Biodiversity
A-Level Biology Topic 18 7:58 English narration · English + 中文 subtitles burned in
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About one point seven million species have been described and given a name.
大约有一百七十万个物种已经被描述并命名。
The best estimates say there are probably eight to ten million out there.
最好的估计认为， 实际存在的可能有八百万到一千万个。
So most of the living world has never been catalogued at all, and some of it will go extinct before anyone writes it down.
所以生物世界的绝大部分从来没有被编目过， 其中一部分还会在有人把它写下来之前就灭绝。
That is the awkward position this topic starts from: we are trying to protect a system we have not finished counting.
这就是本主题尴尬的出发点：我们正在试图保护一个我们还没数完的系统。
So this lesson is about how we classify it, how we sample it, and how we try to keep it.
所以这节课讲的是：我们怎样给它分类、怎样对它取样，以及怎样设法把它保住。
Start with the word species, because it has more than one definition and the exam expects you to know why.
先从"物种"这个词说起，因为它不止一个定义，而考试希望你知道为什么。
The biological species concept says a species is a group whose members can breed together to produce fertile offspring.
生物学物种概念说，物种是一群能够互相交配并产生可育后代的个体。
That word fertile is what earns the mark: a horse and a donkey can breed, but the mule they produce is sterile, so they are not one species.
"可育"这个词才是得分点：马和驴能交配，但生出的骡子不育，所以它们不是同一个物种。
The morphological one says a species is a group whose members look alike.
形态学物种概念说，物种是一群长得相像的个体。
The ecological one says it is a group that fills the same role in its surroundings.
生态学物种概念说，物种是一群在环境中扮演相同角色的个体。
Why keep more than one definition?
为什么要保留不止一个定义？
Because the biological test cannot be used on a fossil, or on anything that does not reproduce sexually.
因为生物学的判断标准无法用在化石上， 也无法用在任何不进行有性生殖的生物上。
Classifying living things starts at the very top, with all life sorted into three domains.
在最顶层，所有生命被分成三个域。
Archaea and Bacteria are both prokaryotes, meaning their cells have no nucleus.
古菌和细菌都是原核生物，也就是细胞里没有细胞核。
They look alike but differ in three things the examiner likes: their membrane lipids, their ribosomal RNA, and what their cell walls are made of.
它们看起来相似，但在三个方面不同，而考官喜欢这三点： 膜脂、核糖体 RNA，以及细胞壁的组成。
The third domain is Eukarya, which is everything whose cells have a nucleus.
第三个域是真核生物域， 也就是所有细胞里有细胞核的生物。
Inside Eukarya, classification sorts organisms into the taxonomic hierarchy — groups that get smaller and smaller and more closely related: kingdom, phylum, class, order, family, genus, and finally species.
在真核域内部，生物被分成越来越小、 亲缘关系越来越近的类群：界、门、纲、目、科、属，最后是种。 要记住这个顺序。
Learn that order. Viruses sit outside all of it; they are classified by the nucleic acid they carry, DNA or RNA, and whether it is single-stranded or double-stranded. And the Eukarya split into four kingdoms: Protoctista, mostly single-celled; Fungi, which absorb their food and have cell walls of chitin; Plantae, which photosynthesise; and Animalia, which feed on other organisms.
真核域又分成四个界：原生生物界，大多是单细胞的； 真菌界，靠吸收获取食物，细胞壁由几丁质构成； 植物界，进行光合作用；动物界，以其他生物为食。
Two definitions first.
先看两个定义。
An ecosystem is all the living things in an area together with their non-living surroundings.
生态系统是一个区域内所有生物连同它们的非生物环境。
A niche is the particular role and place of one species within that ecosystem.
生态位是某一个物种在这个生态系统中特定的角色和位置。
Now, biodiversity is measured at three levels, and a question that says measure biodiversity wants you to name one.
而生物多样性是在三个层次上衡量的，题目说"衡量生物多样性"时，希望你说出其中一个层次。
The first level is the number and range of different ecosystems and habitats — a coral reef is the classic picture of a rich one.
第一个层次是不同生态系统和栖息地的数量与范围。
The second is the number of species present and their relative abundance — how common each one is.
第二个是物种的数量以及它们的相对丰度——每种有多常见。
The third is the genetic variation within each species.
第三个是每个物种内部的遗传变异。
Notice that abundance matters, not just the species list: two areas with exactly the same species can still differ in diversity.
注意丰度也重要，不只是物种名单： 两个区域即使物种完全相同，多样性仍可能不同。
You cannot count every organism in an area, so you take samples.
你不可能把一个区域里的每一个生物都数一遍，所以要取样。
Quadrats are square frames placed on the ground, and you count or estimate the species inside them.
样方是放在地面上的方框，你数一数或估算框内的物种。
Transects mean counting along a line across the area: a line transect records whatever touches the line, while a belt transect counts everything within a strip.
样带是指沿着一条穿过该区域的线来计数： 线样带记录碰到这条线的东西，带状样带则统计一条带子内的所有东西。
Those two work for things that stay put.
这两种适用于不动的生物。
For animals that move, you use mark, release and recapture: catch some, mark them, let them go, then later catch a second sample and see what fraction of it is marked.
对会移动的动物，就用标志重捕法： 抓一些，做上标记，放回去，之后再抓第二批，看其中有多大比例是带标记的。
Whichever method you use, random sampling — choosing your positions by chance — is exactly what stops you biasing the result.
不管用哪种方法，都要随机取样——用随机的方式选位置，正是防止你把结果弄偏的关键。
Here is the calculation.
来看这个计算。
Sixty snails are caught, marked and released.
抓到六十只蜗牛，做上标记后放回。
Later, eighty snails are caught and fifteen of them are marked.
之后又抓到八十只，其中十五只带标记。
The method assumes the marked ones have mixed back in evenly. To estimate the population, multiply the two sample sizes and divide by the number marked in the second sample.
要估算种群数量，就把两次样本量相乘，再除以第二次样本中带标记的数目。
Sixty times eighty is four thousand eight hundred, divided by fifteen gives three hundred and twenty snails.
六十乘八十等于四千八百，除以十五得到三百二十只蜗牛。
Now, this estimate rests on assumptions, and a good answer will state them.
这个估算建立在一些假设之上，好的答案会把它们写出来。
There must be no births, deaths or migration between the two catches.
两次捕捉之间不能有出生、死亡或迁移。
The marks must stay on, and must not make the animal easier for a predator to spot.
标记必须留得住， 而且不能让动物更容易被捕食者发现。
And enough time must pass for the marked animals to mix back in evenly.
还必须留出足够的时间让带标记的个体重新均匀混合。
Break one and the estimate breaks with it.
违反其中任何一条，估算就跟着失效。
A mark that attracts predators lowers the number recaptured, which makes the population look bigger than it is.
会招来捕食者的标记会降低重捕到的数目，从而让种群看起来比实际更多。
To turn all that counting into a single number, you use Simpson's index of diversity.
要把这些计数变成一个数字，就用辛普森多样性指数。
A higher value means the area is more diverse.
数值越高，说明这个区域越多样。
And there is a reason that matters: a more diverse ecosystem is usually more stable, because no single loss can break it.
这一点之所以重要是有原因的：更多样的生态系统通常更稳定，因为没有哪一项损失能把它摧毁。
If one species disappears from a rich community, others can fill the gap; in a community of two or three species, the same loss is a collapse.
如果一个物种从一个丰富的群落中消失，其他物种可以填补空缺； 而在只有两三个物种的群落里，同样的损失就是崩溃。
To link the spread of a species to something in its environment, you use a correlation — Spearman's rank or Pearson's — and the factor you test is either biotic, meaning living, or abiotic, meaning non-living.
要把某个物种的分布与环境中的某个因素联系起来，就用相关性分析—— 斯皮尔曼等级相关或皮尔逊相关——而你检验的因素要么是生物因素， 也就是有生命的，要么是非生物因素，也就是无生命的。
First, why species die out: climate change, competition — often from a species we brought in — hunting by humans, and the damage or loss of their habitats.
先说物种为什么会灭绝：气候变化、竞争——往往来自我们带进来的物种—— 人类的猎杀，以及栖息地的破坏或丧失。
We try to protect biodiversity because other species give us food, medicines and materials, because they help keep ecosystems stable, and because many people think they have value in themselves.
我们努力保护生物多样性， 因为其他物种给我们提供食物、药物和材料，因为它们有助于维持生态系统稳定， 也因为许多人认为它们本身就有价值。
There are two ways to conserve species, and you should be able to give both with an advantage.
保护分为两条路，你应该能各给一条并说出一个优点。
In-situ conservation protects a species in its own habitat, through national parks and marine reserves, so the species keeps its niche and its normal behaviour.
就地保护是在物种自己的栖息地里保护它，通过国家公园和海洋保护区， 让物种保住自己的生态位和正常行为。
Ex-situ conservation keeps and breeds it away from the wild — zoos, botanic gardens, seed banks and frozen zoos of stored cells and eggs — which is safer for the individuals but does not save the habitat.
迁地保护则是把它带离野外来饲养和繁育——动物园、植物园、种子库， 以及储存细胞和卵的"冷冻动物园"——这对个体更安全，但没有保住栖息地。
For rare mammals, assisted reproduction helps: in vitro fertilisation, embryo transfer and surrogacy.
对稀有哺乳动物，辅助生殖能帮上忙：体外受精、胚胎移植和代孕。
Conservationists also control invasive species.
保护工作者还要控制入侵物种。
And two organisations to name: the IUCN lists how threatened each species is, and CITES controls the international trade in endangered species.
还有两个要点名的组织： 世界自然保护联盟列出每个物种受威胁的程度，濒危野生动植物种国际贸易公约管制国际贸易。
One more thing about the mark-recapture estimate: a mark that attracts predators lowers the recapture count and therefore overestimates the population.
关于标记重捕估算还有一点： 如果标记让个体更容易被捕食者发现，重捕数就会偏低， 于是对种群数量的估计就会偏高。
Four things the examiner keeps asking for.
考官反复要的四点。
First, when you define a species, the word fertile has to be there — breeding alone is not enough.
第一，给物种下定义时，"可育"这个词必须出现——只说能交配是不够的。
Second, with Simpson's index, say that a higher value means more diverse, and link it to stability.
第二，讲辛普森指数时，要说数值越高越多样，并把它与稳定性联系起来。
Third, when asked about conservation, give in-situ and ex-situ methods and an advantage of each, not just a list of places.
第三，问到保护时，就地保护和迁地保护的方法都要给，各说一个优点， 而不是只列出一串地方。
Fourth, the Lincoln index rests on assumptions — state them, because a mark scheme almost always has a mark sitting there for exactly that.
第四，凡是用到标志重捕法，都要把假设写出来—— 因为评分标准里几乎总有一分就在那里。
So there it is.
就是这样了。
A word for a group, a hierarchy to file it in, a way to count what you cannot count, and two ways to keep what is left.
一个用来指称类群的词，一套用来归档它的层级， 一种数清你数不完的东西的方法，以及两条保住剩下的路。
Learn these eight terms and the whole topic holds together.
记住这八个术语，整个主题就串起来了。
See you in the next lesson.
下节课见。