行星地球第五集：奇幻沙漠 Deserts

[00:39.36]A third of the land on our planet is desert. [00:43.32] [00:52.20]These great scars on the face of the Earth [00:55.16] [00:55.28]appear to be lifeless, [00:57.00] [00:57.12]but surprisingly none are. [01:00.12] [01:06.40]In all of them life manages somehow [01:09.44] [01:09.52]to keep a precarious hold. [01:12.16] [01:32.32]Not all deserts are hot. [01:34.76] [01:35.16]Fifty-mile-an-hour winds blowing in from Siberia [01:38.60] [01:38.72]bring snow to the Gobi Desert in Mongolia. [01:42.60] [02:07.40]From a summer high of 50 degrees centigrade [02:10.80] [02:10.92]the temperature in midwinter can drop to minus 40, [02:14.84] [02:14.96]making this one of the harshest deserts of all. [02:18.88] [02:27.48]Few animals can survive these extreme changes. [02:32.36] [02:38.48]Wild Bactrian camels, [02:40.84] [02:40.96]one of the rarest mammals on the planet. [02:43.48] [02:43.60]And perhaps the hardiest. [02:46.20] [02:47.36]Their biggest problem is the lack of water, [02:49.80] [02:49.92]particularly now, in winter, [02:51.56] [02:51.68]when the little there is is locked up as ice. [02:54.96] [03:01.72]Surprisingly, snow here never melts. [03:05.72] [03:05.92]The air is just too cold and too dry for it to do so. [03:10.52] [03:15.48]The sun's rays turn it straight into vapour. [03:19.08] [03:19.16]It evaporates. [03:21.12] [03:28.80]But it is the only source of water, [03:31.80] [03:31.88]so Bactrian camels eat it. [03:34.80] [03:35.64]Elsewhere in the world [03:37.04] [03:37.16]a camel at a waterhole can drink as much as 200 litres during a single visit. [03:43.48] [03:45.36]Here the strategy is to take little and often. [03:48.72] [03:48.84]And with good reason, [03:50.20] [03:50.32]for filling the stomach with snow could be fatal. [03:53.92] [03:59.24]The camels must limit themselves [04:01.20] [04:01.32]to the equivalent of just 10 litres a day. [04:05.12] [04:21.04]Winter is the time for breeding. [04:23.84] [04:28.28]This extraordinary performance is a male Bactrian camel's way [04:32.52] [04:32.64]of attracting the attention of a passing female. [04:35.96] [04:37.96]In summer the camels can't stray far from waterholes. [04:42.28] [04:42.56]But now, with mouthfuls of snow lying everywhere [04:45.96] [04:46.04]they can travel widely in search of mates. [04:49.32] [04:53.24]Today less than a thousand of these desert specialists remain in the wild. [04:59.20] [04:59.32]The Gobi, hostile though it is, is their last stronghold. [05:04.60] [05:10.64]There's no other desert quite like the Gobi, [05:13.84] [05:13.96]but why is this place a desert? [05:16.96] [05:18.20]There is one simple and massive cause - [05:21.32] [05:21.44]the Himalayas. [05:22.88] [05:27.28]Clouds blowing from the south hit this gigantic barrier. [05:31.24] [05:31.36]As they're forced upwards [05:32.72] [05:32.84]so they empty their moisture on the mountain slopes, [05:36.12] [05:36.20]leaving little for the land on the other side. [05:39.88] [05:44.24]From the space deserts are very conspicuous. [05:48.40] [05:50.68]Dunes of sand hundreds of miles long streak their surface. [05:55.44] [06:00.04]With no cloak of vegetation to conceal them [06:03.08] [06:03.20]strange formations are exposed in the naked rock. [06:07.68] [06:13.12]Africa's Sahara is the largest desert of all. [06:16.88] [06:17.00]It's the size of the United States [06:19.24] [06:19.32]and the biggest source of sand and dust in the entire world. [06:24.24] [07:02.56]Sandstorms like these appear without warning [07:05.80] [07:05.88]and reduce visibility for days over areas the size of Britain. [07:10.72] [07:19.32]Dromedaries, single-humped camels, take these storms in their stride. [07:24.84] [07:27.80]The heaviest sand rises only a few metres above the ground, [07:32.20] [07:32.32]but the dust can be blown 5,000 metres up into the sky. [07:37.52] [07:42.36]The ferocious wind, armed with grains of sand, [07:46.12] [07:46.24]is the agent that shapes all deserts. [07:49.92] [07:58.60]Reptiles have armoured scaly skins that protect them from the stinging grains. [08:05.36] [08:07.36]For insects the bombardment can be very severe indeed. [08:12.28] [08:14.04]The only escape is below the surface. [08:17.12] [08:33.44]As the winds rise and fall, swallow and eddy [08:37.48] [08:37.60]so they pile the sand into dunes. [08:41.00] [08:44.52]These sand scenes can be hundreds of miles across. [08:48.52] [08:52.64]In Namibia the winds have built some of the biggest dunes in the world. [08:57.32] [09:04.72]Star dunes like these can be 300 metres high. [09:08.80] [09:08.88]Grains, swept up the flanks, are blown off the crests of the ridges [09:13.52] [09:13.60]so it's only the tops that are moving. [09:16.24] [09:22.84]The main body of these dunes may not have shifted for 5,000 years. [09:28.92] [09:39.76]Few rocks can resist the continuous blast of the sand carrying wind. [09:46.00] [09:46.16]These outcrops are standing in Egypt's White Desert. [09:50.92] [09:51.28]But they will not do so for much longer. [09:54.12] [09:55.56]They're being inexorably chiseled away [09:57.96] [09:58.04]and turned into more sand. [10:01.00] [10:38.64]Now lumps of heavily eroded rocks have been marooned in a sea of sand. [10:45.72] [11:02.00]These jagged pyramids a hundred metres tall [11:05.52] [11:05.64]were once part of a continuous rocky plateau. [11:09.12] [11:09.24]The blasting sand will eventually eliminate them altogether. [11:13.68] [11:21.64]The relentless power of the wind ensures that the face of a desert is continually changing. [11:27.88] [11:27.96]But there is one constant presence - [11:31.24] [11:38.72]the desert sun. [11:40.56] [11:54.60]The sun's heat and power to evaporate water [11:58.04] [11:58.16]has had a profound effect on the bodies and habits of everything that lives here. [12:04.08] [12:07.68]This sun potentially is a killer. [12:11.08] [12:12.36]And the red kangaroos must acknowledge that. [12:16.28] [12:17.36]Right now, while the sun is low, [12:19.52] [12:19.64]there's no immediate cause for concern. [12:22.76] [12:23.32]But this situation won't last long. [12:26.40] [12:27.16]Australia is the world's most arid continent with blistering daytime temperatures. [12:33.40] [12:40.08]Every hour the temperature rises by five degrees centigrade. [12:45.92] [12:52.24]Soon the heat will reach a critical point. [12:56.48] [13:05.56]Any kangaroo out in the open is in serious danger of overheating. [13:11.52] [13:16.76]In the full sun the temperature on the ground soars to 70 degrees. [13:22.92] [13:27.00]By midday the radiation is so intense they must take shelter. [13:32.52] [13:52.04]In the shade they're shielded from much of the sun's energy [13:55.52] [13:55.64]but their body temperature can still rise. [13:58.48] [14:01.08]So they lick saliva on to their forearms [14:04.60] [14:04.68]where there is a network of blood vessels close to the surface of the skin [14:08.60] [14:08.68]and, as the saliva evaporates, their blood is cooled. [14:13.20] [14:15.72]This thermal image shows just how effective the process is. [14:20.32] [14:20.44]The blue areas on the body are the cooler parts. [14:24.36] [14:32.80]As the saliva dries it has to be replaced [14:36.44] [14:36.56]and this is a real drain on the kangaroo's body fluids. [14:40.68] [14:50.60]Even in the shade the earth is baking hot [14:54.40] [14:55.00]so the kangaroos dig away the warmed topsoil [14:58.36] [14:58.44]to get at the cooler ground beneath. [15:01.56] [15:23.20]By staying in the shade and licking to control their body temperature [15:27.68] [15:27.80]kangaroos manage to get through the hottest part of the day [15:30.88] [15:31.00]without heat stroke. [15:32.56] [15:33.48]But for the majority of desert animals [15:35.92] [15:36.00]this strategy would not be enough for survival. [15:39.36] [16:01.56]The extraordinary ears of the fennec foxes of Africa radiate heat [16:06.36] [16:06.48]but the animals have another way of keeping cool. [16:09.80] [16:19.04]They spend their days underground [16:21.32] [16:21.44]and only emerge at sunset. [16:24.36] [16:35.92]Darkness brings huge changes. [16:39.16] [16:39.28]In the Sahara the temperature can drop as much as 30 degrees during the night, [16:44.00] [16:44.12]so it's cool enough to allow these desert fox cubs to play. [16:48.68] [16:56.00]All sorts of creatures now appear [16:58.52] [16:58.64]including some really unexpected ones. [17:01.76] [17:04.60]Toads have permeable skins [17:06.80] [17:06.88]and would quickly die from desiccation out in the daytime heat. [17:10.72] [17:10.80]It's only now that they can leave shelter. [17:13.32] [17:16.20]The same is true for scorpions, [17:18.08] [17:18.20]even though their shells are actually watertight. [17:21.68] [17:33.32]In fact, most small desert creatures are nocturnal. [17:37.08] [17:37.20]so it's only now that you can judge [17:39.32] [17:39.44]just how much life there can be in the desert. [17:42.56] [17:43.44]But moisture, lost even at night, has to be replaced sometime somehow [17:49.00] [17:49.12]and that problem dominates the lives of all desert dwellers. [17:53.60] [18:01.48]The Atacama in Chile. [18:03.72] [18:03.80]This is the driest desert in the world. [18:07.20] [18:08.76]Some parts may not see rain for fifty years [18:11.68] [18:11.76]and with such a record [18:13.08] [18:13.16]you'd expect the place to be completely barren. [18:16.84] [18:21.60]These are South America's camels, guanacos. [18:25.80] [18:27.20]They're very good at conserving moisture [18:29.44] [18:29.56]but they nonetheless need a regular supply of water. [18:32.80] [18:45.28]They get it partly from cactus flowers [18:48.44] [18:48.52]but that explanation raises another question. [18:52.04] [18:54.20]How do the cacti survive without rain? [18:57.64] [19:04.16]Hot winds suck all the moisture from the surface of the land. [19:09.28] [19:10.60]Clearly there must be something else that takes the place of rain. [19:15.52] [19:16.76]The secret is a cold sea current that runs parallel to the land. [19:22.56] [19:38.84]The cold water cools the moist warm air above it [19:42.12] [19:42.24]and that produces banks of fog. [19:45.16] [20:07.44]At the same time wind blowing on to the shore sweeps the fog inland. [20:14.32] [20:28.52]Before long the cacti are dripping with dew. [20:33.16] [20:41.32]The fog is so regular [20:43.24] [20:43.36]that moisture loving lichens are able to grow on the cacti [20:46.80] [20:46.92]and they absorb liquid like a sponge. [20:50.12] [20:52.48]In the land of almost no rain [20:55.12] [20:55.24]these precious drops are life-savers for many different creatures. [21:00.80] [21:09.04]Further inland the air remains so warm that its moisture does not condense [21:14.76] [21:14.84]so this slender strip of desert is virtually the only part of the Atacama [21:19.32] [21:19.44]where life can exist. [21:21.40] [21:25.84]Without the fog, [21:27.24] [21:27.48]this land, too, would be empty. [21:30.08] [21:43.24]The guanacos make the most of the dew [21:46.32] [21:46.44]but it will not remain for long. [21:49.16] [21:49.84]In an hour or two the sun will have burnt it off [21:52.64] [21:52.76]and dry the surface of the cacti. [21:55.24] [22:03.76]The Sonoran desert in Arizona is not quite so dry as the Atacama - [22:09.04] [22:09.16]some rain does fall. [22:11.28] [22:12.08]But it is infrequent [22:13.72] [22:13.80]and when it does arrive [22:15.24] [22:15.32]animals and plants have to be ready to make the most of it. [22:19.04] [22:20.44]And it's coming. [22:22.12] [22:30.44]When the summer monsoon blows in [22:32.52] [22:32.64]the giant saguaros, one of the biggest of all cacti, [22:36.04] [22:36.12]are ready to take full advantage of it. [22:38.76] [23:11.60]After a rainstorm the saguaro's long shallow root system sucks up the water [23:18.00] [23:18.08]and the pleats on its trunk enable it to expand rapidly. [23:22.96] [23:30.56]When full, a saguaro stem can store up to five tonnes of water [23:35.76] [23:35.88]and that's enough to see it through many months of drought. [23:40.20] [23:44.24]The trunks of these huge plants provide homes for the gila woodpecker. [23:49.72] [23:51.68]But birds are not the only animals to benefit from the presence of the cacti. [23:58.08] [24:13.76]During four weeks of the summer [24:15.64] [24:15.76]the saguaros bloom at night to attract visitors. [24:21.20] [24:24.08]The pollen and nectar with which these flowers are loaded [24:27.24] [24:27.36]attract long-nosed and long-tongued bats. [24:31.48] [24:46.08]The bats left Mexico a few days earlier to escape the heat of summer [24:51.04] [24:51.16]and are on their way north to the southern United States. [24:55.24] [24:57.00]To get there, they have to cross the Sonoran desert. [25:00.96] [25:06.52]But the desert is so big [25:08.56] [25:08.68]that for most of the year they would be unable to cross it. [25:12.16] [25:12.28]Now, with the saguaro in bloom, [25:14.56] [25:14.68]they can refuel on the way. [25:17.36] [25:26.80]So the saguaro's success in developing a way to store water [25:30.64] [25:30.76]is now crucial to most of the animals that live or even travel through this land. [25:38.24] [26:15.44]The scarcity of rain determined the shape of this icon of the desert [26:19.96] [26:20.08]but water, scarce thought it is, [26:22.04] [26:22.16]has also, like the wind, shaped the land itself. [26:26.16] [26:42.64]In the deserts of Utah [26:44.44] [26:44.56]ancient rivers flowing across sandstone country steadily widen their canyons [26:50.04] [26:50.16]until now the land between them has been reduced to spires and pinnacles. [26:56.32] [27:24.72]With little or no soil to retain the water on the surface of the land [27:29.76] [27:29.88]life here is scarce indeed. [27:32.88] [27:34.44]And when resources are limited, [27:36.40] [27:36.52]conflict is never far away. [27:39.00] [27:49.20]These are Nubian ibex [27:51.80] [27:51.92]and they are squaring up for a duel. [27:54.60] [28:00.60]And when trouble starts, [28:02.00] [28:02.12]a smart ibex knows that the best thing to do is to gain higher ground. [28:07.40] [28:24.04]These are actually subordinate male ibex, [28:27.20] [28:27.32]but their fights are nonetheless serious. [28:29.72] [28:29.80]Losing one might mean never getting the chance to breed ever. [28:34.84] [28:56.28]When competitors are evenly matched as they are here, [28:59.68] [28:59.80]duels can last for an hour. [29:02.68] [29:21.68]In this heat the effort is trully exhausting. [29:25.84] [29:27.60]But victory here will gain important ranking points on a male's way to the top. [29:33.84] [30:03.84]There's so much at stake [30:05.44] [30:05.56]that not all play fair. [30:07.76] [30:22.40]The battle has produced the winner, [30:24.72] [30:24.84]but the ultimate prize is not his yet. [30:28.72] [30:32.04]That currently belongs to the dominant male ibex. [30:36.16] [30:38.24]His rank earns him the loyalty of a harem of females [30:42.16] [30:42.28]and they follow him closely as he travels across this desert [30:45.76] [30:45.88]searching for foof and water. [30:47.96] [30:50.04]He doesn't have to waste time looking for mates - [30:53.00] [30:53.12]they're his for the taking, [30:54.64] [30:54.76]so he can concentrate with them on keeping fit and healthy. [30:59.44] [31:14.16]Lizards are desert specialists. [31:17.12] [31:20.08]But here, their numbers are extraordinary. [31:23.76] [31:28.96]These crevices in South Africa contain the highest density of lizards in the world. [31:34.96] [31:44.40]They're called flat lizards for obvious reasons, [31:47.92] [31:48.04]and they flaunt their multi-coloured bellies in territorial disputes. [31:52.84] [31:55.20]He's made his point, [31:56.64] [31:56.76]and now it's time to find some food. [31:59.56] [32:01.32]As the day warms up, [32:02.80] [32:02.92]the lizards move away from their cracks [32:05.08] [32:05.20]and head down to the bottom of the gorge. [32:07.76] [32:08.96]Their goal is the river. [32:11.08] [32:13.40]There is no food at the edge, [32:15.72] [32:15.84]but this desert river holds a secret. [32:19.40] [32:25.64]Each day blackfly rise from turbulent stretches of the river. [32:30.96] [32:31.04]This is what the lizards have come for. [32:33.80] [32:48.32]The black fly never land, [32:50.36] [32:50.48]so the lizards have to leap for their food. [32:54.00] [33:05.76]In one day each of these acrobatic little lizards may catch 50 flies. [33:11.92] [33:21.68]There are plenty of flies to go round, [33:23.92] [33:24.04]even with hundreds of lizards competing for them. [33:27.36] [33:40.68]Away from these rapids flat lizard populations are found in much smaller numbers. [33:46.68] [33:46.80]But here one unusual abundance has produced another. [33:51.60] [34:02.20]Deserts are created by the lack of water, [34:05.08] [34:05.20]but what actually kills animals here is not heat or thirst, [34:09.32] [34:09.44]but lack of food. [34:11.32] [34:29.00]So how on earth does a plant-eater this size [34:32.96] [34:33.08]survive in a place apparently totally devoid of vegetation? [34:38.64] [34:44.64]Elephants in Namibia are the toughest in Africa. [34:48.60] [34:48.72]And they need to be. [34:50.60] [35:07.00]What little food exists is so dispersed [35:10.12] [35:10.24]that these elephants walk up to 50 miles a day [35:13.16] [35:13.28]as they travel up the dry river channels searching for something to eat. [35:18.80] [35:32.88]At times the task looks truly helpless. [35:36.96] [36:10.28]Elephants may seem out of place in this landscape, [36:13.88] [36:14.00]but they're not the only ones. [36:16.56] [36:32.32]Amazingly, lions live here, too. [36:35.92] [36:40.28]In savanah country huge herds of games support prides [36:44.20] [36:44.32]containing 20 lions or more. [36:46.80] [36:46.92]But to live here lions have had to change their habits - [36:50.64] [36:50.76]prides are much smaller [36:52.40] [36:52.52]and their home ranges are very much bigger. [36:55.44] [36:56.40]And there's an added problem - [36:58.76] [37:00.04]their food is always on the move. [37:03.48] [37:11.76]Like the elephants, [37:13.12] [37:13.24]the lions must travel great distances to find enough to live on. [37:18.00] [37:32.60]But lions can't go everywhere - [37:35.28] [37:35.40]they won't attempt to cross this field of sand dunes [37:39.56] [37:39.68]and the oryx know it. [37:41.52] [38:39.00]The lions must wait for the oryx to leave the safety of the dunes, [38:42.92] [38:43.04]which eventually they must to find food and water. [38:45.88] [38:46.00]And then the lions will ambush them. [38:48.36] [39:51.44]The elephants have found some of their favourite food. [39:55.68] [40:02.00]Grasses are the staple diet of all elephants, [40:04.92] [40:05.04]but this herd concentrates on digging up the roots, [40:08.28] [40:08.40]which have more nutrition and moisture than the stems. [40:12.36] [40:12.48]It's the sort of behaviour [40:13.84] [40:13.96]that can make all the difference in a place of serious shortages. [40:18.00] [40:18.68]Yet all this can change in an instant. [40:22.32] [40:38.76]The fortunes of many deserts are ruled by distant rains. [40:43.36] [40:43.48]This water fell as rain in mountains more than a hundred miles away. [40:48.64] [40:53.92]It's known as a flash flood [40:55.72] [40:55.84]and called that because the water may run for just a single day. [41:00.00] [41:00.12]It's an event that only happens once or twice a year at the most. [41:04.64] [41:24.60]The sandy riverbed acts like a giant strip of blotting paper [41:28.84] [41:28.96]sucking up the water as soon as it appears. [41:32.12] [41:41.24]But every square metre of soil moistened by this river [41:44.56] [41:44.68]will increase the chances of survival for those that live here. [41:49.00] [41:54.40]Waterholes are filled temporarily. [41:57.64] [42:02.12]Elsewhere in Africa elephants drink every day, [42:05.48] [42:05.60]but the lack of water here [42:07.08] [42:07.20]means that desert elephants can only refill their tanks once every four or five days. [42:13.64] [42:14.72]Within a week the flash flood has produced a flush of green, [42:18.84] [42:18.96]more than enough to draw the oryx out of the dunes. [42:23.08] [42:24.88]It's a rare chance for them to build up their food reserves. [42:28.80] [42:37.08]The flood has made life easier for the lions, too. [42:41.44] [42:43.72]The flesh of this oryx will keep the family going for a week at the most. [42:48.60] [42:48.72]But for a while the hunting will be easier, [42:51.08] [42:51.20]now that river channel has turned green. [42:54.20] [42:58.00]The good times for lions and oryx are brief, [43:00.72] [43:00.88]but these are the short moments [43:02.80] [43:02.92]that make it possible to live in deserts the year round. [43:06.88] [43:12.40]Death Valley is the hottest place on Earth. [43:15.36] [43:15.48]Yet even this furnace can be transformed by water. [43:19.56] [43:29.72]A single shower can enable seeds that have lain dormant for 30 years or more [43:34.56] [43:34.68]to burst into life. [43:36.72] [43:44.76]And there hasn't been a bloom like this one for a century. [43:49.00] [44:00.72]The periods of boom in Death Valley are short. [44:04.24] [44:04.36]but they're just frequent enough to keep life ticking over. [44:08.44] [44:16.64]A sudden flush of vegetation is what every desert dweller waits for, [44:21.08] [44:21.20]and when it happens they must make the most of it. [44:24.36] [44:44.00]There is no other species on the planet [44:46.24] [44:46.36]that responds as quickly and as dramatically to the good times as the desert locust. [44:53.04] [44:53.36]Eggs that have remained in the ground for 20 years begin to hatch. [44:58.28] [45:10.40]The young locusts are known as hoppers, [45:12.44] [45:12.56]for at this stage they're flightless. [45:14.80] [45:15.04]They find new feeding grounds by following the smell of sprouting grass. [45:21.36] [45:26.96]Normally it takes four weeks for hoppers to become adults, [45:30.76] [45:30.88]but when the conditions are right as now [45:33.28] [45:33.40]their development switches to the fast track. [45:37.00] [45:41.80]As the vegetation in one place begins to run out [45:44.88] [45:45.00]the winged adults release pheromones - scent messages, [45:48.40] [45:48.52]which tell others in the group that they must move on. [45:51.92] [46:03.40]And when groups merge, [46:04.92] [46:05.04]they form a swarm. [46:07.20] [46:28.88]An adult locust eats its entire body weight every day, [46:32.84] [46:32.96]and a whole swarm can consume literally hundreds of tonnes of vegetation. [46:38.28] [46:39.76]They have to keep on moving. [46:41.96] [46:43.36]The swarm travels with the wind - [46:45.48] [46:45.56]it's the most energy-saving way of flying. [46:48.84] [46:52.28]Following the flow of wind means that they're always heading toward areas of low pressure, [46:57.60] [46:57.72]places where wind meets rain and vegetation starts to grow. [47:02.88] [47:04.72]As they fly, [47:05.68] [47:05.80]swarms join up with other swarms [47:08.36] [47:08.48]to form gigant璱c plagues several billions strong [47:12.28] [47:12.40]and as much as 40 miles wide. [47:14.84] [47:16.80]They will consume every edible thing that lies in their path. [47:21.20] [47:34.00]This is one of planet Earth's greatest spectacles. [47:38.00] [47:38.12]It's rarely seen on this scale [47:40.00] [47:40.12]and it won't last long. [47:42.08] [47:42.20]Once the food is gone, [47:43.80] [47:43.92]the steady roar of a billion beating locust wings will once again be replaced [47:49.60] [47:49.72]by nothing more than the sound of the desert wind. [47:54.20]

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