加载中…地球脉动第一季05 沙漠 中英对照字幕
(2017-05-13 19:32:30)A
third of the land
on
our planet is desert.
These
great scars
on
the face of the Earth
appear to be lifeless,
but
surprisingly none are.
In
all of them life manages somehow
to
keep a precarious hold.
Not
all deserts are hot.
Fifty-mile-an-hour winds
blowing in from Siberi
bring
snow to the Gobi Desert
in
Mongolia.
From
a summer high
of 50
degrees centigrade
the
temperature in midwinter
can
drop to minus 40,
making this one
of
the harshest deserts of all.
Few
animals can survive
these
extreme changes.
Wild
Bactrian camels,
one
of the rarest mammals
on
the planet.
And
perhaps the hardiest.
Their
biggest problem
is
the lack of water,
particularly now, in winter,
when
the little there is
is
locked up as ice.
Surprisingly, snow here never
melts.
The
air is just too cold
and
too dry for it to do so.
The
sun's rays turn it
straight into vapour.
It
evaporates.
But
it is the only source
of
water,
so
Bactrian camels eat it.
Elsewhere in the world
a
camel at a waterhole can drink
as
much as 200 litres during a single visit.
Here
the strategy is to take little and often.
And
with good reason,
for
filling the stomach
with
snow could be fatal.
The
camels must limit themselves
to
the equivalent
of
just 10 litres a day.
Winter is the time
for
breeding.
This
extraordinary performance
is a
male Bactrian camel's way
of
attracting the attention
of a
passing female.
In
summer the camels can't stray
far
from waterholes.
But
now, with mouthfuls
of
snow lying everywhere
they
can travel widely
in
search of mates.
Today
less than a thousand of these desert
specialists remain in the wild.
The
Gobi, hostile though it is,
is
their last stronghold.
There's no other desert
quite
like the Gobi,
but
why is this place a desert?
There
is one simple
and
massive cause -
the
Himalayas.
Clouds blowing from the south
hit
this gigantic barrier.
As
they're forced upwards
so
they empty their moisture
on
the mountain slopes,
leaving little for the land
on
the other side.
From
the space
deserts are very conspicuous.
Dunes
of sand hundreds of miles long
streak their surface.
With
no cloak of vegetation
to
conceal them
strange formations are exposed
in
the naked rock.
Africa's Sahara is
the
largest desert of all.
It's
the size
of
the United States
and
the biggest source of sand
and
dust in the entire world.
Sandstorms like these
appear without warning
and
reduce visibility for days
over
areas the size of Britain.
Dromedaries, single-humped camels,
take
these storms in their stride.
The
heaviest sand rises only
a few
metres above the ground,
but
the dust can be blown
5,000
metres up into the sky.
The
ferocious wind,
armed
with grains of sand,
is
the agent that shapes all deserts.
Reptiles have armoured scaly skins
that
protect them from the stinging grains.
For
insects the bombardment
can
be very severe indeed.
The
only escape
is
below the surface.
As
the winds rise and fall, swallow and eddy
so
they pile the sand into dunes.
These
sand scenes can be
hundreds of miles across.
In
Namibia the winds have built
some
of the biggest dunes in the world.
Star
dunes like these
can
be 300 metres high.
Grains, swept up the flanks,
are
blown off the crests of the ridges
so
it's only the tops
that
are moving.
The
main body of these dunes
may
not have shifted for 5,000 years.
Few
rocks can resist the continuous
blast
of the sand carrying wind.
These
outcrops are standing
in
Egypt's White Desert.
But
they will not do so
for
much longer.
They're being inexorably
chiseled away
and
turned into more sand.
Now
lumps of heavily eroded rocks
have
been marooned in a sea of sand.
These
jagged pyramids
a
hundred metres tall
were
once part
of a
continuous rocky plateau.
The
blasting sand will eventually
eliminate them altogether.
The
relentless power of the wind ensures
that
the face of a desert is continually changing.
But
there is one constant presence -
the
desert sun.
The
sun's heat and power
to
evaporate water
has
had a profound effect on the bodies
and
habits of everything that lives here.
This
sun potentially is a killer.
And
the red kangaroos
must
acknowledge that.
Right
now, while the sun is low,
there's no immediate cause
for
concern.
But
this situation won't last long.
Australia is the world's most arid
continent
with
blistering daytime temperatures.
Every
hour the temperature rises
by
five degrees centigrade.
Soon
the heat will reach
a
critical point.
Any
kangaroo out in the open
is in
serious danger of overheating.
In
the full sun the temperature
on
the ground soars to 70 degrees.
By
midday the radiation is
so
intense they must take shelter.
In
the shade they're shielded
from
much of the sun's energy
but
their body temperature
can
still rise.
So
they lick saliva
on to
their forearms
where
there is a network of blood vessels
close
to the surface of the skin
and,
as the saliva evaporates,
their
blood is cooled.
This
thermal image shows
just
how effective the process is.
The
blue areas on the body
are
the cooler parts.
As
the saliva dries
it
has to be replaced
and
this is a real drain
on
the kangaroo's body fluids.
Even
in the shade
the
earth is baking hot
so
the kangaroos dig away
the
warmed topsoil
to
get at the cooler ground beneath.
By
staying in the shade and licking
to
control their body temperature
kangaroos manage to get
through the hottest part of the day
without heat stroke.
But
for the majority
of
desert animals
this
strategy would not be
enough for survival.
The
extraordinary ears
of
the fennec foxes of Africa radiate heat
but
the animals have
another way of keeping cool.
They
spend their days underground
and
only emerge at sunset.
Darkness brings huge changes.
In
the Sahara the temperature can drop
as
much as 30 degrees during the night,
so
it's cool enough to allow
these
desert fox cubs to play.
All
sorts of creatures now appear
including some really unexpected
ones.
Toads
have permeable skins
and
would quickly die from desiccation
out
in the daytime heat.
It's
only now that
they
can leave shelter.
The
same is true for scorpions,
even
though their shells
are
actually watertight.
In
fact, most small desert
creatures are nocturnal.
so
it's only now
that
you can judge
just
how much life
there
can be in the desert.
But
moisture, lost even at night,
has
to be replaced sometime somehow
and
that problem dominates
the
lives of all desert dwellers.
The
Atacama in Chile.
This
is the driest desert
in
the world.
Some
parts may not see rain
for
fifty years
and
with such a record
you'd
expect the place
to be
completely barren.
These
are South America's camels,
guanacos.
They're very good
at
conserving moisture
but
they nonetheless
need
a regular supply of water.
They
get it partly
from
cactus flowers
but
that explanation
raises another question.
How
do the cacti
survive without rain?
Hot
winds suck all the moisture
from
the surface of the land.
Clearly there must be something
else
that
takes the place of rain.
The
secret is a cold sea current
that
runs parallel to the land.
The
cold water cools
the
moist warm air above it
and
that produces banks of fog.
At
the same time wind blowing
on to
the shore sweeps the fog inland.
Before long the cacti
are
dripping with dew.
The
fog is so regular
that
moisture loving lichens
are
able to grow on the cacti
and
they absorb liquid
like
a sponge.
In
the land of almost no rain
these
precious drops are life-savers
for
many different creatures.
Further inland the air remains so
warm
that
its moisture does not condense
so
this slender strip of desert
is
virtually the only part of the Atacam
where
life can exist.
Without the fog,
this
land, too, would be empty.
The
guanacos make
the
most of the dew
but
it will not remain
for
long.
In an
hour or two
the
sun will have burnt it off
and
dry the surface of the cacti.
The
Sonoran desert in Arizona
is
not quite so dry as the Atacama -
some
rain does fall.
But
it is infrequent
and
when it does arrive
animals and plants have
to be
ready to make the most of it.
And
it's coming.
When
the summer monsoon blows in
the
giant saguaros,
one
of the biggest of all cacti,
are
ready to take
full
advantage of it.
After
a rainstorm the saguaro's long shallow
root
system sucks up the water
and
the pleats on its trunk
enable it to expand rapidly.
When
full, a saguaro stem can store
up to
five tonnes of water
and
that's enough to see it
through many months of drought.
The
trunks of these huge plants
provide homes for the gila
woodpecker.
But
birds are not the only animals
to
benefit from the presence of the cacti.
During four weeks of the summer
the
saguaros bloom at night
to
attract visitors.
The
pollen and nectar
with
which these flowers are loaded
attract long-nosed and long-tongued
bats.
The
bats left Mexico a few days earlier
to
escape the heat of summer
and
are on their way north
to
the southern United States.
To
get there, they have
to
cross the Sonoran desert.
But
the desert is so big
that
for most of the year
they
would be unable to cross it.
Now,
with the saguaro in bloom,
they
can refuel on the way.
So
the saguaro's success
in
developing a way to store water
is
now crucial to most of the animals
that
live or even travel through this land.
The
scarcity of rain determined
the
shape of this icon of the desert
but
water, scarce thought it is,
has
also, like the wind,
shaped the land itself.
In
the deserts of Utah
ancient rivers flowing across sandstone
country
steadily widen their canyons
until
now the land between them
has
been reduced to spires and pinnacles.
With
little or no soil to retain
the
water on the surface of the land
life
here is scarce indeed.
And
when resources are limited,
conflict is never far away.
These
are Nubian ibex
and
they are squaring up
for a
duel.
And
when trouble starts,
a
smart ibex knows that the best thing
to do
is to gain higher ground.
These
are actually
subordinate male ibex,
but
their fights
are
nonetheless serious.
Losing one might mean
never
getting the chance to breed ever.
When
competitors are evenly
matched as they are here,
duels
can last for an hour.
In
this heat the effort
is
trully exhausting.
But
victory here will gain important
ranking points on a male's way to the
top.
There's so much at stake
that
not all play fair.
The
battle has produced the winner,
but
the ultimate prize
is
not his yet.
That
currently belongs
to
the dominant male ibex.
His
rank earns him the loyalty
of a
harem of females
and
they follow him closely
as he
travels across this desert
searching for foof and water.
He
doesn't have to waste time
looking for mates -
they're his for the taking,
so he
can concentrate with them
on
keeping fit and healthy.
Lizards are desert specialists.
But
here, their numbers
are
extraordinary.
These
crevices in South Africa contain
the
highest density of lizards in the world.
They're called flat lizards
for
obvious reasons,
and
they flaunt their multi-coloured
bellies in territorial disputes.
He's
made his point,
and
now it's time
to
find some food.
As
the day warms up,
the
lizards move away
from
their cracks
and
head down
to
the bottom of the gorge.
Their
goal is the river.
There
is no food at the edge,
but
this desert river
holds
a secret.
Each
day blackfly rise
from
turbulent stretches of the river.
This
is what the lizards
have
come for.
The
black fly never land,
so
the lizards have to leap
for
their food.
In
one day each of these acrobatic
little lizards may catch 50 flies.
There
are plenty of flies
to go
round,
even
with hundreds of lizards
competing for them.
Away
from these rapids flat lizard populations
are
found much smaller numbers.
But
here one unusual abundance
has
produced another.
Deserts are created by the lack of
water,
but
what actually kills animals
here
is not heat or thirst,
but
lack of food.
So
how on earth does
a
plant-eater this size
survive in a place apparently
totally devoid of vegetation?
Elephants in Namibia
are
the toughest in Africa.
And
they need to be.
What
little food exists
is so
dispersed
that
these elephants walk
up to
50 miles a day
as
they travel up the dry river channels
searching for something to eat.
At
times the task
looks
truly helpless.
Elephants may seem out of place
in
this landscape,
but
they're not the only ones.
Amazingly, lions live here, too.
In
savanah country huge herds
of
games support prides
containing 20 lions or more.
But
to live here
lions
have had to change their habits -
prides are much smaller
and
their home ranges
are
very much bigger.
And
there's an added problem -
their
food is always on the move.
Like
the elephants,
the
lions must travel great distances
to
find enough to live on.
But
lions can't go everywhere -
they
won't attempt to cross
this
field of sand dunes
and
the oryx know it.
The
lions must wait for the oryx
to
leave the safety of the dunes,
which
eventually they must
to
find food and water.
And
then the lions
will
ambush them.
The
elephants have found
some
of their favourite food.
Grasses are the staple diet
of
all elephants,
but
this herd concentrates
on
digging up the roots,
which
have more nutrition
and
moisture than the stems.
It's
the sort of behaviour
that
can make all the difference
in a
place of serious shortages.
Yet
all this can change
in an
instant.
The
fortunes of many deserts
are
ruled by distant rains.
This
water fell as rain
in
mountains more than a hundred miles away.
It's
known as a flash flood
and
called that because the water
may
run for just a single day.
It's
an event that only happens
once
or twice a year at the most.
The
sandy riverbed acts like
a
giant strip of blotting paper
sucking up the water
as
soon as it appears.
But
every square metre of soil
moistened by this river
will
increase the chances
of
survival for those that live here.
Waterholes are filled
temprorarily.
Elsewhere in Africa
elephants drink every day,
but
the lack of water here
means
that desert elephants can only
refill their tanks once every four or five
days.
Within a week the flash flood
has
produced a flush of green,
more
than enough to draw
the
oryx out of the dunes.
It's
a rare chance for them
to
build up their food reserves.
The
flood has made life easier
for
the lions, too.
The
flesh of this oryx will keep
the
family going for a week at the most.
But
for a while
the
hunting will be easier,
now
that river channel
has
turned green.
The
good times
for
lions and oryx are brief,
but
these are the short moments
that
make it possible to live
in
deserts the year round.
Death
Valley is the hottest
place
on Earth.
Yet
even this furnace
can
be transformed by water.
A
single shower can enable seeds
that
have lain dormant for 30 years or more
to
burst into life.
And
there hasn't been a bloom
like
this one for a century.
The
periods of boom
in
Death Valley are short.
but
they're just frequent enough
to
keep life ticking over.
A
sudden flush of vegetation is
what
every desert dweller waits for,
and
when it happens
they
must make the most of it.
There
is no other species
on
the planet
that
responds as quickly and as dramatically
to
the good times as the desert locust.
Eggs
that have remained in the ground
for
20 years begin to hatch.
The
young locusts
are
known as hoppers,
for
at this stage
they're flightless.
They
find new feeding grounds
by
following the smell of sprouting grass.
Normally it takes four weeks
for
hoppers to become adults,
but
when the conditions
are
right as now
their
development switches
to
the fast track.
As
the vegetation in one place
begins to run out
the
winged adults release pheromones -
scent
messages,
which
tell others in the group
that
they must move on.
And
when groups merge,
they
form a swarm.
An
adult locust eats
its
entire body weight every day,
and a
whole swarm can consume
literally hundreds of tonnes of
vegetation.
They
have to keep on moving.
The
swarm travels
with
the wind -
it's
the most energy-saving
way
of flying.
Following the flow of wind means that
they're
always heading toward areas of low
pressure,
places where wind meets rain
and
vegetation starts to grow.
As
they fly,
swarms join up with other swarms
to
form gigantic plagues
several billions strong
and
as much as 40 miles wide.
They
will consume every edible thing
that
lies in their path.
This
is one of planet Earth's
greatest spectacles.
It's
rarely seen on this scale
and
it won't last long.
Once
the food is gone,
the
steady roar of a billion beating
locust wings will once again be
replaced
by
nothing more
than
the sound of the desert wind.
我们这个星球上有1/3的陆地是沙漠
在地球表面的这些巨大伤痕上面
看上去好像没有生命存在
其实并非如此
在所有的沙漠里
都有生命以各种方式艰难而顽强地生存着
并非所有的沙漠都酷热
以每小时80公里的速度从西伯利亚吹来的风
给蒙古的戈壁沙漠带来降雪
这里夏天的气温高达50摄氏度
而到了严冬季节可以降到零下40度
因此这片沙漠是全世界所有沙漠中最严酷的地区
能适应气温如此极端变化的动物少之又少
野生双峰驼是地球上最珍稀的
也可能是最能吃苦耐劳的哺乳动物之一
它们最大的问题是缺少饮水
尤其是在现在这种隆冬季节
仅有的一点水也结成了冰
您不会想到这儿的雪从不融化
因为空气太寒冷太干燥无法让雪融化
太阳的光芒让它直接变成了水蒸气
使它蒸发掉
但雪是仅有的水源
所以双峰驼不得不吃雪
在世界其它地方
一只骆驼一次可以喝近200升水
而在这里它们只能采取少吃多餐的策略
理由很简单
肚子里装满雪会有致命的危险
它们必须控制在每天10升左右
冬天是它们繁殖的季节
这种奇特的表演是雄性双峰驼为了吸引路过的异性
夏天，双峰驼不能离水源太远
但是现在是冬天，到处都有点雪
它们就可以走得更远去寻找配偶
今天这种野生状态的沙漠动物在全世界范围内不足1000只
虽然戈壁沙漠太严苛
却是他们最后的堡垒
没有任何沙漠跟戈壁沙漠一样
但是这里为什么会是沙漠呢？
只有一个简单而重要的原因
喜马拉雅山
从南方吹来的云层在这里撞上巨大屏障
被迫上移的时候
云层中的水汽尽空
遗留在山坡上
使得高山另一侧的大地水汽稀少
从太空中看去
沙漠是非常巨大的
大风吹积成的沙丘长达数百公里
横亘在沙漠的表面
没有植被遮掩
它们在裸露的岩石中间形成奇怪的形态
非洲的撒哈拉沙漠是世界上最大的沙漠
它的面积几乎与中国的面积相当
在这里经常是狂风大作
像这样的沙尘暴会骤然而起
让你在几天时间内在相当于英国国土面积的范围内
什么也看不见
单峰驼却能在这样的沙尘暴中行走自如
较大的沙粒只能上升几米
但沙尘则可以被吹到五千米的高空
所有沙漠的形成
都是通过夹裹着沙尘的狂风
爬行动物有硬衣的保护
可以使自己免遭沙尘针刺般的打击
对昆虫来说
这种轰炸很可能是致命的
唯一的办法是钻到沙子里面
风起风息，风旋风转
把沙子吹成了沙丘
这种沙海有的直径可达几百公里
在纳米比亚
风形成了世界上最大的一片沙丘
像这样的星状沙丘可能高达300米
两侧的沙砾被吹起来
吹过山脊
所以只有沙丘顶部在移动
这些沙丘的主体可能已经五千年没有移动过
很少岩石能够承受飞沙走石不间断的打击
这是埃及白沙漠上露出地表的岩石
但是它们不会存在太久了
它们一直被风沙无情地打磨着
早晚会变成一粒粒沙子
现在有许多严重遭到蚀刻的巨石孤立在沙海之中
这些100公尺高的嶙峋塔形巨石
曾经隶属于峰峰相连的高原
狂烈飞沙最后终会将它们消磨殆尽
风的无情力量会使沙漠的面貌不断地改变
但是有一样东西却亘古不变
这就是沙漠上的太阳
太阳的热和力使这里的水蒸发了
对生活在这里的动植物形体和习性产生了深刻的影响
这里的太阳是一个潜在的杀手
红大袋鼠一定会同意这一点
现在太阳还很低，所以不必担心
但是这种情况不会持续太久
澳大利亚是世界上最干旱的大陆
这里白天的气温如烤炉般灼热
气温每小时升高5摄氏度
气温很快就会升到危险的高度
任何在阳光下的袋鼠
都会因为过高的温度而处于危险之中
正午时分
地面温度升到了70摄氏度
中午的阳光实在太强了
它们必须找个地方躲避
虽然树荫可以挡住阳光
但它们身体的温度还在不断上升
所以它们用唾液给自己的手臂降温
因为手臂上有靠近皮肤表面的血管
唾液蒸发的同时血液也同时得以冷却
这张热敏图清楚显示出这个过程的效果
身体的蓝色区域温度明显降低
唾液干掉了以后就必须再行补充
这对袋鼠体内的水分来说是很大的消耗量
就算在树荫下
地表温度仍然滚烫
所以袋鼠会刨开高热的表土
接近比较清凉的底土
躲到树荫下
舔自己的皮肤来降温
袋鼠通过这些办法度过一天中最热的时间
而没有中暑
但对于大多数沙漠动物来说
这些策略还不足以让它们熬过酷热
非洲的耳廓狐可以用巨大的耳朵散热
但它们还有其他的避热高招
它们白天在地下度过
到太阳落山之后才出来
夜幕带来的变化是巨大的
在撒哈拉
夜间温度可以骤降30度
这些沙漠狐的幼仔可以出来玩耍了
所有的生物都出来了
包括那些你根本想不到的动物
蟾蜍的皮肤是裸露的
没有任何保护
在白天的高温下水分蒸发会让它们很快死去
只有夜晚它们才能离开洞穴
蝎子的情况同样如此
尽管它们的外骨骼是防水的
事实上所有的小型沙漠动物都是夜间活动的
因此只有晚上你才真正知道
沙漠上有多少生命在活动
但是失去的水分必须在一定的时间内补充回来
所有的沙漠动物都是如此
这里是智利的阿塔卡马沙漠
它是世界上最干燥的沙漠
有些地方可能长达50年不下雨
在这种情况下
你可能会认为这里是不毛之地
这是南美洲的野生羊驼
它们非常善于储藏水分
但即使如此
它们从不放过任何一个补充水分的机会
它们的一部分水分来自仙人掌花
但这种解释引出另外一个疑问
没有雨水
这种植物是如何生存的呢？
热风吸干了地表所有的水分
显然一定有某种东西起到了雨水的作用
秘密就是与海岸方向平行的低温洋流
寒冷的海水使水面暖湿空气温度下降
从而形成云层
同时风向海岸吹拂
把这些雾气吹到了内陆
仙人掌上不久就布满了露水
雾总是定时出现
以致喜欢潮湿的地衣可以在仙人掌上生长
在一个几乎没有雨水的陆地上
这些珍贵的雾滴是许多生物的救命水
深入内陆
空气仍然炎热
导致水汽没有办法凝结
所以这片细长的沙漠地带
是阿塔卡马沙漠唯一可以让生物存活的地区
没有雾这片土地上也不会有生物
羊驼充分利用了这些露珠
但露珠停留的时间不会太久
一两个小时之后太阳就会把仙人掌表面晒干
美国亚利桑那州的沙漠
并不像阿塔卡马沙漠那样干燥
它有时也会降雨
但是这种情况并不多见
所以一旦下雨
动物和植物都必须做好准备充分利用它
夏天的季风吹来的时侯
最大的仙人掌之一：巨人柱
已经做好了准备
暴雨过后
巨人柱长长的浅根系统会吸收水分
它躯干上的皱褶使它可以很快地膨胀
巨人柱的枝干可以储存多达5吨的水
这让它足以忍受几个月的干旱
巨人柱巨大的躯体为啄木鸟提供了安家之所
但从仙人掌植物中获益的不只是鸟类
巨人柱在夏天有4周的开花期
时间是晚上
这时会吸引不少光顾者
它的花朵上的花粉和花蜜吸引了当地的长舌蝠和长吻蝠
这些蝙蝠几天前离开墨西哥以躲避那里的高温
现在正在前往美洲北部的途中
为此它们必须穿过这片沙漠
但是这个沙漠太大
一年中的其他时间它们无法飞过
有了开花的巨人柱
它们就可以途中补充食物
所以巨人柱进化出的成功储水方式
对于绝大多数生活在这里
甚至是路过这里的动物至关重要
稀少的雨量使得沙漠代表性植物长得奇特有趣
虽然雨水并不常见
仍然像风一样创造着地理景观
在犹他州的沙漠里
流过这片石灰岩地区的古老河流
持续不断地扩大着河谷
直到河谷之间的地面变成了如今这种尖塔林立的地貌
由于缺乏或者说没有土壤来保持水分
这里很少见到生物
资源稀少，冲突就要发生了
这是云山羊
它们正在摆好架势准备决斗
决斗开始前
聪明的云山羊知道最重要的是
占据有利地形
这些公山羊实际上属于第二等级
但它们的争斗仍然激烈
失败可能意味着永远失去繁殖下一代的机会
像目前这种势均力敌的情况下
决斗可能持续一小时之久
在这个酷热的时候这种争斗十分耗费体力
但胜利会使一只公山羊拥有建立自己家族的机会
因为胜负太重要
所以并不是所有决斗者都能公平竞争
胜利者产生了
但最高礼物还不能属于它
目前它属于地位最高的公山羊
它的地位使它有一群忠诚的雌性追随者
它们紧跟着它
随着它跋涉沙漠
寻找食物和水
它不用浪费时间去找配偶
因为它可以随意挑选
有它们在身边
它现在只注意自己的体魄和健康
蜥蜴是沙漠的常住民
在这里它们的数量出奇的多
在南非的这些沟沟缝缝里
有世界上分布最密集的蜥蜴群
它们被称为扁头蜥
它们通过炫耀自己多彩的肚子
来解决领土争端
它成功了
现在该是寻找食物的时候了
气温升高之后
这些蜥蜴从沟缝里跑开
躲到了谷底
它们的目标是河流
河边并没有食物
但这条沙漠河中隐藏着一个秘密
每天会有蚋蝇从水流急湍的河段飞起来
蜥蜴就是冲着它们来的
蚋蝇从不落到地面
所以蜥蜴只得跳着捕捉食物
它们像杂技表演一样
一只小蜥蜴一天可以吃掉50只蚋蝇
蚋蝇的数量很多
尽管有数百只蜥蜴在捕食
在远离这些湍急水流的地方
扁头蜥的数量要少得多
但是在这里
一个数量众多的物种带来了另一个物种的大量繁衍
沙漠是因为缺水形成的
但在这里令动物死亡的不是炎热或干旱
而是缺少食物
那么如此大型的以植物为食的动物
是如何在这样一个没有植物的地方生存的呢？
纳米比亚的大象是所有非洲大象中最能吃苦耐劳的
它们也必须如此
食物如此稀少
它们不得不每天沿着干涸的河道行走近百公里
寻找食物
有时候看起来真的是毫无希望
在这片土地上大象似乎已经很难生存
但它们并不是这里唯一的大型动物
令人意外的是
这里也有狮子
在稀树草原地区
一个狮群的数量可以多达20只
但在沙漠里就必须改变自己的习惯
狮群变小，领地范围变大
而且还有别的问题
食物总在移动之中
和大象一样
这些狮子必须长途跋涉才能找到足够的食物
但是狮子并不能哪儿都去
它们不会穿过沙丘
长角羚知道这一点
狮子必须等待长角羚离开沙丘地带
才可以乘机伏击它们
大象终于找到了一些自己喜爱的食物
草是所有大象的主食
但这群大象却专心挖草根
因为草根比草茎营养更丰富，水分更多
但是这一切可以很快地改变
许多沙漠动物的命运由远处的雨水决定
这些水是远在100多公里之外的降雨带来的
它被称为闪电洪水
因为水可能只流一天
而且一年之内最多出现一两次
这些沙漠河床像一条巨大的吸水纸
水一出现就被它们吸走了
但被这条河流滋润的每一寸土地
都会给这里的生存者增加生存的机会
水坑装满了水
但只是暂时的
在非洲的其它地方大象每天都喝水
但是这里水源缺乏
这就意味着这些沙漠大象
只能每隔四五天喝一次水
一周时间还不到
闪电洪水带来的生机
足以把长角羚引出沙丘
这是它们增加食物储备的难得的机会
洪水也让狮子的生活变得容易些了
这只长角羚的肉会让这家维持一周的时间
现在捕猎已经较为容易
因为沿河一带已经变绿了
狮子和长角羚的美好时光是短暂的
但正是一年中的这段短暂时光
让它们得以在沙漠里生存
美国西南部的死谷是地球上最热的地方
但即使这样的地方
水也可以改变它的面貌
一场阵雨会让在地下沉睡了30多年的种子发芽
这里没有这样的景象已经一个世纪了
死谷的繁荣是短暂的
但只要它出现
生命就可以延续下去
植物突然繁茂生长是沙漠所有的动物都翘首企盼的
没有其他任何物种
对这种美好时光的反应速度
比沙漠蝗虫更快
在土地上等待了20年的虫卵开始孵化
蝗虫幼虫称为若虫
暂时它们还不会飞
它们通过嗅到嫩草气味找到新的食物
通常一只若虫需要经过4周才能变为成虫
如果条件适宜
譬如像现在
它们的生长速度会加快
一个地方的植物吃光之后
长翅膀的成虫会释放信息素
用味道告诉大家必须离开了
集中在一起之后
它们就形成蝗虫群
成年蝗虫每天的食量相当于自己的体重
一个蝗虫群每天可以吃掉几吨的食物
它们必须不断地迁移
蝗虫随着风前进
这是最省力的飞行方式
随风飞行意味着它们总是向气压低的地方飞
也就是风和雨相遇植物开始生长的地方
飞行过程中
几个蝗虫群相遇
形成巨大的虫害群
数量达几十亿只
宽度达六七十公里
它们会把沿途的一切可吃的东西都吃光
这是地球上最壮观的场景之一
这种场景并不多见
而且不会持续太久
食物吃完之后
再次取代几十亿只蝗虫拍击翅膀的声音的
不是别的
是沙漠的风声
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