The Living World: Biodiversity
AP Environmental Science Topic 2 7:58 English narration · English + 中文 subtitles burned in
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Transcript
A volcano pushes a new island up out of the sea.
一座火山把一个新岛推出海面。
At first there is nothing on it — bare rock, no soil, no life at all.
起初岛上什么都没有——只有裸岩，没有土壤，没有任何生命。
Fifty years later, it is green.
五十年后，它变绿了。
So how does life arrive?
那么，生命是怎样到来的？
And how many species will this island finally hold?
这座岛最终又能容纳多少物种？
Both answers are in this topic.
两个答案都在这个专题里。
Unit Two is about biodiversity: what it is, why it matters, what disturbs it, and how a community rebuilds itself.
第二单元讲的是生物多样性：它是什么，为什么重要， 什么会扰动它，以及群落如何重建自己。
Let's start with what biodiversity actually means.
我们先从生物多样性到底指什么开始。
Biodiversity is the variety of life, and the exam wants three levels.
生物多样性是生命的多样程度，考试要求你掌握三个层次。
Genetic diversity is the variety of genes inside one species.
遗传多样性是一个物种内部基因的多样程度。
More variety means the population can handle environmental stressors better, and a population bottleneck — a sharp drop in numbers — throws that variety away.
多样性越高，种群越能承受压力；而种群瓶颈——数量的急剧下降——会把这种多样性丢掉。
Species diversity is how many different species live there, called species richness, and how evenly the individuals are shared between them.
物种多样性是指有多少不同的物种，也就是物种丰富度，以及个体在各物种之间分布得是否均匀。
Habitat diversity is the variety of habitats in a region.
生境多样性是一个区域内生境的多样程度。
Why does this matter?
这为什么重要？
A species-rich ecosystem is more resilient.
物种丰富的生态系统更有韧性。
When something goes wrong, other species can fill the missing role, so it recovers faster.
出问题时，其他物种可以补上缺失的角色， 所以它恢复得更快。
And when a habitat is lost, species disappear in a predictable order.
而当生境丧失时，物种消失的顺序是可以预测的。
Specialists go first: they have narrow needs and cannot switch.
特化物种最先消失：它们需求狭窄，无法转换。
The tougher generalists, which eat and live almost anywhere, last much longer.
更皮实的广适性物种几乎什么都吃、哪里都能活， 坚持得久得多。
Species that need a large territory also decline as their range shrinks.
需要大领地的物种也会随着活动范围缩小而衰退。
Nature also works for us, for free.
大自然也在免费为我们工作。
We call this ecosystem services, and there are four kinds.
我们把这叫做生态系统服务，一共四类。
Provisioning services are products: food, fresh water, timber.
供给服务提供产品：食物、淡水、木材。
Regulating services keep conditions steady: climate, water filtration, pollination, flood control.
调节服务维持条件稳定：气候、水的净化、传粉、洪水控制。
Cultural services are the non-material value: recreation, beauty, meaning.
文化服务是非物质的价值：休闲、美景、意义。
Supporting services make all the others possible: nutrient cycling, soil formation, photosynthesis.
支持服务使其他各类成为可能：养分循环、土壤形成、光合作用。
Break them, and the bill is real.
一旦破坏，代价是实实在在的。
Lose the pollinators, and farmers must pay for work the bees did for nothing.
失去传粉者，农民就必须为蜜蜂原本免费做的工作付钱。
Back to our island — this is island biogeography.
回到我们的岛。
How many species will it hold?
它能容纳多少物种？
Two rates decide it.
由两个速率决定。
New species keep arriving — that is immigration — and it is easier the closer an island is to the mainland.
新物种不断到来——这是迁入—— 离大陆近的岛更容易。
Resident species keep dying out — that is extinction — and it is slower the larger the island, because it has more room and more resources.
原有物种不断消失——这是灭绝——大岛上更慢，因为空间更大、资源更多。
Where the two lines cross, the number settles.
两条线相交的地方，物种数就稳定下来。
So a large, near island holds the most species; a small, remote one holds the fewest.
所以又大又近的岛物种最多；又小又远的岛物种最少。
Isolation does something else too: it drives evolution.
隔离还会带来另一件事：它推动演化。
Cut off from the mainland, island species often become specialists, because resources are limited.
与大陆隔绝之后，由于资源有限， 岛屿物种常常变得特化。
Many become endemic — found nowhere else on Earth.
许多成为特有种——地球上别处都没有。
That makes them precious, and fragile.
这让它们珍贵，也脆弱。
Now bring in an invasive species, usually a tough generalist. It outcompetes the specialists, and they have nowhere left to go.
这时若引入入侵物种，通常是皮实的广适性物种，它会在竞争中压过特化物种， 而后者已无处可去。
Every species also has a range of tolerance: the span of conditions it can survive — temperature, salinity, acidity, light.
每个物种还有一个耐受范围：它能够存活的条件跨度——温度、盐度、酸碱度、光照。
In the middle is the optimal range, where individuals grow well and reproduce.
中间是最适范围，个体在这里生长良好、能够繁殖。
Toward the edges are the zones of stress: they stay alive, but they struggle.
靠近两端是胁迫区：它们还活着，但很吃力。
Past the limits of tolerance, they die.
超出耐受极限，它们就会死亡。
This decides where an organism can live, and how well it copes when conditions shift.
这决定了一种生物能住在哪里， 以及条件变化时它能应对得多好。
Ecosystems are disturbed all the time. Anthropogenic disturbance — the human-caused kind — is only part of it. Natural disruptions to ecosystems matter just as much, and sometimes hit harder than we do.
生态系统时刻都在被扰动，大自然自己也在扰动它——有时比人类还猛烈。
Sort the events by their timing.
按发生的时间规律来分类。
Periodic events repeat on a regular cycle: seasons and tides.
周期性事件按固定周期重复：季节和潮汐。
Episodic events happen now and then, with no fixed schedule: fires, floods, eruptions.
偶发性事件时有发生，但没有固定时间表：火灾、洪水、火山喷发。
Random events follow no pattern at all, like a meteor strike.
随机事件完全没有规律，比如陨石撞击。
Over geological time the climate has shifted again and again, and sea level rose and fell with the ice.
在地质时间尺度上，气候一次又一次改变， 海平面随冰量的增减而升降。
When conditions move, animals often move too — they migrate.
当条件移动时，动物往往也跟着移动——它们会迁徙。
How do species keep up?
物种如何跟上变化？
An adaptation is an inherited trait that helps an organism survive and reproduce where it lives.
适应是一种可以遗传的性状，帮助生物在它生活的环境中生存和繁殖。
Adaptations come from natural selection: small genetic changes building up over many generations.
适应来自自然选择：微小的遗传变化在许多世代中逐渐累积。
That is the exam's key point — a population adapts, an individual does not.
这是考试的关键点——适应的是种群，个体并不会适应。
When the environment changes, an individual can only move, change its behaviour, or die.
环境改变时， 个体只能迁移、改变行为，或者死亡。
And if change comes faster than a population can adapt, that population declines.
如果变化的速度快于种群适应的速度，这个种群就会衰退。
Some species are even adapted to disturbance: certain pine cones open only in a fire's heat.
有些物种甚至适应了扰动：某些松果只有在火的高温中才会张开。
So how did our island turn green?
那么我们的岛是怎样变绿的？
Through ecological succession: the slow, orderly change of a community over time.
靠生态演替：群落随时间缓慢而有序的变化。
Bare ground is taken first by pioneer species, which build the first thin soil.
裸地首先被先锋物种占据，它们造出第一层薄薄的土壤。
Then come grasses, then shrubs, then trees.
接着是草，然后是灌木，最后是乔木。
In the end the community becomes relatively stable — the climax community.
最终群落变得相对稳定——这就是顶极群落。
There are two kinds, and mixing them up costs marks.
演替有两种，弄混就会丢分。
Primary succession starts on bare rock with no soil at all — after a glacier retreats, after lava cools, or on a brand-new island.
初级演替从完全没有土壤的裸岩开始——冰川退去之后、 熔岩冷却之后，或者在一座全新的岛上。
Lichens are the classic pioneer: they grow straight onto rock and slowly break it down.
地衣是典型的先锋物种：它们直接长在岩石上， 慢慢把岩石分解。
Because the soil has to be built from nothing, primary succession is slow.
因为土壤必须从零开始形成，初级演替很慢。
Secondary succession follows a disturbance that leaves the soil behind — a fire, or an abandoned field.
次级演替发生在土壤被保留下来的扰动之后——一场火灾，或一块撂荒的农田。
The soil and the seeds are still there, so it is much faster.
土壤和种子都还在，所以快得多。
Two species roles matter far more than their numbers suggest.
有两种物种角色，其重要性远超它们的数量。
A keystone species holds the structure of the community together; remove it, and much of the community collapses.
关键种维系着群落的结构； 把它移除，群落的很大一部分就会崩溃。
Sea otters are the standard example.
海獭是标准的例子。
Otters eat sea urchins, and urchins eat kelp — so without otters, the urchins strip the kelp forest bare.
海獭吃海胆，海胆吃海带——所以没有海獭，海胆就会把海带林啃光。
An indicator species is different.
指示物种则不同。
Its presence, or its health, tells you the condition of the whole ecosystem.
它的存在或健康状况能告诉你整个生态系统的状况。
Lichens vanish where the air is polluted, so bare rock with no lichen is a warning.
空气污染的地方地衣会消失，所以光秃秃没有地衣的岩石就是一个警告。
Let's put a number on diversity.
我们给多样性一个数字。
A pond holds three fish species: five fish, three fish, and two fish, so ten in total.
一个池塘里有三种鱼：五条、三条和两条，一共十条。
Simpson's Diversity Index is one, minus the sum of each species' share squared.
辛普森多样性指数等于一，减去各物种所占比例的平方之和。
The shares are zero point five, zero point three, and zero point two.
三个比例是零点五、零点三和零点二。
Square each share and add them up: that comes to zero point three eight.
把每个比例平方再相加，得到零点三八。
So the index is one minus zero point three eight, which is zero point six two.
所以指数是一减零点三八，等于零点六二。
Close to one means high diversity; close to zero means one species dominates.
接近一表示多样性高； 接近零表示某一个物种占绝对优势。
Two things push the index up: more species, and numbers spread more evenly.
有两件事会把指数推高：物种更多， 以及各物种的数量分布得更均匀。
Three ways to keep your marks.
三个保住分数的方法。
First, when you are asked about biodiversity, name all three levels — genetic, species and habitat — and link high diversity to resilience.
第一，被问到生物多样性时，要说全三个层次——遗传、物种和生境—— 并把高多样性与韧性联系起来。
Second, remember that specialists are more vulnerable than generalists when a habitat changes.
第二，记住当生境改变时，特化物种比广适性物种更脆弱。
Third, never confuse primary with secondary succession: primary starts on bare rock with no soil and is slow; secondary keeps its soil and is fast.
第三，绝不要混淆初级演替和次级演替：初级从没有土壤的裸岩开始，很慢； 次级保留了土壤，很快。
Get those right, and Unit Two is yours.
做对这些，第二单元就是你的了。