- 5 hours ago
How the world's most spectacular grasslands flourish, despite being short of one essential nutrient - nitrogen. The secret lies with the animals: in Kenya, the white rhinos create nitrogen hotspots; in Brazil, the fruit-eating maned wolves garden the cerrado grassland; and in Australia, the bandicoots and rat kangaroos manage the outback.
氮的稀缺与关键作用：草原生态系统普遍缺乏氮元素，而氮是构成蛋白质的基础
本集深入探讨了非洲及全球草原生态系统中生命之间错综复杂的依存关系，揭示了看似无关的物种如何通过氮元素的循环紧密相连。
氮的稀缺与关键作用：草原生态系统普遍缺乏氮元素，而氮是构成蛋白质的基础
本集深入探讨了非洲及全球草原生态系统中生命之间错综复杂的依存关系，揭示了看似无关的物种如何通过氮元素的循环紧密相连。
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00:0310 million species live on planet earth each one is remarkable but none can survive on its own
00:18life depends upon connections unexpected invariably complex beautiful relationships
00:28between millions of plants and animals
00:38this time in our grasslands i want to show you why these antelope need these rhino
00:47why this lone wolf needs an ant
00:53and the incredible secret that lies buried beneath them all
00:59connections like these create the planet's great ecosystems they're vital for all life
01:08i want to show you our world as you've never seen it before
01:34central kenya
01:38and this is whistling acacia grassland
01:51the whistling actually comes from the breeze blowing across the holes in these hollow gauls
01:59and they're home to one of the grasslands most intriguing creatures
02:17this lizard is the aptly named dwarf gecko and this one has laid a couple of eggs
02:25down here in this gall on the tree the perfect place for a nest you might think
02:34but this tiny lizard can only safely lay its eggs here thanks to one of africa's
02:40largest most iconic and bizarre animals the giraffe
03:01the story of why the gecko needs the giraffe is wonderful
03:07and it will take us on a journey across the world to some of our most breathtaking grasslands
03:29they are habitats which occupy a quarter of the land surface of our planet
03:50they support some of our most spectacular wildlife
03:58and yet grasslands lack one of the basic ingredients for life
04:07a truly precious element
04:13it's this
04:17nitrogen
04:19a colourless tasteless gas
04:22it's only when we chill it down to 196 degrees below zero that we can actually see it
04:30we can see it
04:31i can tell you that almost everything that happens out here in this grassland ecosystem
04:37is governed by a craving for this rare resource
04:41in fact i'd go so far as to say that grasslands can only function
04:46because they can cope so ingeniously with so little of it
04:51so for once perhaps this will make the ecologist's job a little easier
04:55because to understand how grasslands work all we need to do is follow the nitrogen
05:14australia
05:30the grasslands here have some of the lowest levels of nitrogen on the planet
05:37which makes the local wildlife perfect for demonstrating why it's so important
05:48nitrogen is the magic ingredient
05:51when you add it to carbohydrate you get protein and protein is what you need to make living tissue
05:59protein is the building block for life all life
06:12a baby eastern grey kangaroo
06:20when he was born he weighed less than a one pence piece
06:33his dad on the other hand is one of the heaviest marsupials in the world
06:41which means that joey has a lot of growing up to do
06:54in fact from birth to adulthood he'll increase his body size by one hundred thousand times
07:07and all of that will be built with protein
07:13and you can't make protein without nitrogen
07:23right now he gets most of his protein from his mother and she gets all of her protein
07:31from a diet of grass
07:37at the moment these kangaroos are just hanging out
07:41and they look like a typical population of kangaroos
07:44and i can assure you they're not because this is not your typical australian grassland
07:54it's anglesea golf club near melbourne
08:07kangaroos were here when the golf course was built 60 years ago
08:12but since then their population has boomed
08:19at a recent count there were 359 of them
08:24that's nearly 20 for each hole
08:32and the secret of their success is down to these immaculate fairways
08:43this grass is so green and so lush because it's had something added to it
08:51this nitrogen fertilizer and i know it's a product of the industrial revolution that's therefore
08:59led to all of this lushness but it certainly proves a point the power of nitrogen in a grassland
09:13it's added once a month it seeps into the soil is quickly taken up by the roots
09:20and then converted into proteins for stronger healthier grass
09:29great for golf
09:31but perfect for the joey
09:36the extra protein means that he can grow faster become healthier and live longer
09:45and that's the power of nitrogen
10:05on the plains of east africa there's an animal that you might think is the least
10:12likely to be affected by nitrogen levels in the grassland
10:28nitrogen or indeed the lack of it plays a decisive role in any grassland ecosystem
10:33it's even intrinsically entwined with the ecology of large predators like these lions
10:45this close and i'm in no doubt that these animals are at the very top of their food chain
10:54but to find out how nitrogen affects predators
10:58i need to start at the bottom of the food chain
11:09and it all begins with a very special plant
11:15one of the great unsung heroes of our planet
11:21grass
11:26the really great thing about grass is that it's simple compared to other flowering plants shrubs
11:33and trees it requires a minimal protein to build so it can take all of the nitrogen from its roots
11:40and
11:40put it not into bulky structures but organize it efficiently yet simply to maximize photosynthesis
11:47to produce its food so grass leaves have protein at much lower concentrations
11:56but this gives grazing animals a real problem
12:03impala are one of africa's commonest antelope a favorite prey of big cats
12:16but their biggest challenge is a much more basic one
12:22how to survive on nothing more than grass
12:28the key is being very fussy about the sort of grass you eat
12:32because some grass is richer in nitrogen than other grass
12:38this impala is actually cherry picking the leaves with the highest concentration of nitrogen
12:47even so she'll have to work hard to extract it
12:54just watch her neck closely
12:58any moment now she's going to regurgitate a ball of partially digested grass
13:08and she'll process it all over again
13:16it's called chewing the cud
13:20by digesting grass not once
13:24but twice these animals can extract as much of the nitrogen as possible
13:30but it still only works on the very best grass
13:34antelope like these impala can only survive on a diet of grass which is relatively high
13:40in nitrogen grass like this we call it sweet grass
13:47this long grass on the other hand is known as sour grass because the nitrogen in its leaves occurs at
13:56even lower concentrations
14:02antelope can't live here they simply can't get enough nitrogen out of sour grass to survive
14:23antelope
14:24but there is an animal here that can change that
14:28a very rare one
14:30this is what i've been looking for
14:36the animal that has deposited this dung
14:39processes grass through its gut in a very different way than
14:42antelope and i'm sure i can demonstrate that here
14:47is some antelope poo and if i take just one of these and break it open here
14:54you can see that what remains is pretty much nothing more than dust
15:02whereas this on the other hand is very different
15:06it's very different if i break this one in half the grass is still very visible
15:15so
15:31so
15:45The white rhino.
15:47What an animal.
15:53Their sheer scale.
15:54I mean, they're the closest thing, perhaps, that we have on the planet to mammalian dinosaurs.
15:59And, of course, they have a reputation for being pretty feisty, so I'm being very careful
16:05and not moving too quickly and keeping my voice down.
16:09In fact, this one's getting a little bit close, so I'm going to stop moving and talking altogether.
16:19Their eyesight is poor, so, if I'm still, they may not notice me.
16:31But they do have a great sense of smell.
16:40If the wind changes, they could charge.
17:03Absolutely fabulous things.
17:15Now, one of these animals has got no horns.
17:18It hasn't lost them in a scrap.
17:20They've been deliberately removed by the park rangers here, as these horns can have a tremendous value.
17:30Rhinos with horns are prized by poachers.
17:33The month before I arrived, they killed one in this very park.
17:40Cutting off her horn could save her life.
17:43And it's hoped that this practice will save the entire species.
17:52Losing a species like this would have a drastic impact on the entire grassland ecosystem.
18:02Her huge size allows her to live on grass where antelope just can't.
18:09Her body houses a massive gut, a fermentation chamber, so she can make up for the low quality of sourgrass
18:18by digesting vast quantities of it.
18:27All of which means she produces a lot of dung, which she drops in a very specific place, a midden.
18:44For rhinos, this is important.
18:47It's how they communicate.
18:54Rhinos come here from far and wide.
18:58By smelling the pile, they can tell who's around, how they're doing, and who is ready to mate.
19:09It's a sort of faecal Facebook, but with added value.
19:17It brings nitrogen-rich manure from all over the grassland to one place.
19:27So the grass growing here becomes increasingly sweeter, until, as if by magic, rhinos have created a grazing lawn of
19:38sweet grass.
19:39Perfect for fussy eaters like Impala.
19:57Rhinos are ecosystem engineers upon which many other animals depend.
20:04That surely is their true value.
20:07That's why they're worth much more alive than dead.
20:21Right across the world, it's sweet grass that is crucial for most grassland grazers.
20:28And it's this quest for sweet grass that drives one of the largest movements of animals on our planet.
20:47Every year, one and a half million wildebeest migrate 3,000 kilometres.
20:53All because of nitrogen.
20:58And this is when its impact reaches the top of the food chain.
21:28Of course, it's a pot of good health in these animals.
21:31Let's go!
21:52So what has a lion kill got to do with nitrogen?
21:56Well, basically two things.
21:58You see, the lions here have identified the richest source of nitrogen available out here in the grassland, large herbivores.
22:09You see, their bodies are made up of principally proteins, and that protein is where the nitrogen is.
22:18Then it's the sweetgrass which dictates where lions like to hunt.
22:29King of beasts they may be, but their lives are ruled by nitrogen.
22:37But how would a grassland predator survive if there were no sweetgrass to hunt in?
23:06Depending on where you are in the world, the grassland always has a special name.
23:10In Africa, the savannah. In the US, it's the prairies.
23:15Here, in South America, it's the cerrado.
23:23There's one animal that lives here that I'd love to show you.
23:26It's really special.
23:27I've never seen one in the wild,
23:29but it's a truly fantastic animal by anyone's standards.
23:36This is MS National Park.
23:42The name comes from the Brazilian for this bird.
23:47The ria.
23:57But it's not a ria that I'm after.
24:02What I'm after is a wolf.
24:09Maine wolves are one of the largest predators in South America.
24:13But their home ranges, their territories, are huge.
24:17So they're not easy to find or see.
24:34This wolf has a problem on her paws.
24:43You see, these grasslands are so low in nitrogen,
24:48that they can't support herds of grazing animals,
24:52like impala or wildebeest.
25:01in fact the wolf's survival here is all thanks to a relationship with one of the cerrado's
25:08smallest inhabitants it's this an ant and the story of why the wolf needs the ant is an amazing
25:19one and it's all down to the way that grassland ecosystems survive against the odds
25:27other species of canids dogs wolves hunt in packs but her life is a lonely one
25:39there's just not enough nitrogen here for even two main wolves so one of them has to go
26:17hunting on her own
26:18means that even the old deer is off the menu
26:24instead she has to catch more modest prey
26:30so
26:32so
26:37so
26:38so
26:43so
26:47so
26:51so
26:56so
27:02so
27:04so
27:05so
27:10so
27:22she may have walked miles
27:25so
27:26so
27:36so
27:37so
27:41so
27:42so
27:42so
27:43so
27:51this is a libera fruit from the word lobo which means wolf and wolves love these things
28:02they make
28:04they make the perfect midnight snack
28:07they make the perfect midnight snack
28:23but they are packed with vitamins but they are packed with vitamins and carbohydrates
28:28so
28:29so eating fruit means she needs to catch fewer mice
28:33and that actually means that she can survive in this nitrogen-starved grassland
28:40so
28:46but it gets even better
28:50the maned wolf is also helping the fruit
28:56wolves
28:56wolves like to mark their territory
28:58and maine wolves are no exception and they like to choose high spots to do so
29:04but of course here on the flat serrado
29:07such places are in short supply so the one they turn to is this and this mound of soil here
29:13is the nest of leaf cutter ants
29:21when the wolf poos on the mound it has an immediate effect
29:37so
29:38soon most of the colony are out in force
29:49the ants will salvage anything they'll even try what's left of a mouse
30:05but one of them has just found the real prize
30:11a seed from the libera fruit
30:32the nutrient-rich coating from these seeds will be fed to the ant larvae
30:57and buried in the safety of the nest those seeds will germinate they've even got added fertilizer
31:06so what we have here is an interaction between three totally disparate species
31:12the ant the fruit and the wolf
31:15now the ants they get food delivered straight to their door
31:19the fruit has found something to disperse its seeds and something else to ensure that they germinate perfectly
31:26and the wolves well they're effectively farming their own food
31:32what a story what a story it's stories like that that mean that you can always entertain a lifelong fascination
31:40with nature
31:40and you know every single leaf cutter ant nest out here on the serrado has a libera fruit tree growing
31:48next to it
31:49it works
31:51it works and you know the most exciting thing for me is that none of the players in this system
31:56the ant the fruit or the wolf has any knowledge or regard for what it's doing
32:01and yet it works perfectly fantastic
32:13these connections are not just important for the creatures concerned
32:18because of the way they move precious nitrogen around
32:22they are crucial for the entire grassland ecosystem
32:43relationships between species evolve over millennia
32:48but they can be destroyed in just a matter of years
32:53and it's happened a lot in australia's grasslands
33:10this is mount rothwell research center in victoria
33:15it's 400 hectares of high security grassland
33:26scientists here are trying to restore animal relationships and thus mend a broken ecosystem
33:38it's surrounded by 11 kilometers of fencing
33:467500 volts protect the perimeter
33:55and it's patrolled every day
34:03i'm feeling a little imprisoned
34:05but then this fence is not designed to keep things in it's actually designed to keep things out
34:12you see in here is the largest piece of native australian grassland left
34:18in this region and this fence is helping keep out a whole load of animals that they don't want in
34:25here
34:28this moggy is or was someone's pet
34:32but she's also an alien predator that didn't evolve in this ecosystem
34:40you see australia has no native cats of any kind down under she's an unstoppable killer along
34:50along with feral dogs and foxes
34:58these carnivores would do anything to get their jaws and claws on what's in here
35:15this is a brush-tailed rock wallaby
35:19and because of these introduced predators he is now one of the rarest mammals in the world
35:25and the extinction of just a few key creatures like him has had a catastrophic impact
35:33crucial ecological relationships are in tatters here and as a result this native grassland is now
35:41far more endangered than any tropical rainforest the question is if you remove all of the alien animals
35:49and reintroduce the key native ones could australia's grasslands be brought back to life
35:57on the inside of this fence a relatively small community of animals and plants has been saved
36:02but i've got to say some of them really are pretty odd and to stand any chance of seeing them
36:09and we're going to have to wait until after dark
36:32welcome to the weird world of the native aussie night
36:58this is a southern brown bandicoot and i'm getting a superb view of it i'm so close
37:06and i can actually see the saliva glistening in its mouth as it's chewing its food
37:14it's one of about 20 different species of bandicoot and they all come equipped with this long
37:21pointed and sensitive snout and they're principally insectivores meat eaters but they do play a very
37:30important role when it comes to maintaining the plant community in this ecosystem he spends all of
37:39his time digging for food by the end of the night he'll have covered the grassland with lots of snout
37:46shaped conical pits
37:50here's one of the pits here and when the wind blows grass seeds are caught in here
37:57along with a lot of other detritus all of which is rich in nitrogen so they become a perfect place
38:05for
38:34germination
38:35and if you look carefully you can hear it munching on some roots or tubers that it's eating
38:46all of these animals are so tame that it's no wonder foreign cats and foxes have almost wiped them out
38:57as it feeds it effectively plows this hard soil allowing air and water in vital for the plants that live
39:07here
39:11but so that the herbivores don't do too much damage this miniature serengeti also has its own mini lion
39:42this is an eastern quoll
39:46a pocket-sized marsupial predator
39:51don't be fooled by size he can take prey much larger than himself
39:58what's important is that unlike cats and foxes quolls have co-evolved with their prey
40:04so they are a critical part of this ecosystem
40:14the rothwell experiment is working rare species of plants birds and mammals the entire grassland
40:22is making a comeback here
40:31the secret to healthy grasslands is having the right species in the right place
40:38and there's one animal that benefits grasslands more than any other on earth
40:55these monoliths dominate the cerrado in south america
40:59there can be as many as 40 000 in one square kilometer
41:08from the outside these things appear entirely lifeless
41:13in fact you could sit down here alongside one and see nothing move all day
41:19but living on the inside is perhaps the most important animal in this entire ecosystem
41:31they are grassland's secret weapon in the battle for nitrogen
41:43termites half-blind distant cousins of cockroaches
41:51there are so many millions living in this grassland
41:54but their combined weight is far greater than that of all the mammals living here put together
42:03and yet you would have no idea they were even here if they didn't build these huge mounds
42:13they build them as cooling towers to keep the temperature of the colony on the inside
42:18at an optimum 30 degrees centigrade and they're remarkably efficient normally accurate plus or minus
42:26a single degree not bad for a colony of primitive insects but then building these mounds all over the landscape
42:34is the least of their accomplishments
42:42because one way or another termites are fundamentally important to almost all life here
42:58especially the strangest of all the giant anteater
43:05these animals are related to armadillos and sloths they're part of a group called edentates which
43:12basically means without teeth but whilst armadillos and sloths do have rudimentary teeth
43:18if you were to perform a dental examination on one of these guys which would be difficult because
43:23their mouth is so small you'd find no teeth at all
43:29but it's not teeth he needs to unlock the termite secret
43:37if you look at its front feet you can see it's got these two huge claws on each one of
43:42them
43:43and they can break into just about any substrate it also uses them for defense it's said that if
43:49it's attacked by a larger predator it will stand back on its tail and lash out with those claws
44:00he uses his tongue which is over 50 centimeters long flicking it in and out at around 150 times a
44:08minute
44:16giant anteaters eat 35 000 termites a day nevertheless you could be forgiven for thinking that no matter
44:25how many they hoover up or how quickly they could never sustain an animal this size but they do
44:33by weight termites are the most protein rich food that you can find there's more protein in these little
44:40insects than there is in beans nuts cheese chicken even roast beef and what's interesting is that termites
44:49occur in ecosystems all over the world and wherever they do there are creatures like this that have
44:56evolved to feed exclusively on termites the reason termites are so rich in protein is all down to some
45:14very intimate relationships the first is with a fungus deep within the mound the fungus breaks down dead
45:24grass so the termites can digest it not much else could eke a living out of this stuff
45:41the second is more remarkable still you see termites can obtain nitrogen directly from the air
45:53and they achieve this through another symbiotic relationship an even more intimate one because
45:59living inside their digestive systems are amazing nitrogen trapping bacteria and it's this that gives
46:07termites their special powers so using the nitrogen they get from that bacteria the nutrients they get from
46:14the fungus they're able to turn this dead woody material into a productive food source this stuff which is so
46:25low
46:25in nitrogen so low in protein they can turn into edible protein and that's how they can form these vast
46:33colonies
46:37the termite mound becomes a nitrogen hot spot that's good for anteaters but also for all the surrounding plants and
46:47for all the other animals living here
47:01the incredible actions of termites nurture grasslands all over the world
47:10including kenya's whistling acacia savannah
47:23kenya's whistling
47:31here the impact of termites goes right to the top
47:37all the way to the world's tallest land animal
48:05when i was a child we thought there was just one type of giraffe
48:10but now we know there were six different species
48:15this one's a reticulated giraffe
48:30he can reach anything but of all the plants here he's chosen to eat the whistling acacia tree
48:42but just look at it it's one of the best offended plants in the whole of africa
49:04but why are these trees so well armed against browsing animals like these giraffes
49:14well the answer you won't be surprised comes down to nitrogen you see these trees leaves
49:21are absolutely packed with nitrogen that's why they're the botanical equivalent of fort knox
49:31at the base of many of these acacias are colonies of african termites
49:37the acacia's roots are boring under the mounds tapping into a rich well of nitrogen
49:44as a result these acacias become an oasis in a nitrogen desert
49:50so it's really no wonder that the thorns alone don't deter giraffes from tucking in
49:59so not to be beaten the tree employs another line of defense its own private army
50:08now just watch this i'm going to pretend to be a giraffe browsing on this branch here so i'm going
50:16to pull at the leaves shake it about a bit try and not get jabbed by the thorns here but
50:24just look at
50:24this very quickly a whole mass of these chromatogaster ants swarms out and covers my hand and i can tell
50:35you that if you were a giraffe browsing on this you wouldn't want these things all over your tongue
50:42each ant is armed with a chemical weapon it's capable of squirting venom
50:50each gall houses a separate colony of ants and each branch might have a dozen or more of these galls
51:07so the only way a giraffe can get an ant-free meal is to grab a quick snack and then
51:13move on
51:32the tree has evolved to produce these hollow thorns and they are the perfect structures for the ants
51:39to make their colonies inside and in return for the plant's investment it gets these insects as vigorous
51:46defenders
51:57but this is just the beginning of a truly amazing web of relationships
52:12these are patas monkeys
52:23as grassland specialists they're the fastest running primates in the world
52:29in the 100 meters sprint they beat any olympian by three seconds
52:45to fuel their energetic lifestyles their favorite food is chromatogaster ants and their larvae
52:59the trick is finding them in just the right position
53:06he could easily have his eye out on those thorns
53:38the Kirchis
53:38and this is the right place
53:39and this is the right place
53:51Once ripped open, the ants abandon the gall.
53:55But this act of primate vandalism only serves to enrich this mini-ecosystem.
54:04You see, a short while later, a new occupant has moved in.
54:16Here, protected from predators and the harsh sun, a dwarf gecko has laid two eggs.
54:32Because she isn't a threat to either them or the acacia, the ants mostly ignore her.
54:45After four months, a perfect miniature gecko hatches out.
55:03It's completely defenceless, but luckily, it's found itself in the perfect nest.
55:16It couldn't possibly appreciate all of the creatures here on the grassland that have come together to put a safe
55:25roof over its head.
55:34The giraffe eats the acacia trees, so it continually produces those galls, which form the gecko's home.
55:41The acacia tree needs the ants to protect its leaves and keep it healthy,
55:45and in turn, the patas monkey needs those ants and their larvae as food.
55:51And all of this can only happen because the tree has managed to accumulate nitrogen
55:57from a grassland that's evolved to prosper despite the fact that this element is always in short supply.
56:12It's often said that you can only tell who your real friends are in times of need.
56:20This is also true in nature.
56:26You might ask if it really matters if an animal becomes extinct.
56:34Well, so intricate are the connections in the natural world
56:38that there's no way to predict the impact of adding or removing species until it's too late.
56:53Who would have thought that a baton would need a bandicoot?
56:58That a leafcutter ant would need a maned wolf?
57:02Or indeed, that a gecko would need a giraffe?
57:06Now, the complex web of relationships that we've seen have evolved over millions of years,
57:14but we've only scratched the surface of a myriad of stories that, when they come together,
57:20make these grasslands functional ecosystems.
57:25And it's here that I've learned to see the real beauty in nature.
57:30You see, for me, it's not in the minute detail.
57:34It's in the bigger picture.
57:36Because this works perfectly.
57:40That is beautiful.
57:44That is beautiful.
57:47That is beautiful.
58:00If you'd like to know more about the fascinating web of links between species,
58:05the Open University has produced some material both to inform and inspire you.
58:11For your free copy or to find out more about Open University programmes,
58:16ring 0845 366 0254
58:21or go to the website bbc.co.uk slash nature
58:26and then follow the links to Open University.
58:30And join me next time when I'll be travelling
58:33to the world's greatest seasonal forest.
58:40Nicky Chapman helps a couple escape to the country in North Wales tomorrow afternoon,
58:44seeking out the dream home at 20 past four.
58:47For now though, we're all done here on BBC HD, so good night.
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