Ecology
AP Biology Topic 8 7:53 English narration · English + 中文 subtitles burned in
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Transcript
This reef holds more kinds of life in one hectare than a whole province of farmland.
这片珊瑚礁一公顷之内的生命种类，比整整一个省的农田还多。
Every fish depends on the coral, and the coral on tiny algae inside it.
这里的每一条鱼都依赖珊瑚， 而珊瑚依赖住在它体内的微小藻类。
Now look at this hillside.
现在看看这片山坡。
The trees are gone, the soil washes away, and the animals that lived here have nowhere to go.
树没有了，土壤被冲走， 原本生活在这里的动物无处可去。
One system holds itself up. The other fell apart.
一个系统能自己撑住自己，另一个却崩塌了。
This is Unit Eight, in six steps.
这就是第八单元，分六步。
First, how living things answer the world around them.
第一，生物如何回应周围的世界。
Then energy: how it enters and how fast it runs out.
然后是能量：它如何进入， 又消耗得多快。
Then population growth.
然后是种群增长。
Then the limits that stop it.
然后是阻止增长的各种限制。
Then whole communities, and who lives with whom.
然后是整个群落， 以及谁和谁生活在一起。
And last, biodiversity, and what disturbs it.
最后是生物多样性，以及什么在扰动它。
First, responses to the environment.
世界在变，生物不会一动不动。
Living things do not sit still while the world changes. They react, and reacting keeps them alive and keeps the inside steady.
它们会作出反应，而反应让它们活下去，也让体内保持稳定。
Some behaviour is innate: built in, never learned.
有些行为是先天的：生来就有，不需要学习。
Other behaviour is learned.
另一些行为是学来的。
In a plant, a tropism turns growth toward a useful cue, such as light.
在植物身上，向性让生长转向有用的信号，比如光。
And signals, like a bee's dance, change what a group does.
而信号，比如蜜蜂的舞蹈， 会改变一整群动物的行为。
Energy enters almost every ecosystem as sunlight.
几乎每一个生态系统的能量都以阳光的形式进入。
Producers capture it.
生产者把它捕获下来。
Consumers get it second hand, and each feeding step is a trophic level.
消费者是间接获得能量的，而每一个取食环节就是一个营养级。
Now watch the numbers.
现在看这些数字。
The producers capture ten thousand units.
生产者捕获一万单位。
The animals that eat them get one thousand.
吃它们的动物得到一千单位。
The next level gets a hundred.
再上一级是一百。
The one above, ten.
再上一级，只有十。
About ninety percent is lost at every step, as heat.
每一级大约损失百分之九十，都变成了热。
Energy and matter behave differently, and the exam wants that difference.
能量和物质的表现不一样，而考试要考的正是这个区别。
Energy flows one way: in as sunlight, out as heat, gone.
能量是单向流动的： 以阳光进入，以热散失，然后就没有了。
Matter cycles.
物质则是循环的。
Here is carbon.
看碳。
Plants pull it from the air by photosynthesis, respiration puts it back, and decomposers return the carbon in dead bodies.
植物通过光合作用把它从空气中取走，呼吸作用又把它送回去， 分解者把遗体中的碳归还回去。
The same atoms are used again and again.
同样的原子被一次又一次地使用。
A food chain is a single line.
食物链只是一条线。
A food web joins many chains together.
食物网把许多条链连在一起。
At the bottom sit the producers, grass and clover, which make food from sunlight.
最下面是生产者——草和三叶草， 它们用阳光制造食物。
Above them the primary consumers eat those plants: the rabbit, the insect, the mouse.
它们上面是初级消费者，吃这些植物：兔子、昆虫、老鼠。
Above them again come the fox and the bird.
再往上是狐狸和鸟。
Now follow the mouse.
现在跟着这只老鼠看。
It feeds the bird, and the fox.
它既养活了鸟，也养活了狐狸。
Now population ecology.
现在来数个体。
A population is all the members of one species in one place.
种群是同一地方同一物种的全部个体。
It grows with births and immigration, and shrinks with deaths and emigration.
出生和迁入让它增长， 死亡和迁出让它减少。
With unlimited resources growth is exponential: the J-curve, doubling and doubling.
资源无限时，增长是指数式的：J形曲线，一倍又一倍。
Real populations cannot.
真实的种群做不到。
Food and space run short, and logistic growth takes over: the curve flattens into an S at the carrying capacity.
食物和空间会短缺，曲线就在环境容纳量处变平，成为S形。
Let's use that equation.
我们来用一下这个公式。
A population has a maximum growth rate of zero point five per year, a carrying capacity of one thousand, and four hundred individuals now.
一个种群的最大增长率是每年零点五，环境容纳量是一千， 现在有四百个个体。
How fast is it growing?
它增长得有多快？
Pause here and try it yourself.
先暂停，自己试一试。
Write the equation first: the growth rate equals the maximum rate, times the number now, times the bracket.
先写公式：增长率等于最大增长率，乘以现在的个体数，再乘以那个括号。
Now put the numbers in.
现在代入数字。
One thousand minus four hundred, over one thousand, gives zero point six.
一千减四百，除以一千，得到零点六。
So that is zero point five, times four hundred, times zero point six: one hundred and twenty individuals per year.
所以就是零点五，乘以四百，再乘以零点六： 每年一百二十个个体。
And notice what that bracket does: near one when the population is small, zero at the ceiling.
再注意那个括号的作用：种群小的时候它接近一，到了上限就等于零。
So what stops the growth?
那么，是什么让增长停下来？
Split the limits with one question: does crowding matter?
用一个问题把各种限制分开：拥挤重不重要？
Density-dependent factors get stronger as the population packs together: competition, predators, disease.
密度制约因素会随着种群变得拥挤而加强：竞争、捕食者、疾病。
Density-independent factors hit just as hard at ten individuals or ten thousand.
非密度制约因素不管是十个个体还是一万个个体，打击都一样重。
A flood, a fire, a frost.
一场洪水、一场火灾、一次霜冻。
Together they push a population around the carrying capacity.
它们共同让种群在环境容纳量附近上下波动。
Predation is one of those density-dependent factors, and you can watch it work.
捕食就是其中一种密度制约因素，而且你可以看着它起作用。
When the rabbits are plentiful the foxes have plenty to eat, so fox numbers climb.
兔子多的时候， 狐狸有充足的食物，狐狸的数量就上升。
But more foxes means more rabbits caught, and the rabbit curve falls.
但狐狸越多，被抓的兔子越多，兔子的曲线就下降。
The foxes then go short of food and fall too.
狐狸接着缺少食物，数量也跟着下降。
The rabbits recover, and the cycle begins again.
兔子恢复，整个循环重新开始。
Zoom out to community ecology: every species interacting in one area.
把视野放大到整个群落：在同一区域相互作用的所有物种。
They compete, they eat one another, and some live together permanently: symbiosis, sorted by who gains.
它们互相竞争，互相取食， 还有一些长期生活在一起：这就是共生，按谁获益来分类。
In mutualism both partners win: the clownfish gets shelter, the anemone gets cleaned.
互利共生中双方都得益：小丑鱼获得庇护，海葵得到清洁。
In commensalism one gains, the other is unaffected.
偏利共生中一方得益，另一方不受影响。
In parasitism one gains, the other is harmed.
寄生中一方得益，另一方受害。
And one species can matter beyond its numbers: a keystone species.
而有的物种，重要性远超它的数量：这就是关键种。
Remember that word.
记住这个词。
Biodiversity is the variety of life, and the exam wants it at three levels.
生物多样性是生命的多样，而考试要求你从三个层次来看它。
Ecosystem diversity is the number of different habitats in a region.
生态系统多样性是一个区域内不同生境的数量。
Species diversity is how many different species live in one of them.
物种多样性是其中一个生境里有多少不同的物种。
And genetic diversity is the variety inside one species.
而遗传多样性是同一个物种内部的变异。
More variety means more ways to survive a shock.
多样越丰富，度过冲击的办法就越多。
Here is what a keystone species means.
这就是关键种的含义。
On this coast, sea otters eat sea urchins, and sea urchins eat kelp.
在这片海岸，海獭吃海胆，海胆吃海带。
There were never many otters, so they look unimportant.
海獭从来就不多， 所以看上去无关紧要。
Now take the otters away.
现在把海獭拿走。
Nothing eats the urchins, so the urchin population explodes.
再没有东西吃海胆，海胆的数量就爆发式增长。
The urchins graze the kelp down to bare rock.
海胆把海带啃到只剩光秃秃的岩石。
The kelp forest was home to hundreds of species, so they disappear with it.
而这片海带林曾经是几百个物种的家，它们也随之消失。
That is a trophic cascade: take out one animal near the top, and the whole system empties.
这就是营养级联：把顶端的一个动物拿走，整个系统就被掏空。
Why does that matter?
这为什么重要？
A community with many species and many genes is more resilient — it recovers faster from a shock — and hands us services for free: clean water and pollination.
物种多、基因多的群落从冲击中恢复得更快， 还免费为我们提供各种服务：干净的水和传粉。
Now the disruptions.
现在说扰动。
An invasive species arrives with no predators and outcompetes natives.
入侵物种来到新地方，没有天敌，把本地物种竞争掉。
Habitat loss takes away the place itself.
生境丧失干脆把地方本身拿走。
And climate change moves conditions away from species that cannot follow. A disturbance can also trigger ecological succession, where the community rebuilds itself over time.
气候变化则让环境条件从那些跟不上的物种身边移走。
Some damage stays invisible until it reaches the top.
有些伤害在到达顶端之前一直看不见。
A pesticide washes into a lake at a level too low to harm anything.
一种农药被冲进湖里，浓度低到不足以伤害任何生物。
Plankton absorb it and never lose it.
浮游生物吸收了它，而且再也排不出去。
A small fish eats hundreds of plankton and keeps all their poison.
一条小鱼吃掉几百个浮游生物，把它们的毒全部留下。
A big fish eats many small fish.
一条大鱼又吃掉许多小鱼。
By the time an osprey eats it, the same poison sits in one body, hundreds of times more concentrated.
等到鱼鹰吃下它时，同样多的毒就集中在一个身体里， 浓度高出几百倍。
Ecosystems are not fixed; they repair themselves.
生态系统不是固定不变的，它们会自我修复。
After a disturbance clears the ground, life returns in a set order.
一次干扰把地面清空之后， 生命会按固定的顺序回来。
Pioneers come first: mosses and lichens that live on bare rock and slowly build soil.
先锋物种最先到：苔藓和地衣，它们能在裸岩上生活， 慢慢造出土壤。
Grasses follow, then shrubs, then trees, each stage changing conditions for the next.
接着是草，然后是灌木，然后是乔木，每一个阶段都为下一个阶段改变了条件。
The end point is the climax community.
终点叫做顶极群落。
Five marks students give away every year.
每年学生都会白白丢掉的五分。
First, use the ten percent rule, and say where the rest went: lost as heat.
第一，用百分之十规则，并说清楚剩下的能量去了哪里： 以热的形式散失。
Second, keep the two growth curves apart: J is exponential, S levels off at the carrying capacity.
第二，把两条增长曲线分清楚：J形是指数增长，S形在环境容纳量处变平。
Third, ask whether a limiting factor depends on crowding.
第三，问一问这个限制因素是否取决于拥挤。
Fourth, name the symbiosis and say who gains.
第四，说出共生的名称，并说明谁获益。
Fifth, follow the effect right through the food web, instead of stopping at one species.
第五，把影响一路追踪穿过整个食物网，而不是只停在一个物种上。