggs for three weeks [12:29.39] [12:29.49]and throughout this time she will be very vulnerable. [12:33.54] [12:46.81]An Arctic fox surveys the colony. [12:49.63] [12:49.78]She's been waiting for the geese all winter. [12:53.04] [12:58.12]Sneaking up unnoticed is impossible. [13:01.73] [13:10.84]Perhaps fortune will favor the bold. [13:14.43] [13:28.49]The colony is well defended. There are no easy pickings here. [13:33.28] [13:44.08]She's driven away from every nest, but hunger compels her to continue. [13:49.60] [14:16.82]There are more eggs here than she can possibly eat now [14:20.33] [14:20.33]but the nesting season is short [14:22.93] [14:23.14]so she stashes much of her plunder for later in the year [14:26.54] [14:26.58]when all the geese have gone. [14:29.13] [14:40.21]Further south, other bigger predators prowl the tundra. [14:45.36] [14:47.21]Wolves. [14:48.42] [14:48.58]For them, finding food on the plains is an even greater challenge. [14:52.99] [14:53.18]Not only is their prey seasonal it's also hard to find. [14:56.85] [14:56.91]They've been searching for days without a sign. [15:00.46] [15:06.99]Somewhere in this immense landscape there is food for them. [15:12.36] [15:18.56]This is it. Caribou. [15:21.35] [15:23.26]Travelling thirty miles a day [15:25.25] [15:25.35]they can cover nearly two thousand miles during the summer months. [15:30.87] [15:37.68]The wolves will starve if they don't find the caribou [15:41.05] [15:41.40]but it's no easy task to locate prey that never stops traveling. [15:46.71] [16:06.23]Biting flies and the quest for new pasture [16:09.19] [16:09.25]drives the migration ever onwards. [16:12.47] [16:47.73]A wolf has finally picked up the trail. [16:51.65] [16:52.49]The caribou are close. [16:55.08] [16:59.18]At last, a chance. [17:01.67] [17:04.42]The hunt is on. [17:07.27] [17:17.20]The wolf panics the herd, and the weak and young are singled out. [17:22.15] [17:30.14]A calf is separated from it's mother. [17:33.65] [18:09.54]At the goose colony it's high summer and eggs are hatching. [18:14.38] [18:16.57]The young all emerge within a day or two. A marvel of timing. [18:20.99] [18:21.07]The colony is now home to a million goslings. [18:25.62] [18:40.83]The fox is still gathering all she can get. [18:45.12] [18:48.93]Sometimes one mouth simply isn't enough. [18:53.37] [19:26.89]One will have to do. [19:29.78] [19:35.13]Not all food is stored. [19:37.15] [19:37.37]Some is needed right now. [19:39.77] [19:41.21]She has seven hungry cubs to feed. [19:44.53] [19:50.61]As their appetites grow, the mother must work tirelessly to raise her family. [19:55.16] [19:55.51]Only fat healthy cubs will survive the Arctic winter. [20:00.03] [20:15.11]The vast majority of the goslings are still flourishing. [20:18.74] [20:19.05]Their parents lead them down to the safety of the water [20:21.98] [20:22.07]as soon as they're strong enough to make the journey. [20:25.23] [20:30.64]For the foxes boom time has come to an end [20:34.38] [20:34.71]but the mother has given her cubs the best possible start in life. [20:39.61] [20:41.77]The geese will continue grazing the tundra [20:44.29] [20:44.37]until the summer ends, and they're forced to head south for the winter. [20:49.11] [20:56.85]At these latitudes the Sun's rays are week [20:59.91] [20:59.97]and grass can only grow here for a few months a year [21:03.09] [21:03.41]but further south summers are longer and the grasslands flourish. [21:09.10] [21:22.80]The prairies of North America. [21:25.84] [21:27.52]This rich pasture once supported the greatest herds ever seen on our planet. [21:33.97] [21:43.20]There were once sixty million bison [21:45.75] [21:46.00]but no animal is immune to intensive hunting by man [21:49.56] [21:49.73]or the destruction of it's habitat [21:51.94] [21:52.25]and a century ago the bison were reduced to barely a thousand. [21:57.28] [21:57.87]Now, thanks to rigorous protection, the species is recovering. [22:02.91] [22:07.67]The growing season is long [22:09.57] [22:09.70]and the grass here can support herds all year around. [22:13.82] [22:19.29]Male bison weigh in at one ton. [22:22.84] [22:38.33]In high summer the bulls are fat from the rich grazing [22:41.62] [22:41.73]and in prime condition but only a few will mate. [22:45.10] [22:45.29]Exactly which few is about to be decided. [22:48.86] [23:19.82]On temperate plains around the world summer is a time for growth and reproduction. [23:26.69] [23:28.89]Now the grass produces it's flowers. [23:32.53] [23:44.56]New colors also come to the plains. [23:48.72] [24:24.24]The northern flowering is mirrored by the grasslands of the southern hemisphere. [24:29.74] [24:29.99]And nowhere is more impressive than on the velt of South Africa. [24:35.21] [25:21.39]Not all temperate plains are so rich and colorful in the summer. [25:27.31] [25:33.93]This is midsummer on the Tibetan Plateau [25:37.57] [25:37.60]the highest great plain in the world. [25:40.71] [25:41.96]Despite the conditions, grass survives, and in sufficient quantities [25:46.26] [25:46.34]to support the highest of all grazing herds [25:49.78] [25:49.98]those of the wild yak. [25:52.28] [25:55.22]Even in summer life is hard [25:57.85] [25:58.01]temperatures rarely rise above freezing, and the air is thin. [26:02.90] [26:09.14]It's also exceptionally dry for one very big reason [26:14.41] [26:17.56]the Himalayas. [26:19.45] [26:21.57]The great mountain range acts as a barrier [26:24.54] [26:24.67]preventing clouds moving in from the south [26:27.28] [26:27.41]and this casts a giant rain shadow that leaves Tibet high and dry. [26:33.29] [26:51.18]Grass clings to life [26:53.41] [26:53.51]even as desiccating winds remove what little moisture remains in the soil. [26:59.49] [27:08.20]So long as grass can survive, so can grazers. [27:12.85] [27:17.56]Wild ass. [27:19.46] [27:27.92]The males are fighting to win territories [27:30.56] [27:30.84]those that hold the best, are more likely to attract a herd of females. [27:35.96] [27:51.39]It's a frisky business. [27:54.46] [27:58.69]That counts as a victory [28:00.38] [28:00.55]but he can't assume the females will actually turn up. [28:04.34] [28:09.03]Female asses are mysterious creatures. [28:12.56] [28:15.01]They come and go as they please and much of their behavior seems unfathomable [28:20.08] [28:20.28]to an outsider. [28:22.12] [28:33.22]They're the great nomads of the plateau [28:35.58] [28:35.86]and will often trek vast distances across these parched plains in search of oases. [28:42.43] [28:43.71]But when they do find paradise [28:45.97] [28:46.09]they're liable to feed and drink for just a few hours [28:49.37] [28:49.50]and then head back to the dust for no apparent reason. [28:53.93] [28:58.17]Wild ass are the most conspicuous pioneers of this high frontier [29:03.18] [29:03.34]but the most numerous grazer in Tibet lives underground. [29:08.02] [29:11.27]Pika [29:11.97] [29:12.40]A relative of the rabbit. It too feeds on grass. [29:16.44] [29:21.94]On the exposed plateau [29:23.97] [29:24.12]pikas never stray far from their burrows [29:26.76] [29:26.95]but even so, squatters will move in given half a chance. [29:32.40] [29:44.53]While ground peckers and snow finches can be a nuisance [29:48.40] [29:48.48]they're worth tolerating,because they provide a valuable early warning system. [29:53.98] [30:06.92]The bizarre Tibetan fox. The pika's nemesis. [30:11.65] [30:34.82]When stalking, it keeps below the skyline [30:38.42] [30:38.73]perhaps helped by it's curious body shape. [30:42.23] [30:42.85]But why the square head? [30:45.17] [31:36.72]In summer, the Tibetan plateau heats up [31:40.03] [31:40.15]drawing in warm wet air from the south [31:43.14] [31:43.40]but the water never arrives. [31:45.89] [31:46.08]As the moist air approaches it's forced upwards by the Himalayas [31:50.20] [31:50.28]and condenses into huge rain clouds. [31:53.99] [32:08.11]These clouds drop all their water on the southern side of the mountains. [32:12.96] [32:13.25]The very peaks that keep Tibet dry [32:15.92] [32:15.94]are responsible for the monsoon rains falling farther south, and the greening of India. [32:22.44] [32:31.94]Here, soaked by rain and bathed in tropical sun [32:35.64] [32:35.76]grass reaches it's full potential. [32:39.31] [32:41.16]Elephant grass is the tallest in the world. [32:45.20] [32:57.00]Grass that towers over an elephant, can conceal all sorts of surprises. [33:03.30] [33:11.34]The male lesser florican. [33:13.62] [33:13.80]It's hard work getting noticed when you go courting in the high grass. [33:19.73] [33:32.21]The long grass plains of tropical India are home to some of the largest [33:36.66] [33:36.66]grass eating mammals on our planet... [33:38.86] [33:39.53]and some of the smallest. [33:42.33] [33:43.82]Pygmy hogs are no bigger than rabbits. [33:46.86] [33:46.86]They're the tiniest and rarest of all wild pigs. [33:51.29] [33:55.54]The female is busy collecting grass but not for eating. [34:00.73] [34:07.99]She's building a nest. [34:10.15] [34:24.22]Each piglet may be small enough to fit in the palm of a hand [34:28.37] [34:28.61]but she does have ten. [34:30.91] [34:44.42]This is how grass can grow given unlimited sunshine and water [34:49.85] [34:50.27]but on most tropical plains across our planet [34:53.26] [34:53.37]the wet season is followed by a dry one. [34:56.81] [35:06.40]On the African savannas, grazers are marching in search of grass and water. [35:13.02] [35:33.07]Without rain, these plains can become dust bowls. Grass can now loose it's hold. [35:40.40] [35:49.63]Elephants are in immediate danger. [35:52.30] [35:52.42]They must drink almost daily. [35:55.60] [36:00.69]Driven on by thirst, they march hundreds of miles across the parched plains. [36:06.81] [36:24.29]Relying on memory, the matriarchs lead their families to those special water holes [36:30.51] [36:30.59]that saved them in previous years. [36:33.32] [36:39.70]This one still has water [36:42.14] [36:42.18]but they must share what remains with desperately thirsty animals of all kinds. [36:48.71] [37:05.02]These are tense times. [37:08.02] [37:23.78]The elephants dominate the water hole [37:26.44] [37:26.63]but as night falls the balance of power will shift. [37:31.18] [37:43.64]Thirsty herds continue to arrive throughout the night. [37:47.83] [37:47.97]It's a cooler time to travel. [37:51.01] [38:10.34]In the darkness the tables turn. [38:13.83] [38:17.44]The elephant's night vision is little better than our own [38:21.16] [38:24.42]but lions have much more sensitive eyes. [38:28.65] [38:41.14]The cats are hungry and the elephants seem to sense it. [38:44.60] [39:24.77]Lions don't usually hunt elephants, but desperate times require desperate measures. [39:32.44] [39:32.54]This herd contains calves easier targets. [39:37.19] [39:41.16]But how to reach them? [39:43.91] [39:58.44]The adults encircle their young. [40:00.77] [40:00.77]It's an impenetrable wall of grey. [40:04.39] [40:31.48]A few exhausted stragglers are still arriving. [40:35.61] [40:39.25]One of them is alone. [40:42.51] [41:01.21]But it's too big for the lions to tackle. [41:05.60] [41:09.90]This one looks a little smaller. [41:13.11] [41:25.42]A solitary lion stands no chance [41:28.89] [41:29.50]but the whole pride is here. [41:31.60] [41:35.35]There are thirty of them, and they're specialist elephant hunters. [41:39.84] [42:45.18]This elephant will feed the whole pride for at least a week. [42:50.75] [42:59.83]Elephants know these drinking holes are dangerous, but they have no choice. [43:05.78] [43:06.14]The dramas that play out here are a savage reminder [43:10.04] [43:10.14]of how important water is for all life on these plains. [43:15.25] [43:23.50]As the dry season finally draws to a close [43:26.81] [43:26.89]Africa's baked savannas undergo a radical change. [43:32.10] [43:33.48]Rain sweeps across the continent and grass, the great survivor, rises again [43:40.33] [43:40.79]and the herds return. [43:43.74] [43:45.67]For months they've been scattered over huge areas, clinging to existence around tiny water holes. [43:52.85] [43:53.00]Now, the good times are back. [43:56.95] [44:01.72]A few African savannas are very special. [44:05.54] [44:05.80]Here, rain water from far and wide flows across the flat plains. [44:11.57] [44:11.88]Grass is submerged but still it grows. [44:15.96] [44:36.07]Flooded, burnt, baked and frozen grass can withstand it all. [44:43.12] [44:47.58]After six months of drought, grass replaces dust over great areas. [44:53.95] [45:04.05]Fresh new shoots draw animals from great distances. [45:09.13] [45:19.95]Many undertake epic migrations to catch the boom time. [45:25.32] [45:32.47]Some resourceful animals live here all the year round. [45:37.58] [45:44.37]Baboons are permanent residents but they have to change their behavior [45:48.94] [45:48.99]if they are to gather what they need on the newly flooded plains. [45:53.71] [45:58.12]There's plenty to eat but getting to it can be a little uncomfortable. [46:04.43] [46:52.47]New water poses problems for the youngsters [46:56.22] [46:56.40]but they know where to find a dry seat. [47:00.25] [47:08.74]A juicy snail is ample recompense for sodden paws. [47:14.66] [47:24.90]Having survived the dry barren times animals can now reap the rewards. [47:31.57] [47:37.29]On this seasonal planet, the great plains are lands of feast and famine. [47:43.42] [47:44.16]At their peak they support the greatest gatherings of wildlife [47:48.24] [47:48.39]found anywhere on Earth. [47:51.98]

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