Populations
AP Environmental Science Topic 3 7:44 English narration · English + 中文 subtitles burned in
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Transcript
In 1944, twenty-nine reindeer were released onto a small island.
1944年，二十九只驯鹿被放到一座小岛上。
There were no predators, and the ground was thick with moss.
岛上没有捕食者，地面上长满了苔藓。
Within twenty years there were six thousand.
二十年内，它们变成了六千只。
Then, in a single winter, almost all of them died.
然后，在一个冬天里，几乎全部死光。
What did the island run out of?
这座岛用尽了什么？
And could the same thing happen to us?
同样的事会不会发生在我们身上？
This topic is about populations: how they grow, what stops them, and what our own numbers are doing.
这个专题讲的是种群：它们如何增长，什么会阻止它们， 以及我们自己的人口正在发生什么。
It runs from insects to elephants to eight billion people.
从昆虫到大象，再到八十亿人。
Let's begin.
让我们开始吧。
Start with two ways to make a living.
先看两种生存方式。
A generalist can live in many habitats and eat many different foods — a raccoon, a cockroach.
广适性物种能生活在多种生境、吃多种食物——比如浣熊、蟑螂。
It is flexible, so it copes well with change.
它很灵活，因此能很好地应对变化。
A specialist needs particular conditions, or one particular food — a panda, a koala.
狭适性物种需要特定的条件，或某一种特定食物—— 比如熊猫、考拉。
It is very efficient in a stable habitat.
它在稳定的生境中非常高效。
But when the habitat changes, the generalist adapts and the specialist is in trouble.
但当生境改变时， 广适性物种能适应，狭适性物种就有麻烦了。
Next, two ways to reproduce.
接下来是两种繁殖方式。
An r-selected species has many offspring with almost no care, matures fast, and dies young — insects, weeds.
r-选择物种产下大量后代，几乎不照顾，成熟快、寿命短—— 比如昆虫、杂草。
It colonises new ground quickly, and it booms and busts.
它们能迅速占领新地方，也会大起大落。
A K-selected species has few offspring but cares for them a lot, matures slowly, and lives long — elephants, humans.
K-选择物种后代很少，但照顾得多，成熟慢、寿命长——比如大象、人类。
The trade-off is simple. The r-strategy recovers fast from a crash. The K-strategy does well in a stable, crowded place, but recovers slowly.
取舍很简单：r-策略在崩溃后恢复得快；K-策略在稳定而拥挤的环境中表现好，但恢复很慢。
Those two strategies give very different survivorship curves.
这两种策略给出非常不同的存活曲线。
A survivorship curve plots how many of a group are still alive at each age.
存活曲线表示一群生物中有多少在各个年龄仍然活着。
Type one, like humans: almost everyone reaches old age, and then they die — high, then a steep drop.
第一类，比如人类：几乎所有个体都能活到老年，然后死亡——先保持很高，再急剧下降。
Type two, like many birds: the same chance of dying at every age — a straight line.
第二类，比如许多鸟类：各个年龄死亡的概率相同——一条直线。
Type three, like fish and insects: nearly all die young, and the few survivors live long.
第三类，比如鱼和昆虫：绝大多数在幼年死亡，少数幸存者活得很久。
Notice the link: type three goes with the r-strategy, and type one with the K-strategy.
注意其中的联系：第三类对应r-策略，第一类对应K-策略。
So what holds a population down?
那么是什么把种群压下去的？
Often, something is eating it.
往往是有东西在吃它。
Watch these two lines.
看这两条线。
When prey are plentiful, the predators have plenty to eat, so predator numbers rise.
猎物充足时，捕食者有得吃，数量上升。
More predators means fewer prey, so the prey line falls.
捕食者多了，猎物就变少，猎物曲线下降。
Then the predators go hungry and their numbers fall too — and the prey recover.
接着捕食者挨饿，数量也下降——猎物随之恢复。
Each predator peak comes just after a prey peak.
每一个捕食者的高峰都紧跟在猎物高峰之后。
The two cycle, out of step, forever.
两者不同步地循环，永不停止。
But there is a bigger limit.
但还有一个更大的限制。
The carrying capacity, written as a capital K, is the largest population an environment can support for a long time.
环境容纳量，写作大写的K， 是一个环境能长期供养的最大种群数量。
When resources are briefly plentiful, a population can overshoot K — it grows past what the land can feed.
当资源短暂丰富时，种群可能超过K—— 增长到超出这片土地能养活的数量。
That never lasts.
这绝不会持久。
The resources run out, and the population crashes. We call that a dieback.
资源耗尽，种群随之崩溃， 我们把这叫做骤降。
Most of the time, populations simply fluctuate up and down around K.
多数时候，种群只是在K附近上下波动。
<slow>Now you know what happened to the reindeer.</slow> No predators, and plenty of moss, so they shot far past K — and one hard winter finished it.
现在你知道那些驯鹿发生了什么：没有捕食者，苔藓充足， 于是它们远远冲过了K——一个严冬就结束了一切。
Here are the two growth curves you must be able to draw.
这是你必须会画的两条增长曲线。
With unlimited resources, a population grows exponentially: slow at first, then faster and faster.
资源无限时，种群按指数增长：起初慢，然后越来越快。
That is the J-curve.
这就是J形曲线。
It cannot go on.
它不可能持续。
As resources get tight, growth slows and the curve levels off at K.
当资源变紧张时，增长放缓，曲线在K处趋平。
That is logistic growth — the S-curve.
这就是逻辑斯蒂增长——S形曲线。
Real populations follow the S.
真实的种群走的是S形。
So what sets K?
那么是什么决定了K？
A limiting factor is whatever runs short first: food, water, space, or disease.
限制因素就是最先不够用的那个东西：食物、水、空间或疾病。
Some limits get worse as a population gets denser. Disease spreads faster in a crowd, and competition for food gets harder.
有些限制会随着种群变密而加剧：疾病在拥挤中传播更快，争夺食物也更激烈。
Those are density-dependent.
这些叫做密度制约因素。
Others do not care how many there are. A flood, a fire, or a cold winter kills the same fraction whether the population is large or small.
另一些则不管有多少个体：洪水、火灾或严冬， 无论种群大小，杀死的比例都一样。
Those are density-independent.
这些叫做非密度制约因素。
Now to human populations.
现在来看人类种群。
An age-structure diagram stacks a country's age groups, youngest at the bottom, males on one side and females on the other.
年龄结构图把一个国家的各年龄组叠起来，最年轻的在底部， 男性在一侧，女性在另一侧。
Its shape predicts the future.
它的形状预示未来。
A wide base means lots of children, and predicts rapid growth.
底部很宽意味着孩子很多，预示人口快速增长。
Straight, even bars mean a stable population.
各条形笔直而均匀，表示人口稳定。
A narrow base means fewer children than adults, and predicts decline — the population will shrink.
底部很窄意味着孩子比成人少，人口将会萎缩。
One glance at the shape tells you where a country is heading.
看一眼形状，就知道一个国家走向何方。
One number drives that shape.
有一个数字决定了这个形状。
The total fertility rate is the average number of children a woman has in her lifetime.
总生育率是一位女性一生中平均生育的孩子数。
The replacement level is about two point one — just enough to replace two parents, with a little spare for children who do not survive.
更替水平大约是二点一——刚好够替换两位父母，再留出一点余量，因为有些孩子活不下来。
Above that, the population grows. Below it, the population shrinks.
高于这个数，人口增长；低于这个数，人口萎缩。
Three things bring it down reliably: education, especially of girls; access to family planning; and lower infant mortality, because parents who expect their children to live have fewer of them.
有三件事能可靠地降低它： 教育，尤其是女孩的教育；能够获得计划生育服务；以及更低的婴儿死亡率—— 因为当父母预期孩子能活下来时，他们就会少生。
Three things change a human population: births, deaths, and migration.
有三件事改变人口：出生、死亡和迁移。
Births minus deaths gives the natural growth; migration then adds or subtracts on top of that.
出生减去死亡得到自然增长， 迁移再在此基础上增加或减少。
And numbers are only half the story.
而数量只是故事的一半。
The ecological footprint measures how much land and water one person needs, to supply what they use and absorb what they throw away.
生态足迹衡量一个人需要多少土地和水，来提供他所用的东西并吸收他丢弃的废物。
A small population with a huge footprint can press harder on the planet than a large one with a small footprint.
一个人口不多但足迹巨大的国家，对地球的压力可能超过一个人口众多但足迹很小的国家。
Let's do the calculation the exam always asks for.
我们来做考试必考的那道计算。
A country has a birth rate of thirty per thousand and a death rate of ten per thousand.
一个国家的出生率是每千人三十，死亡率是每千人十。
Subtract: twenty per thousand.
相减：每千人二十。
Divide by ten to turn that into a percentage — two point zero percent per year.
再除以十，把它变成百分数——每年百分之二点零。
Now use the rule of seventy: the doubling time is seventy divided by the growth rate as a percentage.
接着用七十法则：翻倍时间等于七十除以以百分数表示的增长率。
Seventy divided by two is thirty-five, so the population doubles in about thirty-five years.
七十除以二等于三十五，所以人口约在三十五年内翻倍。
If development cut the growth rate to one percent, the doubling time would stretch to seventy years.
如果发展把增长率降到百分之一，翻倍时间就会延长到七十年。
Finally, the pattern most countries follow: the demographic transition.
最后是大多数国家都会经历的模式：人口转型。
In stage one, births and deaths are both high, so growth is slow.
第一阶段，出生率和死亡率都很高，所以增长缓慢。
In stage two, better food and medicine make the death rate fall, but births stay high — so the population grows fast.
第二阶段，更好的食物和医疗让死亡率下降，但出生率仍然很高——于是人口快速增长。
In stage three, births fall too, and growth slows.
第三阶段，出生率也下降，增长放缓。
In stage four, both are low and the population is stable.
第四阶段，两者都低，人口稳定。
Now look at the gap between the two lines in stages two and three.
现在看第二和第三阶段中两条线之间的差距。
A fallen death rate with a still-high birth rate is why a country's population keeps climbing for decades after its health improves.
死亡率已经下降而出生率仍然很高， 这就是为什么一个国家的健康状况改善之后，人口还会继续攀升几十年。
Three ways to keep your marks.
三个保住分数的方法。
First, learn the rule of seventy: the doubling time is seventy divided by the percentage growth rate, and that rate is births minus deaths per thousand, divided by ten.
第一，记住七十法则：翻倍时间等于七十除以百分数增长率， 而这个增长率等于每千人出生数减死亡数，再除以十。
Second, read the pyramid shape — a wide base means fast growth, even bars mean stable.
第二，读懂金字塔的形状——底部宽表示快速增长，条形均匀表示稳定。
Third, and students lose this one every year: a falling growth rate still means growth.
第三，这一点学生每年都失分：增长率下降仍然意味着在增长。
A population only shrinks when deaths beat births.
只有当死亡超过出生时，人口才会减少。