The Living World: Ecosystems
AP Environmental Science Topic 1 8:06 English narration · English + 中文 subtitles burned in
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Transcript
Picture a grassland.
想象一片草原。
Grass covers the ground everywhere you look.
放眼望去，草覆盖着整个地面。
Rabbits and other grazers are common — you can find them all day.
兔子和其他食草动物很常见，一整天都能看到。
But the big cats that hunt them?
但捕猎它们的大型猫科动物呢？
Only a few live in the whole area.
整片区域只有寥寥几只。
Nature builds this same shape again and again. A huge base, and a tiny top.
大自然一次又一次地造出同样的形状：巨大的底部，极小的顶端。
Why?
为什么？
The answer is energy.
答案是能量。
In this lesson we build the whole of Unit One. What an ecosystem is, how living things interact, how matter goes round in cycles, and how energy moves through — and runs out.
这节课我们来搭建第一单元的全部内容：什么是生态系统，生物之间如何相互作用， 物质如何循环，以及能量如何流动——并最终耗尽。
Let's begin.
让我们开始吧。
An ecosystem is every living thing in an area, plus the non-living parts around them — air, water, soil and sunlight.
生态系统是指某个区域内的所有生物，加上它们周围的非生物部分——空气、水、土壤和阳光。
Inside it, species interact in three ways.
在其中，物种之间有三种相互作用。
Predation is one organism eating another.
捕食是一种生物吃掉另一种生物。
Competition happens when two organisms need the same limited resource.
竞争发生在两种生物需要同一种有限资源时。
Symbiosis is a close, long-term partnership between two species.
共生是两个物种之间长期而密切的关系。
It comes in three forms: mutualism helps both, commensalism helps one and does not harm the other, and parasitism helps one and harms the other.
它分为三种：互利共生对双方都有利，偏利共生对一方有利而不伤害另一方， 寄生对一方有利而伤害另一方。
Zoom out, and ecosystems group into biomes.
把视野拉远，生态系统会归并成生物群系。
A biome is a huge region with its own climate and its own typical life.
生物群系是一个巨大的区域，有自己的气候和典型的生物。
Terrestrial biomes are decided almost entirely by two numbers: temperature and precipitation.
在陆地上，两个数字几乎决定了一切：温度和降水。
Cold and dry gives tundra.
又冷又干形成苔原。
Hot and dry gives desert.
又热又干形成沙漠。
Hot and wet gives tropical rainforest, the richest land biome of all.
又热又湿形成热带雨林——所有陆地生物群系中最丰富的一个。
Every plant and animal there is adapted to those exact conditions.
那里的每一种动植物都适应了这些确切的条件。
Water covers most of the planet, so most biomes are aquatic. Aquatic biomes are sorted by salinity.
水覆盖了地球的大部分，所以大多数生物群系是水生的。
Freshwater systems — streams, lakes and wetlands — have almost no salt.
淡水系统——溪流、湖泊和湿地——几乎不含盐。
Marine systems — the open ocean, coral reefs and estuaries — are salty.
海洋系统——开阔海洋、珊瑚礁和河口——是咸的。
Two of them earn marks again and again.
其中两类在考试中反复出现。
An estuary is where a river meets the sea, and a coral reef is warm, shallow and sunlit.
河口是河流与海洋交汇的地方，珊瑚礁则温暖、浅而阳光充足。
Both are among the most productive places on Earth.
两者都是地球上生产力最高的地方之一。
Before the cycles, learn one sentence. <slow>Matter cycles.
在讲各个循环之前，先记住一句话：物质循环，能量流动。
Energy flows.</slow> A single carbon atom can travel round and round for millions of years, from air to plant to soil and back again.
一个碳原子可以往复循环数百万年，从空气到植物，再到土壤，然后回来。
Energy cannot do that.
能量做不到。
Energy arrives from the sun, passes through living things once, and leaves as heat.
能量来自太阳，一次性穿过生物，然后以热的形式离开。
It never comes back, so the sun must keep supplying more.
它永不回头，所以太阳必须不断补充。
Watch carbon move.
看碳如何移动。
Photosynthesis pulls carbon dioxide out of the air and locks it into plants.
光合作用把二氧化碳从空气中带走，锁进植物里。
Respiration and decomposition send it straight back.
呼吸作用和分解作用又把它送回去。
The ocean holds a huge store, and material buried for millions of years became coal, oil and natural gas.
海洋储存着巨量的碳， 而埋藏数百万年的物质变成了煤、石油和天然气。
Burning those fossil fuels releases ancient carbon in seconds — far faster than nature can take it back.
燃烧这些化石燃料在几秒内释放出远古的碳——远快于自然把它收回的速度。
Now nitrogen.
现在看氮。
Seventy-eight percent of the air is nitrogen gas, and almost nothing can use it.
空气中百分之七十八是氮气，而几乎没有生物能利用它。
Nitrogen fixation, by bacteria or by lightning, unlocks it into ammonium.
固氮作用由细菌或闪电完成，把氮气转化为铵。
Nitrification turns that into nitrate, which roots absorb — that step is assimilation.
硝化作用再把它变成硝酸盐，被根吸收—— 这一步叫同化。
Denitrification closes the loop and returns nitrogen gas to the air.
反硝化作用使循环闭合，把氮气送回空气。
Fertiliser skips the bacteria and adds usable nitrogen straight to the soil.
化肥跳过了细菌，直接把可利用的氮加进土壤。
Phosphorus is the odd one out, and examiners love that.
磷是个例外，考官很喜欢考它。
It has no atmospheric, or gas, stage — there is no phosphorus in the air at all, so the whole cycle is slow.
它没有气态阶段——大气中根本没有磷，所以整个循环很慢。
Rock weathers and releases phosphate into soil and water.
岩石风化，把磷酸盐释放到土壤和水中。
Plants take it up, animals eat the plants, and decomposers return it.
植物吸收它，动物吃植物，分解者再把它归还。
In the end it settles as sediment and becomes rock again.
最后它沉积下来，重新变成岩石。
Because it is so slow, phosphorus is often the limiting nutrient: the one that runs out first and stops growth.
正因为很慢，磷常常是限制性营养： 最先耗尽、并使生长停止的那一种。
The hydrologic cycle — the water cycle — moves fastest of all.
水的循环最快。
Evaporation lifts it from the ocean, and plants release it by transpiration.
太阳把海水蒸发，植物通过蒸腾作用把水释放出来。
The vapour rises, cools, and condensation turns it into cloud.
水汽上升、冷却，凝结成云。
Precipitation brings it down as rain or snow.
降水以雨或雪的形式落下。
Some runs off into rivers, some soaks in as groundwater, and all of it finds its way back to the sea.
一部分成为径流汇入河流，一部分下渗成为地下水，最终都会回到海洋。
Energy lifts it up; gravity brings it down.
能量把它抬上去，重力把它带下来。
Now for energy.
现在讲能量。
Producers capture sunlight by photosynthesis.
生产者通过光合作用捕获阳光。
The total energy they capture is the gross primary productivity, G P P.
它们捕获的总能量叫总初级生产力。
But producers are alive too, so they burn some of it in their own respiration.
但生产者也是活的，所以会在自身呼吸作用中消耗掉一部分。
What is left is the net primary productivity, N P P — new plant material, and the only energy the rest of the ecosystem can ever eat.
剩下的就是净初级生产力——新的植物物质，也是生态系统其余部分唯一能吃到的能量。
So N P P equals G P P minus respiration — write it that way.
所以净初级生产力等于总初级生产力减去呼吸消耗。
Let's use it.
我们来用一用。
In a grassland, producers capture twenty thousand kilocalories per square metre each year, and burn eight thousand of that in respiration.
在一片草原上，生产者每年每平方米捕获两万千卡的能量， 其中八千千卡消耗在自身呼吸作用上。
Find the net primary productivity.
求净初级生产力。
Subtract the respiration from the total. Twenty thousand minus eight thousand leaves twelve thousand kilocalories per square metre per year.
用总量减去呼吸消耗：两万减八千，得到每平方米每年一万二千千卡。
Write the units every time — they carry marks.
每次都要写单位——它们是有分的。
And remember, only that twelve thousand is available to the animals above.
记住，只有这一万二千千卡能供上面的动物使用。
Every organism sits at a trophic level — its feeding step in the chain.
每一种生物都处在一个营养级上——它在食物链中的取食层次。
Producers make their own food from sunlight, and form level one.
生产者用阳光自己制造食物，构成第一营养级。
Primary consumers are the herbivores that eat the producers.
初级消费者是吃生产者的食草动物。
Secondary consumers eat the herbivores, and tertiary consumers eat those.
次级消费者吃食草动物，三级消费者再吃它们。
Decomposers are the quiet heroes: bacteria and fungi break down dead material from every level and return the nutrients to the soil.
分解者是默默无闻的英雄：细菌和真菌分解各个层次的死亡物质，把养分归还土壤。
Without them, every cycle would stop.
没有它们，所有循环都会停止。
Here is the rule that shapes the whole unit.
这条规则塑造了整个单元。
Only about ten percent of the energy at one trophic level reaches the level above.
一个营养级的能量，只有大约百分之十能到达上一级。
Where does the other ninety percent go?
另外百分之九十去哪了？
Most of it powers living — moving, growing, staying warm — and escapes as heat.
大部分用于维持生命——运动、生长、保持体温——并以热的形式散失。
Some food is never eaten, and some passes through undigested.
有些食物从未被吃掉，有些则未经消化就排出。
So energy flows and is lost: it is never destroyed, it just stops being useful, which is why the chain cannot go on for ever.
能量没有被消灭，只是不再可用。
Start from the twelve thousand kilocalories of net primary productivity.
从一万二千千卡的净初级生产力开始。
Pause and try the chain yourself.
先暂停，自己算一遍这条链。
Ready?
准备好了吗？
Ten percent reaches the primary consumers: one thousand two hundred.
百分之十到达初级消费者：一千二百。
Ten percent of that reaches the secondary consumers: one hundred and twenty.
其中的百分之十到达次级消费者：一百二十。
The tertiary consumers get about twelve.
三级消费者只得到大约十二。
Three steps, and the energy has shrunk a thousand times.
三步之后，能量缩小了一千倍。
<slow>Now you can answer the grassland question.</slow> Food chains stay short, and top predators stay rare, because almost nothing is left at the top.
现在你能回答草原上的那个问题了：食物链很短，顶级捕食者很少， 因为顶端几乎什么都不剩了。
A food chain is one straight line of who eats whom.
食物链是一条谁吃谁的直线。
Real ecosystems are messier, so we draw a food web: many chains joined together.
真实的生态系统更复杂，所以我们画食物网：许多条食物链连在一起。
Draw the arrows in the direction the energy travels — from the eaten to the eater.
画箭头时要按能量传递的方向——从被吃的一方指向吃的一方。
Point them the wrong way and you lose the mark.
方向画反就会丢分。
And look out for a keystone species: one species that many others depend on.
还要留意关键种：许多其他物种都依赖的一个物种。
Remove it, and large parts of the web can collapse.
把它移除， 食物网的很大一部分就可能崩溃。
Three ways to protect your marks.
三个保住分数的方法。
First, never mix them up: matter cycles, energy flows.
第一，绝不要弄混：物质循环，能量流动。
Second, on the maths questions there is no calculator, so keep the numbers round, show every step, and always write the units.
第二，计算题不能用计算器，所以把数字保持整齐，写出每一步，并且永远写上单位。
Third, remember what makes phosphorus different — no gas stage, very slow, and often the limiting nutrient.
第三，记住磷的特别之处——没有气态阶段，非常慢，而且常常是限制性营养。
Get those right, and Unit One is yours.
做对这三点，第一单元就是你的了。