在生物学系列里 我们花了很多时间来谈论 In our series on Biology, we spent many weeks together
动物和植物的生理结构 talking about the physiology of animals and plants.
细胞如何形成组织 形成器官 And how cells work together to make tissues, to make organs,
组成器官系统 组成大块的肉 to make organ systems, to make us the hunks of meat
和鲜脆的蔬菜. and vegetables that we are.
为了了解整个生物, In understanding the whole organism,
重要的是 知道所有层面上都发生了些什么. it's important to know what's going on at all those levels.
生态学也是一样的 And the same is true for Ecology, only instead of zooming
只是 我们需要放大缩小观察的不是一个生物体 in and out on different levels within a living thing,
而是来观察整个地球 we can zoom in and out on the earth.
根据放大的倍率 Depending on the power of the magnification,
我们可以了解地球的所有事情 we can understand a whole range of things about our planet.
例如 我们可以观察同一种生物中的不同团体 For instance, we can look at groups within a species
是怎么在同一个地方一起生活. and how they live together in one geographic area.
这叫做 种群生态学. That's Population Ecology.
还有 群落生态学 There's also Community Ecology,
它研究的是不同类的生物 如何生活在一起, where you look at groups of different organisms living together
以及它们之间如何相互影响. and figure out how they influence each other.
如果把"眼睛倍率"缩到最小, 那么就成了生态系统生态学, And then, the most zoomed-out we get is Ecosystem Ecology,
它研究的是一个生态系统内 the study of how all living and nonliving things
所有生命和非生命体之间的互动. interact within an entire ecosystem.
现在, 让我们"放大眼光" 来看看"种群生态学" So let's start by zooming in, with Population Ecology.
它主要研究同种动物的个体间如何相互影响, The study of groups within a species that interact mostly with each other,
以此来理解为什么一个地方的一群动物, to understand why those populations are different
和另一个地方的同一群动物不同. in one time and place than they are in another.
现在也许你可能会问自己, How, you may be asking yourself,
学这玩意有用吗? is that in any way useful to anyone ever?
实际上这玩意对所有人都超有用! Well, it's actually superuseful to everybody, always!
让我们来看看它多有用, 举个例子, "西尼罗河病毒" Let's look, for instance, at the outbreak of West Nile virus
爆发于 2012 年的夏天 袭击了得克萨斯州的达拉斯城. that struck Dallas, Texas in the summer of 2012.
在达拉斯城里 有12人死于病毒 In Dallas County, 12 people died from the virus as of the filming
近300人被感染 of this and nearly 300 people had been infected.
但在2011年, 就算是整个得克萨斯州, 也才27例感染了"西尼罗河病毒". But in 2011, the whole state of Texas reported only 27 cases
只有2人死亡. of West Nile, and only 2 deaths.
差距太明显了 怎么会这样? That seems kind of significant. So, what's up?
事实证明 这是一个种群生态学问题. It turns out that this isa population ecology problem.
"西尼罗河病毒"是一种由蚊子传播的疾病, West Nile is a mosquito-borne illness,
这些蚊子的数量在2012年暴增了 and the population of mosquitoes in Dallas in 2012 busted
就像库尔小子打破砖墙一样 through brick walls like the Kool-aid man,
（ 库尔小子是一款饮料的动画人物, 它们的广告就是这个动画人物破墙而出.） spreading West Nile like crazy.
疯狂的传播"西尼罗河病毒". So why did this outbreak happenin 2012 and not the year before?
那为什么是在 2012 年数量暴增 而不是在前一年暴增? And why did it happenin Texas and not in New Jersey?
为什么它发生在德州, 而不是新泽西呢？ The answer is population ecology!
答案是: 种群生态学! Before we start solving any disease outbreak mysteries,
在我们找出疾病爆发的原因之前, we gotta understand the fundamental principles of population ecology.
我们得先学习种群生态学的一些基础知识. For starters, a population is just a group of individuals
首先, "种群" 就是一些同类个体组成的团体. of one species who interact regularly.
而且这些个体间会时常相互影响. How often organisms interact has a lot to do with geography:
有多时常? 和环境有很大关系： You're gonna have a lot more face time with folks you live near
你和住附近的人的碰面次数 than those who live farther away.
肯定比住得远的要多. As a result, individuals who are closer to you will be
那些与你更近的人 the ones you'll compete with for food, and living space,
会和你竞争食物, 生活空间, mates, all that stuff.
交配对象, 所有东西. But in order to understand why populations are different
但为了理解为什么一个种群 from time to time, and place to place, a population ecologist
在不同时间和不同地点的表现不同 种群生态学家 needs to know a few things about a population, like its density.
需要知道关于这个种群的几件事情, 比如密度. In this instance, how many mosquitoes there are in the
在刚刚那个例子里 密度就是有多少蚊子住在达拉斯城 greater Dallas area that might come into contact with each other.
并且蚊子们碰面的几率有多大. A population's density changes due to a number of factors,
种群密度的变化因素很多, all of which are pretty intuitive: It increases when
同时也很简单，当种群密度增加时 new individuals are either are born or immigrate,
无非是因为有新生物出生 或者是有新移民搬进来了, that is, move in, and it decreases because of
减少时 deaths or emigration, or individuals moving out.
无非是因为有生物死亡 或是有人移民去了其他地方. Simple enough. But as a population ecologist,
够简单了吧. 但作为一个种群生态学家, you also need to know about the geographical arrangement
你还需要知道这个种群的地理位置. of the individuals within the population.
你还需要知道这个种群的地理位置. This is their dispersion.
也就是它们的"分布". Like, are the mosquitos all clumped together?
比如 这些蚊子是扎堆聚在一起的吗？ Are they evenly spaced across the county?
它们是均匀分布的吗？ Is there some kind of random spacing?
它们的间距是否随机？ The answer to these questions give scientists
这些问题的答案 a snapshot of a population at a given moment.
给出了一个种群在某一特定时刻的快照 And to figure out a puzzle like the West Nile outbreak,
它可以用来解决一些难题 比如为什么"西尼罗河病毒"会爆发 which involves studying how a population has changed over time,
它涉及到研究一个种群如何随着时间变化 you have to investigate one of population ecology's
所以你必须研究种群生态学里的 central principles: population growth.
一个重要原理: 种群增长. There are all kinds of factors that drive population growth,
有各种因素会引起种群增长 and they can vary radically from one organism to the next.
不同生物的种群增长原因 可能有很大的不同 Things like fecundity, how many offspring an individual can have
比如繁殖能力 生育数量什么的 in a lifetime, make a huge difference in the size of a population.
这些事情使得不同物种的数量有巨大的差异 So, for instance, why do mosquito populations seem to grow
举例来说 为什么蚊子的数量增长如此快？ so quickly, while the endangered black rhino may never recover
而黑犀牛却因为偷猎濒临灭绝, from a single act of poaching?
而且有可能永远恢复不到以前的数量了呢? For starters, mosquitoes can have 2,000 offspring in their two-week lifetime,
首先, 蚊子能活2个星期左右 但能生 2,000 只后代. while the rhino can have, like, 5 in 40 years.
而犀牛 能活40年 但5年才生一胎. Still, a population doesn't usually or even ever grow
尽管如此 但蚊子的数量 to its full potential, and it can't keep growing indefinitely.
不会一直增长 To understand how fast or slow, and high or low,
为了理解一个种群为什么增长得快或慢, a population actually grows, you need to focus
种群数量为什么多或少, 你需要关注 on what's keeping growth in check.
是什么导致了数量变化. These factors are, appropriately, called limiting factors.
这些因素 也可以叫"限制因素" Say you're a mosquito in Dallas in 2011,
假设你是一只蚊子 2011年时住在达拉斯 the year before the outbreak.
那是病毒爆发的前一年. Back then, the growth rate wasn't what it was in 2012.
那时 人口的增长比2012年小多了 So, something was keep in' ya down.
肯定是有什么阻止了数量的增长 To figure out what your limiting factors were,
为了弄清楚限制因素是什么 you have to first narrow down what you need as a mosquito,
咱先来观察单个蚊子需要啥 to live and reproduce successfully.
才能生存和繁殖 First you've got to find your food.
1. 食物 Now, you mosquitos, you eat all kinds of things,
现在 你是一只蚊子 你可以吃各种东西 but in order to reproduce, assuming you're a female,
但让我们假设你是雌性吧, you need a blood meal.
雌性需要吸血. So you have to find a vertebrate and suck some of its blood out.
所以 你必须找到一个脊椎动物 然后吸点血 Presumably there's no shortage of vertebrates
脊椎动物在达拉斯到处都是 walking around Dallas foryou to suck blood out of.
你很容易就能吸到血 I have good friends who are vertebrates in Dallas.
我有一些朋友住在达拉斯 You might even be able to suck some of their blood.
你可能曾经吸过一些他们的血 Next, temperature: Because you mosquitoes are ectothermic,
2. 温度 因为蚊子是变温动物 ( 变温动物无法调节自己的体温 需要通过外界间接调节) it has to be warm in order for you to be active.
你喜欢温暖的天气 And Texas is plenty warm,
而得克萨斯州 是很温暖的 and the winter of 2011 and 2012 was especially balmy.
并且2011, 2012年的冬天特别温暖 In fact, the summer of 2012 was exceptionally hot,
事实上 2012年的夏天异常炎热 which helps speed up the mosquito life cycle.
这有助于加快蚊子的生命周期 So, that's one limiting factor that's been removed
所以 对达拉斯地区的蚊子来说 for Dallas area mosquitoes.
"温度"这个限制因素也不存在了 Moving on to mates:If you're a female mosquito
3. "伴侣" 如果你是一个雌蚊 you need to find a nice male mosquito with a job,
你需要找个雄蚊来交配 and preferably his own car, because you know Dallas
也许他有一部漂亮的跑车 因为你知道达拉斯 is a pretty big city, to mate with.
是个相当大的城市. This isn't actually all that hard because of the way
前面都是开玩笑的 实际上这是很不实际的 that mosquitoes do it, males just gather into a mosquito cloud
现实是 雌蚊在交配季节的每天黄昏时 at dusk every night during mating season, and all a female has to do
会跑到雄性蚊子堆里 is find her local dude-cloud and fly into it
找到她的交配对象 in order to get mated with. Easy cheese!
最后成功交配. Finally, space: And, ah-ha!
4. 空间 啊, 哈! Because here we have another important clue.
因为我们知道 蚊子需要产卵在积水里 Mosquitoes need to lay their eggs in stagnant water.
因为我们知道 蚊子需要产卵在积水里 And if there's anything mosquito larvae hate,
如果说有什么是蚊子幼虫讨厌的 it's a rainstorm flushing outthe little puddle of water
那一定是暴雨 因为那会冲刷它住的小水坑 they've been living in.
那一定是暴雨 因为那会冲刷它住的小水坑 And since Dallas saw a pretty severe drought
达拉斯在 2012 年的夏天干旱的挺严重的 in the summer of 2012, there were lots of pockets
一些小水坑旁边 of stagnant nasty mosquito water sitting around,
都会有超多的蚊子 acting as nurseries for many, many, West Nile-infected mosquitoes.
就像蚊子的托儿所一样 养着一堆小蚊子 So, when we look at this evidence, we find at least
当我们回顾的时候 我们发现 two limiting factors for Dallas mosquito population growth
至少有2个因素成为了达拉斯蚊子增长的限制因素 that were removed in 2011: the constraints of temperature and space.
在2011年被移除了 也就是"温度"和"空间" It was plenty hot and there were lots of egg-laying locations,
那时不仅很暖和 而且还有很多的地方产卵 so the bugs were free to go nuts.
所以蚊子的数量就急速上升了 Population ecologists group limiting factors like these into two
种群生态学家把这些限制因素分为两类 different categories: density-dependent and density-independent.
"密度有关" 和 "密度无关" They do it this way because we need to know whether a population's
这样分类是因为我们需要知道 growth rate is being controlled by how many individuals are in it,
种群的增长率是否和当前数量有关, or whether it's being controlled by something else.
还是说增长率和其他东西有关. And the reason these limitations matter is because they affect
这些"限制因素"之所以重要, what's known as the carrying capacity of the mosquitos habitat:
是因为它影响了蚊子栖息地的 "环境承载力" That's the number of individuals that a habitat
也就是栖息地所能承受的数量. can sustain with the resources that it has available.
也就是栖息地所能承受的数量. So, density-dependent limitations are factors that inhibit growth
那么, "密度有关"的限制是抑制其增长率的一个因素 because of the environmental stress caused by a population's size.
因为环境的压力 是由人口数量造成的. For example, there may simply not be enough food, water,
例如 有可能根本就没有足够的食物, 水 and space to accommodate everyone.
和空间来容纳每一个人. Or maybe because there are so many individuals, a nearby predator
或者 也许是因为有这么多的人 附近的捕食者 population explodes, which helps keep the population in check.
的数量也上升了 这有助于保持人口的数量 Things like disease can also be a density-dependent limitation.
还有疾病之类的东西 也算是一个依赖于密度的限制 Lots of individuals living in close quarters
因为大量的个体的生活距离近 can make infections spread like crazy.
可以使得某些流行病的蔓延非常的快 Now I don't think that the Dallas mosquitoes are going to
现在我不认为 达拉斯的蚊子 run out of vertebrates to dine on anytime soon.
会吸干那些脊椎动物 But, let's say hypothetically that the explosion of local mosquito
但是 让我们假设说 蚊子数量爆炸的地方 populations caused a similar explosion in the number of Mexican
是在墨西哥, free-tailed bats, the official flying mammal of the state of Texas.
那边有无尾蝙蝠. And they eat mosquitoes.
蝙蝠会吃蚊子. That would be a limiting factor that was density-dependent.
这会成为一个限制因素, 而且它是"密度有关"的. More mosquitoes leads to more bats, which leads to fewer mosquitos.
更多的蚊子让更多的蝙蝠有东西吃, 从而导致蚊子变少. It's pretty simple. When density-dependent limitations
这是很简单的 当"密度有关"的限制 start to kick in and start to limit a population's growth,
开始限制人口的增长 that means that the habitat's carrying capacity has been reached.
这意味着栖息地的承载能力已达极限. But the other type of limiting factor, the density independent ones,
当然 也有其他类型的限制因素 也就是那些"密度无关"的. have nothing to do with how many individuals there are
"密度无关"的意思是说 这些限制因素 or how dense the population is.
跟人口数量和密度没有关系. A lot of times these limitations are described in terms
密度无关的一般都是些自然灾害 of some catastrophe: a volcanic eruption, a monsoon, a Chernobyl.
火山爆发 季风 切尔诺贝利 In any case, some crucial aspect of the population's
在任何情况下 lifestyle changes enough that it makes it harder to get by.
一些关键生活方式的改变 会使得这些灾难更难发生. But these factors don't have to be super dramatic.
但这些改变不需要是什么剧烈的变化. Going back to mosquitoes: Say, in 2013, there's a huge thunderstorm,
咱接着谈蚊子 假设说2013年 有很多特别大的雷暴 a real gully-washer, in Dallas everyday for three months.
简直就是水坑杀手, 在3个月时间里, 达拉斯每天都有雷暴. That's going to disturb the clutches of mosquito eggs hanging out
对于蚊卵来说简直就是要命啊 in the stagnant water, so the number born that year
那年的蚊子出生数量 would be substantially smaller.
肯定会非常少. By the same token, if the temperature swing the other way and it
出于同样的原因 如果温度发生变化. was unseasonably cold all summer, the bugs' growth rate would drop.
比如夏天特别冷, 那么蚊子的增长率也会下降. Now, the truth is, there are a billion and a half situations,
事实是 有超多情况会影响人口数量, both big and small, that could lead to a population either reaching
这些情况可大可小, 最后可能导致人口达到环境的承载极限, its carrying capacity or collapsing because of external factors.
或因为外部因素大幅下降. It's a population ecologist's job to figure out what those factors are.
种群生态学家的工作 就是找出这些因素是什么. And that is what math is for!
这就是数学的用途! Our friend math says that any population of anything...
我们的朋友"数学"说 任何有关种群数量的东西... anything, will grow exponentially unless there's
都会成倍增长 除非 some reason that it can't.
由于某些原因 导致它不能成倍增长 Exponential growth means that the population grows
指数增长意味着 随着种群密度的增长 at a rate proportional to the size of the population.
其增长率也随之增长 So here at the beginning of 2012,
假设说 这里是2012年的年初 we might only have had 1,000 mosquitoes in Dallas,
达拉斯可能只有 1,000 只蚊子 but then after, say, one month, we got 3,000.
一个月后 变成了3000只 Now, with 3 times as many reproducing mosquitoes,
后来又变成了当前数量的3倍 the population grew three timesas fast as when there were 1,000.
就像是当初我们从1000变到3000一样 So then there are 9,000, at which point, it's growing three times
于是现在有了9000只 它增长了三倍 as fast as when there were 3,000.And on and on into infinity.
就像之前一样 然后一直一直一直增长到无限多 And in this scenario, the mosquitos are all,
在这种情况下 蚊子们会.. "CARRYING CAPACITY MY CHITIN-COVERED BUTT! THERE'S NO STOPPING US!"
"环境承载力什么的滚球去吧 没人可以阻挡我们!" But you know that doesn't really happen.
但你知道 这事儿从没有发生过 I mean, it can happen for a while, humans have been on an exponential
我的意思是 它不可能一直增长 但它可以持续一段时间 growth curve since the industrial revolution, for example.
举个栗子 自工业革命以来 人类的数量就一直在以指数形式增长 But eventually, something always knocks
但最终 肯定会有某些事情使得 the population size back down.
人口数量回落 That thing might be a density-dependent factor like food scarcity
这事可能是"密度有关"因素 比如粮食短缺 瘟疫 or an epidemic, or a density-independent one,
或者是一些"密度无关"的事情 like an asteroid that takes out the whole continent.
比如小行星撞到大陆上了 Regardless, this exponential growth curve can't go up forever.
无论如何 指数增长曲线不可能一直上升 And when those factors come into play,
总是一些限制因素会发生作用 a population experience only logistic growth.
这样种群密度曲线就会变成S曲线（即广义logistic曲线，意为会达到饱和） This just means that the population is limited to the carrying capacity
意思就是说 栖息地对种群数量的承载能力是有限的 of its habitat, which, when you think about it, ain't too much to ask.
你仔细想想的话 这个其实满合理的 See how this graph flattens up at the top?
看到曲线变平坦没？ The factor that creates that plateau is almost always a
曲线变得平坦的原因 density-dependent limitation.
几乎总是因为被"密度有关"一些限制因素抑制 As you add mosquitoes, eventually the rate of population growth
当蚊子越来越多 最终人口增长率 is going to slow down because they run out of food or space,
会放慢 因为他们的食物或空间用完了. and when we get to where the number levels off,
当我们发现数字变稳定后, that number is the carrying capacity of the
这数字就是栖息地对蚊子数量的承载能力了. mosquito population in that particular habitat.
这数字就是栖息地对蚊子数量的承载能力了. Now, let's apply all of these ideas using a simple equation that will
现在 让我们用一个简单的公式 allow us to calculate the population growth of anything we feel like.
来计算人口的增长率. I know it's math but, wake up because this is important!
我知道这是数学 但别睡着了 因为这很重要！ The city of Dallas is depending on you!
达拉斯城全靠你了! Let's calculate the growth of Dallas mosquito population
让我们来算算, 达拉斯城的蚊子 over a span of two weeks.
在两星期内的繁殖情况. All we have to do to get the rate of growth, that's r,
我们要算的是增长率, 我们用 r 来表示它, is take the number of births...
它等于出生的蚊子数量... births, minus the number of deaths and then divide that
减去死亡的蚊子数量. 然后除以 all by the initial population size.
当前的人口数量 Which we generally just call N.
我们一般叫它 N So, let's say we start with an initial population of 100 mosquitoes,
假设我们一开始有 100 只蚊子, and each of those mosquitoes lives an average of 2 weeks.
这些蚊子平均能活2周 So our deaths, over a span of two weeks, will be 100.
也就是说2周后 死亡蚊子数量是100只 Half of these mosquitoes are going to be female, so 50 of them.
蚊子中有一半是雌性 所以其中有50个 And they can produce about 2,000 babies in their lifetime,
它们一辈子大约能生 2,000 只小蚊子 so that's times 2,000.
所以得乘2000 So 50 mommy mosquitoes times 2,000 babies per mommy
50只母蚊子 乘以2000只后代 and you get births equaling 100,000 little baby mosquitos.
大概会有10万只小蚊子出生 Once we plug in all the numbers into this equation,
一旦把这些数字插入到这个等式中 even though this is totally a hypothetical,
尽管这完全是一个假设 we will see the true scope of Dallas mosquito problem.
我们可以知道蚊子数量的大概范围 So, blink! In two weeks, the population had 100,000 babies
眨眼间! 两周内就会有10万只小蚊子出生 and only 100 of them died.
只有100只老蚊子死亡 So this is a population growth rate, if you do the math, of 999.
那么 人口增长率是999 This means that for every mosquito out there at the beginning
这意味着 每只蚊子在两周后 of two weeks, there will be 999 more at the end of two weeks.
会变成999只 That is a 99,800% increase...by Thor's Hammer!
就像是雷神之锤的魔法让它增长了99,800%一样! Again, these are hypothetical numbers,
再次声明 这些都是假设的数字 but it gives you a sense of how a population can just go out of control
但这可以告诉你，当所有限制因素共同起作用时， when all the factors that we talk about go in its favor.
种群数量是如何失控的 And you guys haven't even seen trouble until you see what the graph
你们还没看到问题所在吧？ of human population looks like over the last couple of millennia.
等我介绍人类人口在过去几千年的增长情况后，你们就知道了 But to find out more about that,
想知道更多知识, you're going to have to join us next week. Until then,
下周记得加入我们. Thank you for watching this episode of Crash Course Ecology.
感谢收看本集生态学速成班. And thanks to everyone who helped put it together.
感谢所有帮忙做节目的人. The table of contents is over there
这里有张表, if you want to go re-watch anything and if you have any questions for us,
但字幕君没翻这张表, 因为字幕君好懒. we're on Facebook and Twitter and
如果你有任何问题, 我们在 Facebook 和 Twitter 上有帐号的 of course, in the comments below.
当然, 你也可以在视频下留言. See you next time.
我们下次再见.
@糖醋陈皮 校对@洋流范儿