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Howling storms and hurricanes,
devastating droughts and raging forest fires

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may come only once or twice in our lives,
so we think of them as exceptional happenings.

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But for those plants
with much longer lives than ours,

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such events may be crucial for survival.

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And those with briefer lives may only be able
to exist at all in the aftermath of such disasters.

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The greatest British hurricane of recent times
struck on the night of October 16th, 1987.

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Ancient woodlands that had stood for centuries
were devastated.

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For individual trees like this 250-year-old beech
it was, of course, a catastrophe.

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But for other plants, it was an opportunity
they'd been waiting for for decades.

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The seeds from which
these young plants are springing

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may have come from adults growing in a clearing
elsewhere in the wood over a century ago.

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Since then, they have lain dormant in the soil,

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but now the light has triggered them into life.

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They won't have this clearing to themselves
for long, so they grow swiftly,

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and the soil, enriched by rotting
woodland leaves, feeds them well.

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In their second year, they flower.
They're foxgloves.

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The seeds from these flowers
will not sprout here, there's no room for them.

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They will have to find a new clearing,
and in their turn may have to wait decades for it.

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Willowherb competes with the foxgloves
for this kind of territory,

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so it, too, needs to reproduce urgently.

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The willowherb seeds,
supported by no more than fluff,

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will be carried in millions,
far and wide, by the wind.

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After a few years, other claimants appear

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and contest the ownership of the clearing -
young birches.

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The willowherb may not be able
to stay here for much longer.

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A few years later, the willowherb has gone.
The birches, it seems, have won.

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But the soil is now losing its richness.

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Most of these birches
started their lives about ten years ago,

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but there were a lot of them
and competition was intense.

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So most of them didn't do well at all.
This one is pretty well dead.

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This is one of the winners
that has overshadowed the rest.

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But even this one will not be
the final holder of this territory.

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That prize will go
to a little seedling like this, an oak.

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Birches, on the timescale of a wood,
have short lives.

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After 40 or 50 years,
they begin to succumb to fungus and disease.

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Oaks grow more slowly but more strongly.

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Eventually, they overtake the ailing birches
and capture the sunshine.

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The social struggle in the woodland is over,
at least for the time being. The oaks rule.

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But, as befits ruling monarchs, they have
to support a multitude of lesser mortals.

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The wide, spreading expanses of leaves,
all full of starches and sugars,

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are rich meadows in which vast numbers
of creatures can graze throughout the summer.

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The oaks protect themselves to some degree
by generating toxins in their leaves,

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but that doesn't deter weevils.

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Or bush crickets.

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Sawfly larvae munch away
within the thickness of the leaves.

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The caterpillar of the yellow-tailed moth.

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The tortrix moth caterpillar
converts a leaf into a tube, eats it from within,

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and then uses it as a shelter
while it changes into an adult.

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The bagworm caterpillar tunnels inside a leaf,

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but shelters inside a portable tube which it hangs
beneath the leaf it's currently consuming.

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The meals the oak provides for these insects
then nourish others,

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when the insects themselves
become meals for bigger diners.

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The great tit with chicks to feed
will collect at least 300 caterpillars a day.

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The banquet only lasts a few months.

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As summer gives way to autumn,
the oaks drop their leaves and halt all activities.

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So do their lodgers within their cocoons,
or hibernating in crevices.

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As the year turns, sunshine warms the soil, and
plants that spent winter as bulbs below ground

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race to make use of the light
before the oaks can regrow their leaves.

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First are the snowdrops.

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As spring proceeds and the sunlight strengthens,
bluebells take over.

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These glorious carpets of uninterrupted blue
are a British speciality.

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20,000 years ago,
glaciers advanced over most of Britain,

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driving many plant species
southwards in front of them.

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When at last the ice melted,
the English Channel began to form

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and became a barrier that prevented many plants
from spreading back from Europe into Britain.

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The bluebell was one of the few that made it.

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In the woodlands of North America,
things are rather different.

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These lilies are growing
in the Appalachian Mountains.

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Spring here is even richer than it is in Britain.

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Instead of just two or three species of flower
in a square yard, as in an English woodland,

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here there are a dozen or even more.

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Glaciers once covered these mountains, too,

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but no arm of the sea cut them off
from the rest of the American continent.

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So when the glaciers melted,

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nothing prevented all the spring plants
from returning from warmer areas down south.

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And here, instead of the uniform carpet
created by bluebells,

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there is one with a rich pattern of many colours.

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Trillium lilies.

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Dutchman's breeches.

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Wild geranium.

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A member of the lily family, the bellwort.

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Blue flox.

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But the glories of spring do not last long.

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In April, English oaks rebuild their canopy.

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The bluebells' time is over.

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In their few weeks of activity, they manufactured
enough food not only to set seed,

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but, below ground, to bud off new bulbs.

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So next spring, if there's room,
they will extend their splendid carpet.

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A young beech reinforces the canopy
by adding another, lower layer.

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Shade returns to the woods.

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In tropical forests, it's never winter,
and the trees are in leaf throughout the year.

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If a plant on the ground beneath
a permanent canopy like this needs sunshine,

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it will have to climb to get it.

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These youngsters search for some kind of ladder
by lashing around with their whip-like tendrils.

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Once one of them gets a grip,
it puts a coil in the tendril to shorten it,

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pulling itself closer to the branch
up which it might climb.

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Other plants ascend by twining their main stem
around their support.

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As a climber gets nearer the canopy,
and the light, it expands its leaves.

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There are no more determined competitors
in this upward scramble

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than the rattans that live in the forests
of South-East Asia and tropical Australia.

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A mature rattan produces
the longest stem of any plant.

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One has been measured at 560 feet.

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The mature plant doesn't develop leaves
on its stem here on the forest floor,

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it only does that up in the canopy.

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This luxuriant growth,

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basking in the full sunshine 200 feet
above the ground, is the crown of the rattan.

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And it makes the plant's character quite plain -
it's a kind of palm.

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The tendrils with which it climbs
are so thin they are easily overlooked.

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But snag one of these on your arm
and it'll rip your clothes and your flesh.

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The tendrils are rigid enough to reach up
and hook onto the branches of established trees.

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They then hold the stouter,
heavier main stem in position

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while it grows upwards
from the fearsomely protected bud at its tip.

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There is, of course, an easier way to get up here.
You can float up, as a seed.

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Other plants among these squatters arrived here
as seeds stuck to the fur of a monkey,

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or within the gut of a bird.

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Orchids have seeds as fine as dust,

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so small they can be lifted
by even the faintest breath of air.

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As a consequence,
even the highest branches of the tallest trees

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may carry spectacular displays
of breathtaking blooms.

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But living up here has its problems.

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Water must be collected and stored.

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And, away from the ground,
it's difficult to get mineral nutriment.

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This orchid deals with those difficulties

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by wrapping its green roots
around the branch it sits on.

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They then intercept rainwater
trickling down the bark

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that carries with it a trace of nourishing dust.

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In some forests at higher altitudes,
there are almost permanent mists,

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and the canopies of all rainforests

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get intermittent supplies of water
from regular storms.

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Many plants store water
as permanent pools in their centres,

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and have little difficulty
keeping them topped up.

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Orchids may keep water in swollen,
bulb-like stems.

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The ponds in the centre of bromeliads
are more than water reservoirs,

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they're a source of nourishment.

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Tiny animals such as mosquito larvae
take up residence in them,

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and their excrement and dead bodies
accumulate at the bottom as a rich sludge.

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So the bigger a plant grows,
the more food it accumulates.

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I'm 200 feet above the ground,
suspended in the branches of a koompassia tree,

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the tallest of all the trees
in the South-East Asian forest.

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And here beside me is a magnificent basket fern.

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As it grew and put out leaves,
so it collected moisture.

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As the leaves got bigger, they in turn
collected other leaves falling from above.

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They decayed to form a rich leaf-mould.

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And as they grew, so more leaves,
and with them, more seeds.

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So that now there's a fig tree growing here.

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So here you have a complete garden,
200 feet up in the forest,

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with no part of it
touching the ground in any way.

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But some of these squatters
can become murderers.

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A young fig tree like this
may arrive here as a seed carried by a bird.

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At first, it grows quite slowly.

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As it gains in strength, its roots crawl
downwards over its landlord's branches.

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Some dangle free but keep on growing.

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Eventually, they reach the ground.

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Now, supplied with nutrients from the soil,
the fig grows really fast.

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The rootlets wrapped around the main trunk
thicken and fuse into a lattice.

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The host tree's fate is now sealed,
for it is in the clutches of a strangler fig.

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As years pass, the fig thickens its roots,
embracing the trunk ever more completely.

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Trees grow by increasing their girth.
For the host tree, that is now impossible.

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But growth is difficult anyway,
because the fig has a huge crown in the canopy

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that cuts off sunshine from its host,

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and its roots in the ground
are stealing most of the soil's nutrients.

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Eventually, the host tree is killed,
and its trunk rots away.

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But the fig does not fall.

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Its roots now form a hollow cylinder that is
quite capable of standing upright by itself.

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This strangler is about 300 years old.

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In fact, it may be misleading
to refer to it as a single tree.

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It's probable that, 300 years ago,
there were several young figs up in the canopy.

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Now, centuries later,
their roots have grown down to the ground,

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they've got rid of the body of their victim,

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and they're clinging to one another in this
extraordinary interlace of pillars and buttresses

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in order to maintain their dominance
of this part of the forest.

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Nor are these monsters
always satisfied with just one victim.

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This 500-year-old, having strangled
its first victim and lost its support,

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toppled sideways into a second,
killed that, and then a third,

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and now its roots are ready to embrace a fourth.

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Dead tree trunks are not wasted,
and neither are dead leaves when they fall.

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Both are food for fungi.

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Some leaves are captured
even before they reach the ground.

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The fungus has constructed a net,
stretched between the twigs of the undergrowth.

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Once they've caught their leaf,
the threads put out white filaments.

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These produce powerful acid
which dissolves the cellulose in the leaves.

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Why doesn't it dissolve the fungus, too?

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Because fungi are not plants,
their bodies don't contain cellulose.

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They're constructed from a material much more
akin to that of animal horn and hooves.

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Fungi are neither plant nor animal, they belong
to a category of life that is all their own.

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Nourished by the liquefied tissues of leaves,
this fungus puts out more threads.

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But fungi do require moisture.

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They can only live out in the open like this
in the moist atmosphere of the rainforest.

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In cooler, drier woodlands, they have
great difficulty living out in the open.

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Instead, they hide in the ground or within
the tissues of the bodies they feast on.

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A fungus has no stem, no root, no leaves.

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For most of the time, it's nothing more
than a tangled tissue of branching threads.

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These produce digestive acids,
absorb the resulting soup,

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and use it to construct more threads and widen
their search for more dead plant tissue.

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But cellulose is very low in nitrogen.

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To get that, some fungi trap living animals.

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The microscopic threads develop tiny lassoes.

199
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These give off a chemical that attracts
microscopic worms, nematodes.

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One of them nuzzles into the ring.

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And the fungus suddenly draws its lasso tight.

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The worms are killed
and the fungus has its nitrogen.

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All this takes place out of sight below ground
or within the body of a dead plant.

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Only when a fungus is ready to reproduce
does it make itself more visible.

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From such apparitions as these come spores,
the fungal equivalent of seeds.

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They're so small that they drift away like smoke.

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00:27:46,007 --> 00:27:50,205
But the appearance of these
spectacular constructions is brief.

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As soon as their spores have been shed,
sometimes after only a few days, they collapse.

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Now they are merely food for maggots.

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So the corpses of plants
do not retain their nutriment forever.

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Some of it is consumed by fungi,

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and the remainder, now in soluble form,

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seeps back into the soil
to sustain the next generation.

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00:28:24,527 --> 00:28:28,361
But it's not always easy
for that new generation to get a start.

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On the north-west coast of America
and in British Columbia,

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fir, spruce and hemlock grow so densely and tall

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that very little light filters down to the world
below, except when one of them dies.

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This giant tree fell about ten years ago.

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The gap it left in the canopy above is still open,

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00:28:51,887 --> 00:28:55,596
but there's still very little light
on the forest floor itself -

221
00:28:55,767 --> 00:28:59,123
these ferns and mosses are so very thick.

222
00:28:59,287 --> 00:29:04,884
But up here on the fallen trunk,
things are very different.

223
00:29:05,047 --> 00:29:07,163
This is the next generation.

224
00:29:07,327 --> 00:29:10,046
Bare bark doesn't hold the moisture.

225
00:29:10,207 --> 00:29:13,119
Thick moss could bury a seedling.

226
00:29:13,287 --> 00:29:16,677
But in moss like this, growing in such a position,

227
00:29:16,847 --> 00:29:21,716
a young plant can get
all the moisture and the light that it needs.

228
00:29:27,127 --> 00:29:32,281
Its threadlike roots grow downwards
over the surface of the trunk.

229
00:29:35,687 --> 00:29:39,805
Even before they reach the ground,
they find plenty of nourishment,

230
00:29:39,967 --> 00:29:43,357
from the soil accumulating
around the clumps of moss,

231
00:29:43,527 --> 00:29:47,839
and from the bark
that is already being broken down by fungi.

232
00:29:54,767 --> 00:29:59,761
Eventually, the roots make contact
with the earth, and, one after the other,

233
00:29:59,927 --> 00:30:04,842
a whole group of vigorous saplings
establish themselves along the trunk.

234
00:30:05,847 --> 00:30:10,716
So, eventually, a row of giant trees
stands in the forest

235
00:30:10,887 --> 00:30:14,766
in a line of almost regimental straightness.

236
00:30:14,927 --> 00:30:20,126
Each is propped up on arching roots
that never were beneath ground,

237
00:30:20,287 --> 00:30:25,839
but which still hold, between them,
the rotting remains of the huge trunk

238
00:30:26,007 --> 00:30:29,841
that 150 years ago nursed them into life.

239
00:30:35,687 --> 00:30:40,203
Southern Australia,
in the rolling mountains outside Melbourne.

240
00:30:40,367 --> 00:30:46,158
It's drier than the American North-West, but
there's still enough rain to support tall forests.

241
00:30:46,327 --> 00:30:49,956
And with them, a rich, dense undergrowth.

242
00:30:53,047 --> 00:30:56,722
These ferns are even taller
than those in British Columbia,

243
00:30:56,887 --> 00:31:01,756
these are tree ferns,
but they're still only understorey.

244
00:31:10,807 --> 00:31:16,643
Above them rise one of the tallest of all trees.

245
00:31:49,727 --> 00:31:55,518
These are called locally mountain ash,
but they're no relation of the European ash.

246
00:31:55,687 --> 00:32:01,045
In fact, they are eucalypts,
and they stand over 300 feet tall.

247
00:32:03,567 --> 00:32:06,559
They carry their tiny seeds in small capsules,

248
00:32:06,727 --> 00:32:10,356
and shed them, a few at a time,
throughout the year.

249
00:32:23,967 --> 00:32:27,755
But in the deep shade,
the seeds stand no chance.

250
00:32:27,927 --> 00:32:31,397
Even if they had landed
on the fallen trunk of an adult,

251
00:32:31,567 --> 00:32:37,039
they would not have been high enough
above the ground to be clear of the tree ferns.

252
00:32:37,207 --> 00:32:39,767
The fact is, in mature forests like this,

253
00:32:39,927 --> 00:32:43,840
the mountain ash
has a severe problem of regeneration.

254
00:32:44,007 --> 00:32:47,204
The solution could not be more dramatic.

255
00:32:50,607 --> 00:32:55,635
It's another of those events
that for human beings are major catastrophes,

256
00:32:55,807 --> 00:32:57,320
a forest fire.

257
00:33:00,927 --> 00:33:06,240
Oil in the bark and leaves of eucalyptus
make them extremely inflammable.

258
00:33:11,367 --> 00:33:13,164
As the flames leap higher,

259
00:33:13,327 --> 00:33:17,957
the seed capsules up in the crowns
are singed and shed their contents.

260
00:33:20,167 --> 00:33:24,319
The fire grows in strength
and becomes a fire-storm.

261
00:33:49,127 --> 00:33:54,281
In the wake of such a huge burn,
little is left alive.

262
00:34:03,047 --> 00:34:10,123
But, safe in the soil, below the worst of the heat,
many of the eucalyptus seeds have survived.

263
00:34:15,367 --> 00:34:18,916
In the sunshine,
they sprout with extraordinary speed.

264
00:34:19,087 --> 00:34:22,602
Few, if any,
of their competitors can match them.

265
00:34:22,767 --> 00:34:25,122
Nourished by the rich dressing of ash,

266
00:34:25,287 --> 00:34:30,520
they grow around the charred logs
as thick and as uniform as a crop of wheat.

267
00:34:37,287 --> 00:34:43,760
Within a year or so, they're so tall that no
other competitors are able to invade their land.

268
00:35:06,367 --> 00:35:09,962
This stump was burning exactly ten years ago.

269
00:35:10,127 --> 00:35:16,475
These saplings around me sprouted from seeds
that germinated immediately after that fire.

270
00:35:16,647 --> 00:35:20,879
The saplings are growing so close together
that their roots interlock,

271
00:35:21,047 --> 00:35:26,075
forming a dense mat into which
few other tree seedlings can get a root-hold.

272
00:35:26,247 --> 00:35:30,320
This land still belongs to the mountain ash.

273
00:35:35,607 --> 00:35:42,479
As the trees begin to spread their canopies,
the less vigorous saplings are thinned out.

274
00:35:42,647 --> 00:35:47,437
Now there is space on the ground
for the tree ferns to return.

275
00:35:47,607 --> 00:35:51,998
Only the giant redwoods of California
exceed the mountain ash in height,

276
00:35:52,167 --> 00:35:56,206
and that may only be
because of the activities of loggers.

277
00:35:56,367 --> 00:36:02,283
Back in the 19th century,
one ash was felled here that measured 435 feet.

278
00:36:02,447 --> 00:36:06,122
That's the tallest tree ever known.

279
00:36:07,567 --> 00:36:11,162
The greatest threat to the survival of this forest

280
00:36:11,327 --> 00:36:17,197
is that it should grow
for hundreds of years without fire.

281
00:36:17,367 --> 00:36:24,000
Should that happen, then these huge trees
will eventually die from sheer old age,

282
00:36:24,167 --> 00:36:30,003
fall and lie buried among the tree ferns
without ever having reseeded.

283
00:36:30,167 --> 00:36:39,121
The paradox is that this magnificent forest
can only survive if it is first almost destroyed.

284
00:36:57,927 --> 00:37:01,886
Another fire, this time in the west of Australia.

285
00:37:02,047 --> 00:37:08,680
Here, the soil is so poor, and the climate so dry,
that tall trees can't grow.

286
00:37:09,527 --> 00:37:13,759
Instead, there's a rich variety of bushy plants.

287
00:37:13,927 --> 00:37:18,523
The grass tree has a shock of long leaves
that burn fiercely but quickly.

288
00:37:18,687 --> 00:37:23,715
It is neither a grass nor a tree,
but a strange relation of the lilies.

289
00:37:28,527 --> 00:37:34,477
What looks like a woody trunk
is a fibrous stem that has a special protection.

290
00:37:35,367 --> 00:37:41,237
The grass tree sheds its leaves each year,
but their bases remain attached to the stem,

291
00:37:41,407 --> 00:37:47,721
and produce a thick gum that glues the whole lot
together into a very effective fire-guard.

292
00:37:48,607 --> 00:37:52,282
Even if it doesn't save every one,
most will survive.

293
00:37:52,447 --> 00:37:57,680
Indeed, every species here has to have
the ability to live through at least a brief fire.

294
00:37:57,847 --> 00:38:01,726
Otherwise, it would quickly
lose its place in the community.

295
00:38:03,767 --> 00:38:08,795
It's 8 months since the fire, and the bush
has recovered in a most dramatic way.

296
00:38:09,487 --> 00:38:13,844
The fire stimulated the various plants
in at least four different ways.

297
00:38:14,007 --> 00:38:17,636
Its sheer heat baked these fruits of the banksias,

298
00:38:17,807 --> 00:38:21,880
opening the windows on the sides,
allowing the seeds to fall to the ground.

299
00:38:22,407 --> 00:38:26,798
The fire also produced
great volumes of ethylene gas,

300
00:38:26,967 --> 00:38:32,405
and that triggered the grass tree,
after it had regrown its burnt leaves,

301
00:38:32,567 --> 00:38:38,881
to put up this huge green spike
which will ultimately carry its flowers.

302
00:38:39,047 --> 00:38:40,719
The smoke of the fire...

303
00:38:41,727 --> 00:38:47,643
..was the stimulus which caused seeds
that had lain dormant in this sand

304
00:38:47,807 --> 00:38:51,436
for the last twenty years or so since the last fire,

305
00:38:51,607 --> 00:38:56,397
it triggered them to produce
the annual plants which are now springing up.

306
00:38:56,567 --> 00:39:02,915
And it was the sudden superabundance
of nutrients produced by the fire in its ash

307
00:39:03,087 --> 00:39:07,319
which allowed the perennial plants
like this lovely cat's-paw

308
00:39:07,487 --> 00:39:11,400
to cover the whole ground with colour.

309
00:39:13,847 --> 00:39:19,444
Within a few years,
the whole community is fully restored.

310
00:39:31,607 --> 00:39:36,397
The fire was an opportunity
for the social struggle to begin again.

311
00:39:36,567 --> 00:39:41,118
Each individual plant
had to fight once more for living space,

312
00:39:41,287 --> 00:39:44,563
profiting or suffering from its speed of recovery,

313
00:39:44,727 --> 00:39:50,040
or its suitability to any slight change
in the terrain that the fire created.

314
00:39:50,207 --> 00:39:55,998
But, overall, the character of the bush
remains unchanged.

315
00:40:00,807 --> 00:40:03,924
The grasslands of East Africa are not so stable.

316
00:40:04,087 --> 00:40:08,046
Here, fire, or its absence,
can trigger great change.

317
00:40:11,087 --> 00:40:13,555
This fire is advancing quite fast

318
00:40:13,727 --> 00:40:18,596
because the grass is so dry that as soon as
the flames reach the leaves,

319
00:40:18,767 --> 00:40:20,997
they are consumed within seconds.

320
00:40:21,167 --> 00:40:25,080
And as long as there's wind behind it, it'll travel.

321
00:40:25,247 --> 00:40:30,367
But in fact, the line of the fire is very thin,

322
00:40:30,527 --> 00:40:34,998
and if I want to, it's quite easy to cross it.

323
00:40:41,527 --> 00:40:49,081
The land certainly looks ravaged and destroyed,
but in fact, little damage has been done.

324
00:40:49,247 --> 00:40:52,922
The leaves of these grasses, certainly,
have disappeared.

325
00:40:53,087 --> 00:40:57,638
But the roots are undamaged.

326
00:40:57,807 --> 00:41:01,402
The heat is at its least intense
close to the ground,

327
00:41:01,567 --> 00:41:07,597
and this part of the root, close to the surface
of the earth, is totally undamaged.

328
00:41:07,767 --> 00:41:11,043
And it's from here
that the new growth will come.

329
00:41:14,807 --> 00:41:19,722
And there are many around
to relish the succulent young leaves.

330
00:41:20,727 --> 00:41:26,279
Grasses can survive such cropping because their
leaves readily break at the base when pulled,

331
00:41:26,447 --> 00:41:30,156
leaving the horizontal stems
and their buds undamaged.

332
00:41:34,847 --> 00:41:40,843
Plants with vertical stems grow from the top.
If cropped, they're likely to be killed.

333
00:41:44,607 --> 00:41:49,761
And if they're pulled, they come up roots and all.

334
00:41:51,367 --> 00:41:57,442
So the great herds of Africa
in effect weed out the grass's competitors,

335
00:41:57,607 --> 00:42:03,603
and it has vast acres to itself,
which suits the grazers as well as the grass.

336
00:42:03,767 --> 00:42:07,043
But something can upset
this happy arrangement.

337
00:42:07,207 --> 00:42:10,165
Another of those environmental catastrophes.

338
00:42:10,327 --> 00:42:13,239
This time it's drought.

339
00:42:14,167 --> 00:42:16,965
The sun turns the soil to dust.

340
00:42:17,127 --> 00:42:23,362
The animals, having eaten the last dry stalks,
move away to look for food elsewhere.

341
00:42:24,487 --> 00:42:27,001
The land is left bare.

342
00:42:27,167 --> 00:42:31,718
Scavengers come to clean up the corpses
of those that starved to death,

343
00:42:31,887 --> 00:42:34,640
but they too will soon leave.

344
00:42:36,487 --> 00:42:40,241
When the rains at last return,
the grass springs again,

345
00:42:40,407 --> 00:42:44,719
as do other plants whose seeds,
one way or another, have landed here.

346
00:42:44,887 --> 00:42:50,678
With no animals to graze them,
acacia seedlings grow into bushes.

347
00:42:50,847 --> 00:42:52,917
And they grow very fast.

348
00:42:53,087 --> 00:42:58,559
Once they are a few feet high, they're tall
enough to survive a fast-moving grass fire.

349
00:42:58,727 --> 00:43:02,276
And their thorns will protect them
from most grazers.

350
00:43:09,487 --> 00:43:12,126
By the time they're about ten years old,

351
00:43:12,287 --> 00:43:16,360
their spreading branches cut off
most of the light from the ground,

352
00:43:16,527 --> 00:43:20,315
their thrusting roots are sucking up
most of the moisture,

353
00:43:20,487 --> 00:43:23,718
and the grass has virtually disappeared.

354
00:43:26,847 --> 00:43:30,601
A patch of acacia scrub has established itself.

355
00:43:30,767 --> 00:43:34,680
All the trees, significantly,
are of the same age and height.

356
00:43:34,847 --> 00:43:38,237
They all took the same opportunity to sprout.

357
00:43:41,807 --> 00:43:45,436
But this situation isn't permanent either.

358
00:44:25,447 --> 00:44:29,042
A hungry elephant
that fancies a mouthful of acacia leaves

359
00:44:29,207 --> 00:44:36,283
has no problem, and no hesitation either,
in pushing over the whole tree to get it.

360
00:44:44,007 --> 00:44:47,602
Strangely, it's not just acacias
that they knock over.

361
00:44:48,487 --> 00:44:51,445
They also push down commiphera trees.

362
00:44:51,607 --> 00:44:55,725
They don't even like their leaves,
and seldom eat them.

363
00:44:55,887 --> 00:44:58,606
So is that just wanton destruction,

364
00:44:58,767 --> 00:45:04,285
or can they possibly know
what the effect of removing the trees will be?

365
00:45:13,767 --> 00:45:15,598
Whether they know it or not,

366
00:45:15,767 --> 00:45:19,442
a group of elephant,
once they have taken up residence,

367
00:45:19,607 --> 00:45:27,116
can turn a promising acacia woodland
back to grass in only four or five years.

368
00:45:29,847 --> 00:45:32,361
The grazers return,

369
00:45:33,047 --> 00:45:39,156
and the elephants, once more, have supplies
of their favourite food, which is grass.

370
00:45:56,207 --> 00:46:03,158
Elephant are, without any doubt, prime factors
in removing thorn scrub from the plain

371
00:46:03,327 --> 00:46:05,841
and allowing grass to spread.

372
00:46:06,007 --> 00:46:09,636
You could argue that, in effect,
they are farming the grass,

373
00:46:09,807 --> 00:46:15,518
knocking over trees they don't like
and browsing the thorn scrub into destruction.

374
00:46:15,687 --> 00:46:19,236
But you could also argue things
the other way round.

375
00:46:19,407 --> 00:46:23,400
You could say it is the grass
which is exploiting the elephants.

376
00:46:23,567 --> 00:46:26,923
By providing them with food year after year,

377
00:46:27,087 --> 00:46:31,160
the grass remain
the dominant plants on the plains.

378
00:46:31,327 --> 00:46:33,557
And if you look at things that way,

379
00:46:33,727 --> 00:46:39,199
then that is a trick which other grasses
have played on a world-wide scale.

380
00:46:41,607 --> 00:46:44,917
Wheat once grew only around the Mediterranean.

381
00:46:45,087 --> 00:46:50,605
10,000 years ago, its seeds were accepted
by a small group of human beings as food.

382
00:46:50,767 --> 00:46:55,443
It was so much to their liking
that they sowed it wherever they settled

383
00:46:55,607 --> 00:46:59,156
and carried it with them
as they overran the earth.

384
00:47:01,047 --> 00:47:04,039
So a few species of grass,

385
00:47:04,207 --> 00:47:09,201
by recruiting the aid of animals,
and in particular ourselves, the human animal,

386
00:47:09,367 --> 00:47:13,076
have succeeded in interrupting
the ecological cycles

387
00:47:13,247 --> 00:47:17,798
that have operated for millions of years
in so many parts of the earth.

388
00:47:17,967 --> 00:47:21,323
They've managed to claim
for their own exclusive use

389
00:47:21,487 --> 00:47:26,515
not only wide open plains
but fertile, well-watered lands

390
00:47:26,687 --> 00:47:31,158
that once supported
rich communities of animals and plants.

391
00:47:31,327 --> 00:47:34,842
These grasses have solved the social struggle.

392
00:47:35,007 --> 00:47:38,795
They have got rid of their competitors.

393
01:09:30,287 --> 01:09:34,075
Eventually, they reach the ground.

394
01:09:37,007 --> 01:09:42,718
Now, supplied with nutrients from the soil,
the fig grows really fast.

395
01:09:50,007 --> 01:09:56,640
The rootlets wrapped around the main trunk
thicken and fuse into a lattice.

396
01:10:04,727 --> 01:10:12,566
The host tree's fate is now sealed,
for it is in the clutches of a strangler fig.

397
01:10:14,127 --> 01:10:20,123
As years pass, the fig thickens its roots,
embracing the trunk ever more completely.

398
01:10:20,287 --> 01:10:25,725
Trees grow by increasing their girth.
For the host tree, that is now impossible.

399
01:10:25,887 --> 01:10:30,642
But growth is difficult anyway,
because the fig has a huge crown in the canopy

400
01:10:30,807 --> 01:10:33,116
that cuts off sunshine from its host,

401
01:10:33,287 --> 01:10:38,315
and its roots in the ground
are stealing most of the soil's nutrients.

402
01:10:41,807 --> 01:10:47,518
Eventually, the host tree is killed,
and its trunk rots away.

403
01:10:47,687 --> 01:10:50,360
But the fig does not fall.

404
01:10:52,087 --> 01:10:59,038
Its roots now form a hollow cylinder that is
quite capable of standing upright by itself.

405
01:11:18,567 --> 01:11:21,604
This strangler is about 300 years old.

406
01:11:21,767 --> 01:11:25,316
In fact, it may be misleading
to refer to it as a single tree.

407
01:11:25,487 --> 01:11:30,561
It's probable that, 300 years ago,
there were several young figs up in the canopy.

408
01:11:30,727 --> 01:11:34,561
Now, centuries later,
their roots have grown down to the ground,

409
01:11:34,727 --> 01:11:37,287
they've got rid of the body of their victim,

410
01:11:37,447 --> 01:11:42,567
and they're clinging to one another in this
extraordinary interlace of pillars and buttresses

411
01:11:42,727 --> 01:11:47,005
in order to maintain their dominance
of this part of the forest.

412
01:11:55,327 --> 01:12:00,003
Nor are these monsters
always satisfied with just one victim.

413
01:12:00,167 --> 01:12:06,197
This 500-year-old, having strangled
its first victim and lost its support,

414
01:12:06,367 --> 01:12:10,838
toppled sideways into a second,
killed that, and then a third,

415
01:12:11,007 --> 01:12:15,000
and now its roots are ready to embrace a fourth.

416
01:12:21,487 --> 01:12:27,517
Dead tree trunks are not wasted,
and neither are dead leaves when they fall.

417
01:12:27,687 --> 01:12:30,485
Both are food for fungi.

418
01:12:30,647 --> 01:12:35,038
Some leaves are captured
even before they reach the ground.

419
01:12:35,207 --> 01:12:40,884
The fungus has constructed a net,
stretched between the twigs of the undergrowth.

420
01:12:47,367 --> 01:12:51,804
Once they've caught their leaf,
the threads put out white filaments.

421
01:13:07,447 --> 01:13:12,202
These produce powerful acid
which dissolves the cellulose in the leaves.

422
01:13:12,367 --> 01:13:14,961
Why doesn't it dissolve the fungus, too?

423
01:13:15,127 --> 01:13:19,962
Because fungi are not plants,
their bodies don't contain cellulose.

424
01:13:20,127 --> 01:13:27,602
They're constructed from a material much more
akin to that of animal horn and hooves.

425
01:13:27,767 --> 01:13:34,764
Fungi are neither plant nor animal, they belong
to a category of life that is all their own.

426
01:13:36,567 --> 01:13:43,439
Nourished by the liquefied tissues of leaves,
this fungus puts out more threads.

427
01:13:43,607 --> 01:13:46,167
But fungi do require moisture.

428
01:13:46,327 --> 01:13:51,355
They can only live out in the open like this
in the moist atmosphere of the rainforest.

429
01:13:53,487 --> 01:13:58,242
In cooler, drier woodlands, they have
great difficulty living out in the open.

430
01:13:58,407 --> 01:14:03,606
Instead, they hide in the ground or within
the tissues of the bodies they feast on.

431
01:14:06,247 --> 01:14:10,559
A fungus has no stem, no root, no leaves.

432
01:14:10,727 --> 01:14:17,326
For most of the time, it's nothing more
than a tangled tissue of branching threads.

433
01:14:17,487 --> 01:14:21,605
These produce digestive acids,
absorb the resulting soup,

434
01:14:21,767 --> 01:14:28,400
and use it to construct more threads and widen
their search for more dead plant tissue.

435
01:14:29,407 --> 01:14:32,956
But cellulose is very low in nitrogen.

436
01:14:34,207 --> 01:14:39,122
To get that, some fungi trap living animals.

437
01:14:39,287 --> 01:14:42,757
The microscopic threads develop tiny lassoes.

438
01:14:42,927 --> 01:14:49,719
These give off a chemical that attracts
microscopic worms, nematodes.

439
01:14:54,047 --> 01:14:57,164
One of them nuzzles into the ring.

440
01:14:59,127 --> 01:15:03,723
And the fungus suddenly draws its lasso tight.

441
01:15:09,287 --> 01:15:13,644
The worms are killed
and the fungus has its nitrogen.

442
01:15:22,487 --> 01:15:28,244
All this takes place out of sight below ground
or within the body of a dead plant.

443
01:15:28,407 --> 01:15:33,800
Only when a fungus is ready to reproduce
does it make itself more visible.

444
01:16:32,527 --> 01:16:38,602
From such apparitions as these come spores,
the fungal equivalent of seeds.

445
01:16:39,927 --> 01:16:45,160
They're so small that they drift away like smoke.

446
01:16:45,327 --> 01:16:49,525
But the appearance of these
spectacular constructions is brief.

447
01:16:49,687 --> 01:16:55,922
As soon as their spores have been shed,
sometimes after only a few days, they collapse.

448
01:17:00,887 --> 01:17:04,880
Now they are merely food for maggots.

449
01:17:05,727 --> 01:17:09,640
So the corpses of plants
do not retain their nutriment forever.

450
01:17:09,807 --> 01:17:12,116
Some of it is consumed by fungi,

451
01:17:12,287 --> 01:17:14,926
and the remainder, now in soluble form,

452
01:17:15,087 --> 01:17:19,444
seeps back into the soil
to sustain the next generation.

453
01:17:23,847 --> 01:17:27,681
But it's not always easy
for that new generation to get a start.

454
01:17:27,847 --> 01:17:31,237
On the north-west coast of America
and in British Columbia,

455
01:17:31,407 --> 01:17:35,480
fir, spruce and hemlock grow so densely and tall

456
01:17:35,647 --> 01:17:41,916
that very little light filters down to the world
below, except when one of them dies.

457
01:17:42,727 --> 01:17:47,243
This giant tree fell about ten years ago.

458
01:17:47,407 --> 01:17:51,036
The gap it left in the canopy above is still open,

459
01:17:51,207 --> 01:17:54,916
but there's still very little light
on the forest floor itself -

460
01:17:55,087 --> 01:17:58,443
these ferns and mosses are so very thick.

461
01:17:58,607 --> 01:18:04,204
But up here on the fallen trunk,
things are very different.

462
01:18:04,367 --> 01:18:06,483
This is the next generation.

463
01:18:06,647 --> 01:18:09,366
Bare bark doesn't hold the moisture.

464
01:18:09,527 --> 01:18:12,439
Thick moss could bury a seedling.

465
01:18:12,607 --> 01:18:15,997
But in moss like this, growing in such a position,

466
01:18:16,167 --> 01:18:21,036
a young plant can get
all the moisture and the light that it needs.

467
01:18:26,447 --> 01:18:31,601
Its threadlike roots grow downwards
over the surface of the trunk.

468
01:18:35,007 --> 01:18:39,125
Even before they reach the ground,
they find plenty of nourishment,

469
01:18:39,287 --> 01:18:42,677
from the soil accumulating
around the clumps of moss,

470
01:18:42,847 --> 01:18:47,159
and from the bark
that is already being broken down by fungi.

471
01:18:54,087 --> 01:18:59,081
Eventually, the roots make contact
with the earth, and, one after the other,

472
01:18:59,247 --> 01:19:04,162
a whole group of vigorous saplings
establish themselves along the trunk.

473
01:19:05,167 --> 01:19:10,036
So, eventually, a row of giant trees
stands in the forest

474
01:19:10,207 --> 01:19:14,086
in a line of almost regimental straightness.

475
01:19:14,247 --> 01:19:19,446
Each is propped up on arching roots
that never were beneath ground,

476
01:19:19,607 --> 01:19:25,159
but which still hold, between them,
the rotting remains of the huge trunk

477
01:19:25,327 --> 01:19:29,161
that 150 years ago nursed them into life.

478
01:19:35,007 --> 01:19:39,523
Southern Australia,
in the rolling mountains outside Melbourne.

479
01:19:39,687 --> 01:19:45,478
It's drier than the American North-West, but
there's still enough rain to support tall forests.

480
01:19:45,647 --> 01:19:49,276
And with them, a rich, dense undergrowth.

481
01:19:52,367 --> 01:19:56,042
These ferns are even taller
than those in British Columbia,

482
01:19:56,207 --> 01:20:01,076
these are tree ferns,
but they're still only understorey.

483
01:20:10,127 --> 01:20:15,963
Above them rise one of the tallest of all trees.

484
01:20:49,047 --> 01:20:54,838
These are called locally mountain ash,
but they're no relation of the European ash.

485
01:20:55,007 --> 01:21:00,365
In fact, they are eucalypts,
and they stand over 300 feet tall.

486
01:21:02,887 --> 01:21:05,879
They carry their tiny seeds in small capsules,

487
01:21:06,047 --> 01:21:09,676
and shed them, a few at a time,
throughout the year.

488
01:21:23,287 --> 01:21:27,075
But in the deep shade,
the seeds stand no chance.

489
01:21:27,247 --> 01:21:30,717
Even if they had landed
on the fallen trunk of an adult,

490
01:21:30,887 --> 01:21:36,359
they would not have been high enough
above the ground to be clear of the tree ferns.

491
01:21:36,527 --> 01:21:39,087
The fact is, in mature forests like this,

492
01:21:39,247 --> 01:21:43,160
the mountain ash
has a severe problem of regeneration.

493
01:21:43,327 --> 01:21:46,524
The solution could not be more dramatic.

494
01:21:49,927 --> 01:21:54,955
It's another of those events
that for human beings are major catastrophes,

495
01:21:55,127 --> 01:21:56,640
a forest fire.

496
01:22:00,247 --> 01:22:05,560
Oil in the bark and leaves of eucalyptus
make them extremely inflammable.

497
01:22:10,687 --> 01:22:12,484
As the flames leap higher,

498
01:22:12,647 --> 01:22:17,277
the seed capsules up in the crowns
are singed and shed their contents.

499
01:22:19,487 --> 01:22:23,639
The fire grows in strength
and becomes a fire-storm.

500
01:22:48,447 --> 01:22:53,601
In the wake of such a huge burn,
little is left alive.

501
01:23:02,367 --> 01:23:09,443
But, safe in the soil, below the worst of the heat,
many of the eucalyptus seeds have survived.

502
01:23:14,687 --> 01:23:18,236
In the sunshine,
they sprout with extraordinary speed.

503
01:23:18,407 --> 01:23:21,922
Few, if any,
of their competitors can match them.

504
01:23:22,087 --> 01:23:24,442
Nourished by the rich dressing of ash,

505
01:23:24,607 --> 01:23:29,840
they grow around the charred logs
as thick and as uniform as a crop of wheat.

506
01:23:36,607 --> 01:23:43,080
Within a year or so, they're so tall that no
other competitors are able to invade their land.

507
01:24:05,687 --> 01:24:09,282
This stump was burning exactly ten years ago.

508
01:24:09,447 --> 01:24:15,795
These saplings around me sprouted from seeds
that germinated immediately after that fire.

509
01:24:15,967 --> 01:24:20,199
The saplings are growing so close together
that their roots interlock,

510
01:24:20,367 --> 01:24:25,395
forming a dense mat into which
few other tree seedlings can get a root-hold.

511
01:24:25,567 --> 01:24:29,640
This land still belongs to the mountain ash.

512
01:24:34,927 --> 01:24:41,799
As the trees begin to spread their canopies,
the less vigorous saplings are thinned out.

513
01:24:41,967 --> 01:24:46,757
Now there is space on the ground
for the tree ferns to return.

514
01:24:46,927 --> 01:24:51,318
Only the giant redwoods of California
exceed the mountain ash in height,

515
01:24:51,487 --> 01:24:55,526
and that may only be
because of the activities of loggers.

516
01:24:55,687 --> 01:25:01,603
Back in the 19th century,
one ash was felled here that measured 435 feet.

517
01:25:01,767 --> 01:25:05,442
That's the tallest tree ever known.

518
01:25:06,887 --> 01:25:10,482
The greatest threat to the survival of this forest

519
01:25:10,647 --> 01:25:16,517
is that it should grow
for hundreds of years without fire.

520
01:25:16,687 --> 01:25:23,320
Should that happen, then these huge trees
will eventually die from sheer old age,

521
01:25:23,487 --> 01:25:29,323
fall and lie buried among the tree ferns
without ever having reseeded.

522
01:25:29,487 --> 01:25:38,441
The paradox is that this magnificent forest
can only survive if it is first almost destroyed.

523
01:25:57,247 --> 01:26:01,206
Another fire, this time in the west of Australia.

524
01:26:01,367 --> 01:26:08,000
Here, the soil is so poor, and the climate so dry,
that tall trees can't grow.

525
01:26:08,847 --> 01:26:13,079
Instead, there's a rich variety of bushy plants.

526
01:26:13,247 --> 01:26:17,843
The grass tree has a shock of long leaves
that burn fiercely but quickly.

527
01:26:18,007 --> 01:26:23,035
It is neither a grass nor a tree,
but a strange relation of the lilies.

528
01:26:27,847 --> 01:26:33,797
What looks like a woody trunk
is a fibrous stem that has a special protection.

529
01:26:34,687 --> 01:26:40,557
The grass tree sheds its leaves each year,
but their bases remain attached to the stem,

530
01:26:40,727 --> 01:26:47,041
and produce a thick gum that glues the whole lot
together into a very effective fire-guard.

531
01:26:47,927 --> 01:26:51,602
Even if it doesn't save every one,
most will survive.

532
01:26:51,767 --> 01:26:57,000
Indeed, every species here has to have
the ability to live through at least a brief fire.

533
01:26:57,167 --> 01:27:01,046
Otherwise, it would quickly
lose its place in the community.

534
01:27:03,087 --> 01:27:08,115
It's 8 months since the fire, and the bush
has recovered in a most dramatic way.

535
01:27:08,807 --> 01:27:13,164
The fire stimulated the various plants
in at least four different ways.

536
01:27:13,327 --> 01:27:16,956
Its sheer heat baked these fruits of the banksias,

537
01:27:17,127 --> 01:27:21,200
opening the windows on the sides,
allowing the seeds to fall to the ground.

538
01:27:21,727 --> 01:27:26,118
The fire also produced
great volumes of ethylene gas,

539
01:27:26,287 --> 01:27:31,725
and that triggered the grass tree,
after it had regrown its burnt leaves,

540
01:27:31,887 --> 01:27:38,201
to put up this huge green spike
which will ultimately carry its flowers.

541
01:27:38,367 --> 01:27:40,039
The smoke of the fire...

542
01:27:41,047 --> 01:27:46,963
..was the stimulus which caused seeds
that had lain dormant in this sand

543
01:27:47,127 --> 01:27:50,756
for the last twenty years or so since the last fire,

544
01:27:50,927 --> 01:27:55,717
it triggered them to produce
the annual plants which are now springing up.

545
01:27:55,887 --> 01:28:02,235
And it was the sudden superabundance
of nutrients produced by the fire in its ash

546
01:28:02,407 --> 01:28:06,639
which allowed the perennial plants
like this lovely cat's-paw

547
01:28:06,807 --> 01:28:10,720
to cover the whole ground with colour.

548
01:28:13,167 --> 01:28:18,764
Within a few years,
the whole community is fully restored.

549
01:28:30,927 --> 01:28:35,717
The fire was an opportunity
for the social struggle to begin again.

550
01:28:35,887 --> 01:28:40,438
Each individual plant
had to fight once more for living space,

551
01:28:40,607 --> 01:28:43,883
profiting or suffering from its speed of recovery,

552
01:28:44,047 --> 01:28:49,360
or its suitability to any slight change
in the terrain that the fire created.

553
01:28:49,527 --> 01:28:55,318
But, overall, the character of the bush
remains unchanged.

554
01:29:00,127 --> 01:29:03,244
The grasslands of East Africa are not so stable.

555
01:29:03,407 --> 01:29:07,366
Here, fire, or its absence,
can trigger great change.

556
01:29:10,407 --> 01:29:12,875
This fire is advancing quite fast

557
01:29:13,047 --> 01:29:17,916
because the grass is so dry that as soon as
the flames reach the leaves,

558
01:29:18,087 --> 01:29:20,317
they are consumed within seconds.

559
01:29:20,487 --> 01:29:24,400
And as long as there's wind behind it, it'll travel.

560
01:29:24,567 --> 01:29:29,687
But in fact, the line of the fire is very thin,

561
01:29:29,847 --> 01:29:34,318
and if I want to, it's quite easy to cross it.

562
01:29:40,847 --> 01:29:48,401
The land certainly looks ravaged and destroyed,
but in fact, little damage has been done.

563
01:29:48,567 --> 01:29:52,242
The leaves of these grasses, certainly,
have disappeared.

564
01:29:52,407 --> 01:29:56,958
But the roots are undamaged.

565
01:29:57,127 --> 01:30:00,722
The heat is at its least intense
close to the ground,

566
01:30:00,887 --> 01:30:06,917
and this part of the root, close to the surface
of the earth, is totally undamaged.

567
01:30:07,087 --> 01:30:10,363
And it's from here
that the new growth will come.

568
01:30:14,127 --> 01:30:19,042
And there are many around
to relish the succulent young leaves.

569
01:30:20,047 --> 01:30:25,599
Grasses can survive such cropping because their
leaves readily break at the base when pulled,

570
01:30:25,767 --> 01:30:29,476
leaving the horizontal stems
and their buds undamaged.

571
01:30:34,167 --> 01:30:40,163
Plants with vertical stems grow from the top.
If cropped, they're likely to be killed.

572
01:30:43,927 --> 01:30:49,081
And if they're pulled, they come up roots and all.

573
01:30:50,687 --> 01:30:56,762
So the great herds of Africa
in effect weed out the grass's competitors,

574
01:30:56,927 --> 01:31:02,923
and it has vast acres to itself,
which suits the grazers as well as the grass.

575
01:31:03,087 --> 01:31:06,363
But something can upset
this happy arrangement.

576
01:31:06,527 --> 01:31:09,485
Another of those environmental catastrophes.

577
01:31:09,647 --> 01:31:12,559
This time it's drought.

578
01:31:13,487 --> 01:31:16,285
The sun turns the soil to dust.

579
01:31:16,447 --> 01:31:22,682
The animals, having eaten the last dry stalks,
move away to look for food elsewhere.

580
01:31:23,807 --> 01:31:26,321
The land is left bare.

581
01:31:26,487 --> 01:31:31,038
Scavengers come to clean up the corpses
of those that starved to death,

582
01:31:31,207 --> 01:31:33,960
but they too will soon leave.

583
01:31:35,807 --> 01:31:39,561
When the rains at last return,
the grass springs again,

584
01:31:39,727 --> 01:31:44,039
as do other plants whose seeds,
one way or another, have landed here.

585
01:31:44,207 --> 01:31:49,998
With no animals to graze them,
acacia seedlings grow into bushes.

586
01:31:50,167 --> 01:31:52,237
And they grow very fast.

587
01:31:52,407 --> 01:31:57,879
Once they are a few feet high, they're tall
enough to survive a fast-moving grass fire.

588
01:31:58,047 --> 01:32:01,596
And their thorns will protect them
from most grazers.

589
01:32:08,807 --> 01:32:11,446
By the time they're about ten years old,

590
01:32:11,607 --> 01:32:15,680
their spreading branches cut off
most of the light from the ground,

591
01:32:15,847 --> 01:32:19,635
their thrusting roots are sucking up
most of the moisture,

592
01:32:19,807 --> 01:32:23,038
and the grass has virtually disappeared.

593
01:32:26,167 --> 01:32:29,921
A patch of acacia scrub has established itself.

594
01:32:30,087 --> 01:32:34,000
All the trees, significantly,
are of the same age and height.

595
01:32:34,167 --> 01:32:37,557
They all took the same opportunity to sprout.

596
01:32:41,127 --> 01:32:44,756
But this situation isn't permanent either.

597
01:33:24,767 --> 01:33:28,362
A hungry elephant
that fancies a mouthful of acacia leaves

598
01:33:28,527 --> 01:33:35,603
has no problem, and no hesitation either,
in pushing over the whole tree to get it.

599
01:33:43,327 --> 01:33:46,922
Strangely, it's not just acacias
that they knock over.

600
01:33:47,807 --> 01:33:50,765
They also push down commiphera trees.

601
01:33:50,927 --> 01:33:55,045
They don't even like their leaves,
and seldom eat them.

602
01:33:55,207 --> 01:33:57,926
So is that just wanton destruction,

603
01:33:58,087 --> 01:34:03,605
or can they possibly know
what the effect of removing the trees will be?

604
01:34:13,087 --> 01:34:14,918
Whether they know it or not,

605
01:34:15,087 --> 01:34:18,762
a group of elephant,
once they have taken up residence,

606
01:34:18,927 --> 01:34:26,436
can turn a promising acacia woodland
back to grass in only four or five years.

607
01:34:29,167 --> 01:34:31,681
The grazers return,

608
01:34:32,367 --> 01:34:38,476
and the elephants, once more, have supplies
of their favourite food, which is grass.

609
01:34:55,527 --> 01:35:02,478
Elephant are, without any doubt, prime factors
in removing thorn scrub from the plain

610
01:35:02,647 --> 01:35:05,161
and allowing grass to spread.

611
01:35:05,327 --> 01:35:08,956
You could argue that, in effect,
they are farming the grass,

612
01:35:09,127 --> 01:35:14,838
knocking over trees they don't like
and browsing the thorn scrub into destruction.

613
01:35:15,007 --> 01:35:18,556
But you could also argue things
the other way round.

614
01:35:18,727 --> 01:35:22,720
You could say it is the grass
which is exploiting the elephants.

615
01:35:22,887 --> 01:35:26,243
By providing them with food year after year,

616
01:35:26,407 --> 01:35:30,480
the grass remain
the dominant plants on the plains.

617
01:35:30,647 --> 01:35:32,877
And if you look at things that way,

618
01:35:33,047 --> 01:35:38,519
then that is a trick which other grasses
have played on a world-wide scale.

619
01:35:40,927 --> 01:35:44,237
Wheat once grew only around the Mediterranean.

620
01:35:44,407 --> 01:35:49,925
10,000 years ago, its seeds were accepted
by a small group of human beings as food.

621
01:35:50,087 --> 01:35:54,763
It was so much to their liking
that they sowed it wherever they settled

622
01:35:54,927 --> 01:35:58,476
and carried it with them
as they overran the earth.

623
01:36:00,367 --> 01:36:03,359
So a few species of grass,

624
01:36:03,527 --> 01:36:08,521
by recruiting the aid of animals,
and in particular ourselves, the human animal,

625
01:36:08,687 --> 01:36:12,396
have succeeded in interrupting
the ecological cycles

626
01:36:12,567 --> 01:36:17,118
that have operated for millions of years
in so many parts of the earth.

627
01:36:17,287 --> 01:36:20,643
They've managed to claim
for their own exclusive use

628
01:36:20,807 --> 01:36:25,835
not only wide open plains
but fertile, well-watered lands

629
01:36:26,007 --> 01:36:30,478
that once supported
rich communities of animals and plants.

630
01:36:30,647 --> 01:36:34,162
These grasses have solved the social struggle.

631
01:36:34,327 --> 01:36:38,115
They have got rid of their competitors.

632
01:36:39,305 --> 01:36:45,709
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