0:00:02 > 0:00:06Our planet is the greatest living puzzle in the universe.
0:00:06 > 0:00:08A collection of worlds within worlds.
0:00:09 > 0:00:13Each one a self-contained ecosystem bursting with life.
0:00:18 > 0:00:19But how do they work?
0:00:20 > 0:00:24The intricate web of relationships...
0:00:24 > 0:00:27and the influence of natural forces...
0:00:27 > 0:00:29makes each microworld...
0:00:29 > 0:00:32complex and unique.
0:00:33 > 0:00:36So to discover their secrets,
0:00:36 > 0:00:39we need to explore them one by one...
0:00:39 > 0:00:42untangle their interlocking pieces...
0:00:44 > 0:00:47..and ultimately reveal the vital piece,
0:00:47 > 0:00:49the key to life itself,
0:00:49 > 0:00:53hidden deep within each of nature's microworlds.
0:01:10 > 0:01:12The Serengeti -
0:01:12 > 0:01:18without doubt one of the most famous wildlife destinations on Earth.
0:01:21 > 0:01:27It's a vast grassland, covering over 23,000 square kilometres,
0:01:27 > 0:01:30which is pretty much the same area as Wales.
0:01:34 > 0:01:36THUNDERCLAP
0:01:36 > 0:01:40It's a landscape exposed to unpredictable rains...
0:01:48 > 0:01:51..fires...
0:01:51 > 0:01:53even occasional droughts.
0:01:55 > 0:01:59But this microworld supports the greatest density of herbivores
0:01:59 > 0:02:01found anywhere in the world...
0:02:11 > 0:02:14..grass-eaters of all shapes and sizes.
0:02:21 > 0:02:25While they rely on the vegetation, they themselves are essential
0:02:25 > 0:02:30for the survival of some of the world's most magnificent predators.
0:02:42 > 0:02:44The big cats...
0:02:58 > 0:03:02..supreme athletes whose hunting abilities have been honed
0:03:02 > 0:03:06over millennia out here on the open plains.
0:03:22 > 0:03:28But despite their predatory prowess, not every hunt leads to a kill.
0:03:29 > 0:03:32This place, with these daily life and death battles,
0:03:32 > 0:03:35seems as old as time itself,
0:03:35 > 0:03:40but a million years ago, this whole area was completely different.
0:03:40 > 0:03:44The Serengeti was once covered with woodland.
0:03:44 > 0:03:49The grasslands we see today were formed just 100,000 years ago.
0:04:00 > 0:04:04Sitting between a chain of volcanoes along its eastern border
0:04:04 > 0:04:07and the shores of Lake Victoria to the west,
0:04:07 > 0:04:14this microworld is a comparatively new, yet complicated, ecosystem.
0:04:14 > 0:04:17Untangling the web of its many working parts
0:04:17 > 0:04:20is not going to be easy.
0:04:22 > 0:04:28We'll need to explore the connections and relationships between animals
0:04:28 > 0:04:31and also between them and the world they live in.
0:04:32 > 0:04:37Because by doing so, we can hopefully discover the single key
0:04:37 > 0:04:40that unlocks the entire Serengeti ecosystem.
0:04:44 > 0:04:46In terms of spectacle and scale,
0:04:46 > 0:04:52there's one species that eclipses all others on the Serengeti grasslands.
0:05:03 > 0:05:05Wildebeest.
0:05:10 > 0:05:14There are an estimated 1.5 million making a living here,
0:05:14 > 0:05:19and evidence suggests their numbers are increasing.
0:05:19 > 0:05:21It's one of the largest congregations
0:05:21 > 0:05:24of a single species of mammal found anywhere on Earth.
0:05:27 > 0:05:32Wildebeest is an Afrikaans name meaning "wild cattle",
0:05:32 > 0:05:35but, actually, these bizarre-looking creatures are antelope -
0:05:35 > 0:05:37not cows at all.
0:05:40 > 0:05:44The herds are so massive they have to constantly move around
0:05:44 > 0:05:46the entire Serengeti system,
0:05:46 > 0:05:48driven by the need for food.
0:05:54 > 0:05:57Such a vast concentration of grazing animals
0:05:57 > 0:05:59has an enormous impact on the environment.
0:06:02 > 0:06:06But they themselves are a vital part of the Serengeti story -
0:06:06 > 0:06:11a living larder for Africa's killer carnivores.
0:06:11 > 0:06:13WILDEBEEST LOW
0:06:13 > 0:06:17Lions are the largest predators in Africa,
0:06:17 > 0:06:21and out here, they're a constant threat to the herds.
0:06:24 > 0:06:26Lions are special.
0:06:28 > 0:06:31Unlike other feline predators, they're social,
0:06:31 > 0:06:34hunting together cooperatively as a pride.
0:06:34 > 0:06:38It's believed that this social behaviour evolved so that together,
0:06:38 > 0:06:43in a team, they could hunt mega-herbivores like buffalo.
0:06:47 > 0:06:52Living in a pride has other advantages too.
0:06:52 > 0:06:54The females, usually sisters,
0:06:54 > 0:06:58are able to help look after each others' cubs.
0:06:58 > 0:07:03So this social behaviour also means they can raise more offspring.
0:07:03 > 0:07:06But more young means more hungry mouths to feed.
0:07:06 > 0:07:11The spoils of the hunt will have to be shared.
0:07:11 > 0:07:15But as an adult wildebeest weighs as much as a lioness,
0:07:15 > 0:07:17there should be enough to go around.
0:07:20 > 0:07:24With so much prey available, the Serengeti is home
0:07:24 > 0:07:29to some of the largest prides of lion found anywhere in the world.
0:07:29 > 0:07:32They make for an imposing, intimidating sight,
0:07:32 > 0:07:35strutting the Serengeti...
0:07:39 > 0:07:42..with characteristic swagger.
0:07:42 > 0:07:46But they don't get things entirely their own way.
0:07:50 > 0:07:52These are spotted hyena.
0:07:53 > 0:07:55They're stocky predators
0:07:55 > 0:07:59with perhaps the most potent bite force of any mammal.
0:08:00 > 0:08:03Though they're fearsome hunters,
0:08:03 > 0:08:07they're better known as being scavengers and thieves.
0:08:10 > 0:08:12LIONESS GROWLS
0:08:12 > 0:08:15And they're not beyond taking on a pride of lion,
0:08:15 > 0:08:18particularly if they're fat and full.
0:08:20 > 0:08:25The hyenas potent stomach acids are capable of dissolving even bones,
0:08:25 > 0:08:29so they clear up what the lions leave behind.
0:08:31 > 0:08:33Predators like lion and hyena
0:08:33 > 0:08:37play a valuable role in balancing the ecosystem.
0:08:37 > 0:08:41Without them, the herd could decimate the plains,
0:08:41 > 0:08:43leaving little food for other foragers...
0:08:48 > 0:08:51..like elephants.
0:08:51 > 0:08:56The largest land mammal has a vast appetite
0:08:56 > 0:08:59and can be extremely destructive.
0:09:00 > 0:09:04Elephants are one of only a few animals that have the ability
0:09:04 > 0:09:07to fundamentally alter their surroundings.
0:09:09 > 0:09:16This leviathan beast is a powerful force in the Serengeti's ecosystem.
0:09:22 > 0:09:25But the working parts to any microworld
0:09:25 > 0:09:29are not just the creatures that live within it.
0:09:31 > 0:09:35Other factors also influence these self-contained systems.
0:09:36 > 0:09:42The weather, water, natural disasters, disease,
0:09:42 > 0:09:46even the very bedrock beneath, all play their part.
0:09:55 > 0:09:56And in the Serengeti,
0:09:56 > 0:10:02there's one invisible factor with an enormous influence -
0:10:02 > 0:10:03the equator.
0:10:08 > 0:10:10The close proximity of the equator
0:10:10 > 0:10:14has a variety of effects on a variety of worlds.
0:10:14 > 0:10:19In the Serengeti, it plays havoc with the weather.
0:10:23 > 0:10:27The rainfall not only varies drastically year-on-year,
0:10:27 > 0:10:31but also from the wetter North to the drier South.
0:10:35 > 0:10:38The more rain, the better the grazing.
0:10:41 > 0:10:43This unpredictability with the weather
0:10:43 > 0:10:47has a tangible effect on the wildebeest.
0:10:47 > 0:10:48THUNDER RUMBLES
0:10:48 > 0:10:51They follow the rain clouds like storm chasers.
0:10:53 > 0:10:55This ensures there's always enough food
0:10:55 > 0:10:58to support one and a half million stomachs.
0:11:01 > 0:11:05But there are even more subtle connections between rain
0:11:05 > 0:11:08and life on the plains.
0:11:13 > 0:11:15Even the lion are affected.
0:11:15 > 0:11:17It's been shown that their success rate in hunting
0:11:17 > 0:11:22is not just determined by the number of prey animals that are present,
0:11:22 > 0:11:25but by the amount of cover there is.
0:11:29 > 0:11:33The longer grass, produced by plenty of rain,
0:11:33 > 0:11:36gives the lions a great advantage.
0:11:38 > 0:11:43The very grass the lions need to cover them as they stalk their prey
0:11:43 > 0:11:47is the same grass that attracts animals here in the first place.
0:11:50 > 0:11:55The more food the lions secure, the more cubs they can have.
0:11:55 > 0:12:00So the lions' breeding success is not just dependent on the herds,
0:12:00 > 0:12:04but on the long grass fed by the rains.
0:12:06 > 0:12:09Of course, the grass has a much more direct connection
0:12:09 > 0:12:12with the herbivores that live here.
0:12:14 > 0:12:18It's the food source that supports the vast wildebeest herds.
0:12:18 > 0:12:22But it's not just them.
0:12:22 > 0:12:25With 28 species of herbivores,
0:12:25 > 0:12:31the Serengeti supports the greatest diversity found anywhere in Africa.
0:12:31 > 0:12:35The herbivores may appear to be in direct competition with one another,
0:12:35 > 0:12:38all trying to eat as much as they can,
0:12:38 > 0:12:41but on closer inspection,
0:12:41 > 0:12:45the grass-eaters all fill an ever so slightly different role.
0:12:48 > 0:12:52Elephants tear out trunkfuls of long grass.
0:13:00 > 0:13:06Wide-faced wildebeest and zebra crop the grass like lawn mowers.
0:13:11 > 0:13:14And behind them, the smaller Thomson's gazelle,
0:13:14 > 0:13:18with their small, pointed faces,
0:13:18 > 0:13:20clip stalks ever shorter.
0:13:22 > 0:13:28With all these animals eating grass, it's remarkable there's any left.
0:13:28 > 0:13:30If the grasses were to recede,
0:13:30 > 0:13:35the trees would return to take over the plains.
0:13:35 > 0:13:38But these trees are also subject to attack.
0:13:40 > 0:13:42On the western edge of the Serengeti plain,
0:13:42 > 0:13:45there are a few hardy acacia trees.
0:13:45 > 0:13:49They're resilient trees with inch-long spines to deter browsers,
0:13:49 > 0:13:53but the animals have learnt to get around these defences.
0:13:56 > 0:14:01At almost ground level, the dik-dik, one of Africa's smallest antelopes,
0:14:01 > 0:14:03picks at the tiny leaves.
0:14:05 > 0:14:07With a small, narrow face,
0:14:07 > 0:14:11it daintily plucks between the lethal-looking spines.
0:14:13 > 0:14:16Impala also have narrow faces
0:14:16 > 0:14:19that allow them to feed from the higher branches.
0:14:23 > 0:14:26But there's another similar-sized antelope
0:14:26 > 0:14:30which employs a different trick so as not to compete with the impala.
0:14:37 > 0:14:39The gerenuk has a specially adapted pelvis,
0:14:39 > 0:14:43which allows it to stand straight up.
0:14:46 > 0:14:52So they're able to get to the leaves that are out of reach to the impala.
0:15:12 > 0:15:17But none of these can compete with nature's leggy champion -
0:15:17 > 0:15:19the giraffe.
0:15:19 > 0:15:22With a long leathery tongue and thick rubbery lips,
0:15:22 > 0:15:26they browse the untouchable leaves at the top of the trees,
0:15:26 > 0:15:31seemingly impervious to the trees' spiny defences.
0:15:34 > 0:15:37So the trees are under attack from every level,
0:15:37 > 0:15:41with none of the species actually competing directly
0:15:41 > 0:15:43with one another for food.
0:15:43 > 0:15:46This niche feeding is what allows
0:15:46 > 0:15:49such a vast array of life to succeed here.
0:15:52 > 0:15:55How important the grass is to this system
0:15:55 > 0:15:58becomes most evident when it disappears
0:15:58 > 0:16:01from over-grazing and lack of rain.
0:16:01 > 0:16:06The migrating herds move on, following the rains,
0:16:06 > 0:16:08leaving behind in their shadow drought.
0:16:11 > 0:16:15For the resident territorial lion, unable to migrate with the herds,
0:16:15 > 0:16:18these are desperate times.
0:16:23 > 0:16:26But it's these harsh conditions that keep the lion numbers
0:16:26 > 0:16:28on the Serengeti under control.
0:16:29 > 0:16:31Only the fittest will survive.
0:16:33 > 0:16:38Without cover, food is hard to come by and it's about to get worse.
0:16:41 > 0:16:43Fire.
0:16:45 > 0:16:50But even this has a valuable part to play in this microworld.
0:16:53 > 0:16:56In other parts of the world, grasslands are found on soils
0:16:56 > 0:17:01that do not have enough nutrients to support trees.
0:17:01 > 0:17:03The Serengeti, though, is different.
0:17:03 > 0:17:07The rich soils here can support trees, and once did.
0:17:09 > 0:17:10So where are they now?
0:17:14 > 0:17:16This was a great puzzle to scientists
0:17:16 > 0:17:20when they first started studying the Serengeti.
0:17:20 > 0:17:22Do these fires keep the trees at bay?
0:17:22 > 0:17:25FIRE CRACKLES
0:17:26 > 0:17:30The answer to that lies in how the fires are started in the first place.
0:17:34 > 0:17:36Unlike many other parks in Africa,
0:17:36 > 0:17:41where as much as 50% of all fires are caused by lightning strikes,
0:17:41 > 0:17:47the fires in the Serengeti are mostly down to another factor...
0:17:55 > 0:17:57..people.
0:17:57 > 0:18:03To create grazing for their cattle, they set fires to burn down the trees
0:18:03 > 0:18:06and allow the fast-growing grasses to become established.
0:18:09 > 0:18:13Grass grows from its roots, not just its tips,
0:18:13 > 0:18:17so even with the surface completely burnt away,
0:18:17 > 0:18:21new prolific growth can begin underground.
0:18:25 > 0:18:30People's deliberate modification of the landscape through fire
0:18:30 > 0:18:32has had massive effects.
0:18:32 > 0:18:36But it's not the only, or the most important, factor
0:18:36 > 0:18:38in the Serengeti we see today.
0:18:42 > 0:18:46The wildebeest herd certainly play their part.
0:18:46 > 0:18:51They keep the grass so short that there's less fuel for fire to spread.
0:18:53 > 0:18:58Fire plays an active role in keeping the trees at bay in some areas,
0:18:58 > 0:19:02but there must be something else that's managing the tree population.
0:19:08 > 0:19:13Elephants decimate the few trees there are, especially in droughts.
0:19:14 > 0:19:17In one scientific study,
0:19:17 > 0:19:21six male elephants entered a stand of acacia trees covering an area
0:19:21 > 0:19:24of less than a square mile.
0:19:24 > 0:19:26And within 24 hours,
0:19:26 > 0:19:3234% of the trees were destroyed and a further 22% damaged beyond repair.
0:19:34 > 0:19:37Six elephants, one day,
0:19:37 > 0:19:41one square mile and 50% destruction.
0:19:41 > 0:19:44It's a significant amount of damage,
0:19:44 > 0:19:46and it's not just adult trees.
0:19:51 > 0:19:56Elephants target and eat young saplings as they grow in the grass,
0:19:56 > 0:19:58kind of like gardeners weeding their borders.
0:20:02 > 0:20:08But fire and elephants are not the reason grasses reign on the plain.
0:20:08 > 0:20:11That's down to something even bigger than an elephant.
0:20:18 > 0:20:20What makes the Serengeti so special
0:20:20 > 0:20:24is down to what lies on the eastern border -
0:20:24 > 0:20:27volcanoes.
0:20:27 > 0:20:30When this region was a dry woodland half a million years ago,
0:20:30 > 0:20:36it had yet to feel the full force of the Earth's fiery power.
0:20:39 > 0:20:43First came eruptions from Kerimasi, which is now extinct.
0:20:45 > 0:20:48But it was followed by Ol Doinyo Lengai,
0:20:48 > 0:20:52a volcano that first came to life half a million years ago
0:20:52 > 0:20:55and continues to regularly erupt.
0:21:07 > 0:21:12These volcanoes spew out hot lava, ash and burning gases
0:21:12 > 0:21:14from the Earth's core.
0:21:14 > 0:21:20The existing woodland didn't stand a chance and burnt to the ground.
0:21:21 > 0:21:27In their place, fast-growing grasses sprang forth and flourished.
0:21:28 > 0:21:34Over time, more and more layers of nutrient-rich volcanic ash piled up.
0:21:36 > 0:21:39They created a very unusual soil.
0:21:43 > 0:21:48The combination of ash and rain has created together something unique
0:21:48 > 0:21:51just below the surface of the Serengeti...
0:21:52 > 0:21:57..a thick impenetrable crust that's less than a metre below the surface
0:21:57 > 0:21:59called the hardpan.
0:22:01 > 0:22:06Made from calcium carbonate, the same mineral as found in cave stalactites,
0:22:06 > 0:22:12it's as hard as concrete and, like a protective underground layer,
0:22:12 > 0:22:15it covers the eastern grasslands.
0:22:19 > 0:22:24This hardpan is just deep enough below the surface for grasses
0:22:24 > 0:22:26and shallow roots to thrive,
0:22:26 > 0:22:29but too shallow to allow tree roots
0:22:29 > 0:22:32to reach the water table deep underground.
0:22:33 > 0:22:38So, like an underground fence, it keeps out the trees,
0:22:38 > 0:22:43leaving the Serengeti's grasses to flourish unhindered.
0:22:43 > 0:22:48So this hard underground pan holds the key to the inner-workings
0:22:48 > 0:22:51of the Serengeti's system.
0:22:51 > 0:22:55But this pivotal piece has another vital role to play
0:22:55 > 0:22:58in this microworld...
0:22:59 > 0:23:04..one that will make the wildebeest risk their very lives.
0:23:06 > 0:23:09Every year, the wildebeest migration swings back
0:23:09 > 0:23:12to these eastern grasslands,
0:23:12 > 0:23:17even though lethal obstacles lie in their path.
0:23:19 > 0:23:23African rivers hide a real danger -
0:23:23 > 0:23:25Nile crocodiles.
0:23:27 > 0:23:32These huge reptiles can measure nearly six metres in length.
0:23:32 > 0:23:36Crocodiles, like the wildebeest, are creatures of habit,
0:23:36 > 0:23:38and they know the herd is coming.
0:23:51 > 0:23:53The tired and thirsty migrating herds
0:23:53 > 0:23:58are desperate to reach the rich grasslands on the eastern side.
0:23:58 > 0:24:01They're willing to face danger just to get a drink.
0:24:38 > 0:24:42But there is method to this seeming madness.
0:24:42 > 0:24:44They have to cross the river regardless of the danger,
0:24:44 > 0:24:47because what lies on the other side
0:24:47 > 0:24:51is lush pasture that holds the key to their survival.
0:24:56 > 0:24:59In their numbers, the herd find a kind of safety.
0:24:59 > 0:25:03The predators are hopelessly outnumbered.
0:25:03 > 0:25:06For every one or two dragged down to their doom,
0:25:06 > 0:25:08a thousand cross unhindered.
0:25:20 > 0:25:23In a herd of over a million individuals,
0:25:23 > 0:25:25comparatively few will perish.
0:25:25 > 0:25:28It's as if the few that are taken
0:25:28 > 0:25:32are sacrificed for the greater good of the herd.
0:25:41 > 0:25:44And in the land of plenty on the other side of the waters,
0:25:44 > 0:25:49the wildebeest have sufficient bounty to give birth to their young.
0:25:56 > 0:26:01The hardpan has created not just rich fertile grassland
0:26:01 > 0:26:03almost devoid of trees,
0:26:03 > 0:26:07but it's also rich in nutrients, especially phosphates.
0:26:09 > 0:26:11Mammals are distinct from other groups of animals
0:26:11 > 0:26:13as they suckle their young with milk.
0:26:16 > 0:26:20And to produce milk, mammals need phosphates -
0:26:20 > 0:26:21lots of them.
0:26:21 > 0:26:25And these eastern plains are loaded with it.
0:26:35 > 0:26:38So the wildebeest come here in vast numbers.
0:26:40 > 0:26:44They risk crocodiles, lion and hyena to give their young
0:26:44 > 0:26:47the very best start in life.
0:26:47 > 0:26:51The female wildebeest eat the phosphate-rich grasses
0:26:51 > 0:26:54and in turn produce the richest and best milk
0:26:54 > 0:26:56for their growing offspring.
0:27:02 > 0:27:05These vital phosphates get passed on
0:27:05 > 0:27:08through the food chain to their young,
0:27:08 > 0:27:10and then on to the predators.
0:27:14 > 0:27:15CUB SQUEAKS
0:27:26 > 0:27:30Life here seems, at times, almost too easy.
0:27:37 > 0:27:41These minerals could ultimately be said to be the reason why
0:27:41 > 0:27:44these plains support some of the largest
0:27:44 > 0:27:46and most diverse concentrations
0:27:46 > 0:27:49of large mammals found anywhere in the world.
0:27:51 > 0:27:55So in a world within a world, where everything plays its role
0:27:55 > 0:27:58and life seems completely interconnected,
0:27:58 > 0:28:02the pivotal piece lies out of sight
0:28:02 > 0:28:05just below the surface. The hardpan,
0:28:05 > 0:28:09rich in vital nutrients that feed the grass,
0:28:09 > 0:28:13which then feed the herds, who in turn feed the predators.
0:28:14 > 0:28:16And the results are plain to see -
0:28:16 > 0:28:20a thriving, bustling microworld
0:28:20 > 0:28:23packed with life.
0:28:23 > 0:28:27The greatest and most famous grassland on our planet -
0:28:27 > 0:28:29the Serengeti.
0:28:48 > 0:28:51Subtitles by Red Bee Media Ltd