你还记得二叠纪-三叠纪灭绝事件吗? You remember back in the days right after the Permian-Triassic extinction event,
那个巨大行星撞到了地球 when that giant flaming asteroid and those
然后那些甲烷爆炸几乎杀光了所有生物? methane explosions killed almost all of the organisms on the planet?
不, 你当然不记得, 因为这发生在 No, of course you don't because that happened
2.52 亿年前, 哺乳动物都还没出现. 252 million years ago, and mammals weren't a thing yet.
但这事跟这集视频有点关系 But that's kind of the point of this episode.
那个小行星... 干扰了地球的生态环境 That asteroid was a... a disturbance to the ecology of the planet.
植物, 动物和土壤在很大程度上损失殆尽, The flora, fauna and soils were largely wiped out,
给那些幸存的生物留下了大量的资源 leaving a blank canvas for the organisms that survived, and there
因为幸存的生物不多, 它们爱占多少占多少. really weren't all that many of them, to fill in as they could.
二叠纪-三叠纪灭绝事件后 What happened after the Permian-Triassic "disturbance"
发生了一个戏剧性的 生态演替 例子, is a dramatic example of ecological succession,
生态演替就是群落受到干扰后, how the makeup of a community changes over time,
随着时间推移发生变化. starting from, like, the day after a disturbance.
只是通常 干扰没那么强大. Just, usually, the disturbance is a little less disturbing.
研究群落如何变化 不只是 The study of how ecological communities change doesn't just
长期的观察它, 或者是看某些大灾难带来的影响 look at huge-long periods of time, or the effects of some apocalypse.
生态演替非常普遍 也许一季内就在公园里发生了 Succession can easily happen over a season in a park, or in just
或在你家花园里几天内就发生了. a few days in a patch of land as small as your garden.
干扰也许会来的很突然, And, this might come as a surprise, but disturbances that
它会改变群落的现状 shake up the status quo within a community actually
但从长远来看 是有益于群落的. serve to make that community better in the long run.
因为就像生命和整个宇宙一样 Because much like life, and the entire universe,
演替都是关于变化 succession is all about change.
变化 使得除氢气外啥都没有的宇宙 And change is how a universe full of nothing
拥有了充满生机的地球. but hydrogen came to include a planet full of life.
生态系统每天都在遭受干扰: Disturbances happen in ecosystems all the time, every day:
野火, 洪水, 风暴. a wildfire, a flood, a windstorm.
在这些不可预知的事件后, 生态学家不断观察到 After these unpredictable events, ecologists kept seeing
生态系统里可预见的, 甚至是有序的变化 predictable, even orderly changes in the ecosystem.
生物如何处理这样的干扰 How life deals with these disturbances is
是理解生态系统的一个重要关键. an important key to understanding ecosystems.
首先 搞清一件事 森林里倒下的一棵树 First, let's note that a tree falling in the forest
和天上掉下来的彗星 这俩都是干扰 and a comet falling in the forest, while both disturbances,
但干扰等级不同. are different levels of disturbance.
演替有好几种类型 Likewise there are a couple of different types of succession.
第1种类型, 小行星撞击后的荒地 The first type, the one that happens after the asteroid hits
或冰川融化后的荒地 or the glacier plows over the landscape or
或 森林火灾/火山 把树烧光 把那地方变成荒地 the forest fire-slash-volcano burns the verdant ecosystem
这都叫做 初级演替: into pure desolation, that's called a primary succession:
生物迁入一个毫无生命的地方就叫初级演替. when organisms populate an area for the first time.
初级演替发生在毫无生命的荒地. The jumping off point for primary succession is your basic,
初级演替发生在毫无生命的荒地. lifeless, post-apocalyptic wasteland.
你可能在想 那些地方听起来很可怕! You're probably thinking, that place sounds terrible!
谁会愿意生活在那里? Who would ever want to live there?
事实上 在那里生活有个巨大的优势 Well, actually, there is one tremendous advantage to desolate
没有竞争. wastelands... no competition.
有很多生物根本不介意 A lot of organisms don't mind settling down in the more
去更荒凉的角落呆着.. inhospitable nooks and crannies of the planet.
这些开拓者通常是原核生物或原生生物 These pioneer species are often prokaryotes or protists,
然后是非维管植物, 也许还会有些 followed by nonvascular plants, then maybe some extra
超顽强的维管植物. super hardy vascular plants.
这些开拓者中有很多 There are tons of organisms that make their living
会死在新殖民地. 那就是它们的工作 colonizing dead places. It's their thing.
就像二叠纪-三叠纪灭绝事件之前, Like before the Permian-Triassic extinction,
有茂密的裸子植物森林 there were these dense forests of gymnosperms,
那时也许到处都是松柏, 银杏, 苏铁这样的植物 probably full of species a lot like the conifers, gingkos
这三种植物今天仍然还在. and cycads we still have today.
但小行星撞击后 大片森林死亡 取而代之的是 But after the asteroid hit, the big forests died and were replaced
石松植物 by lycophytes,
这种植物很像已经灭绝的 simpler vascular plants like the now-extinct
鳞木, 以及现代的石松. scale trees and today's club mosses.
虽然它们在以前的好时代时 While they might have had a hard time competing
很难和那些更复杂的植物竞争. with the more complicated plants during the good times.
但现在其他古生代植物都死的差不多了 The rest of the Paleozoic Flora barely survived extinction,
以前有好几十种银杏 of all the dozens of species of gingko that were around
但现在只有一种还存在 , back then, only one still exists,
它的基因完全孤立了, 变成了活化石 completely genetically isolated, a living fossil.
重要的是要记住 It's important to remember that when we talk about
当我们讨论初级演替时 primary ecological succession, we're talking about plants,
我们基本上就是在讨论植物 pretty much exclusively.
因为植物统治世界, 记得吗? Because plants rule the world, remember?
没有植物, 就不会有动物群落 Without plants, the animals in a community don't stand a chance,
参与初级演替的物种往往是植物 and primary successional species are often plants
有些植物可以利用风传播种子, 比如石松, 苔藓 that have wind borne seeds, like lycophytes, or mosses
和地衣, 风把它们的孢子到处传播. and lichens that have spores that blow in and colonize the area.
初级演替的结果是 And the outcome of a primary successional landscape is to build,
土壤变得更有营养了, 因为随着时间的推移 or rebuild, soils, which develop over time as the mosses,
那些苔藓, 小草和小植物会生长, 死亡和分解. grasses and tiny little plants grow, die and decompose.
土壤一旦准备好了, 大点的植物就可以搬进去了 Once the soils are ready, slightly bigger plants can move in,
这时 我们叫做 次生演替. at which point, we move onto secondary succession.
再之后 红木森林也可以在这里生长了 And then it's game on: a whole redwood forest
再之后 红木森林也可以在这里生长了 could develop out of that.
初级演替需要很长很长的​​时间: But primary succession takes a long, long time:
比如数百甚至数千年. like hundreds, maybe thousands of years in some places.
事实上, 在二叠纪-三叠纪灭绝事件后, 过了差不多四五百万年. In fact, the recovery of these big gymnosperm forests after the
那些大裸子植物森林才恢复回来. Permian-Triassic extinction event took about 4 or 5 million years.
泥巴对你来讲也许很乏味 Dirt may seem unglamorous to you, but it is alive
但它变成 富含生命 美丽, 复杂, 良好的土壤 是需要时间的！ and beautiful and complicated, and making good soil takes time!
次生演替 不只是初级演替使那块地方变得可居住后 Now, secondary succession isn't just the next act after primary
会发生的事. succession has made a place livable after some disaster.
它通常也是群落遭受小型干扰后的响应. It's usually the first response after a smaller disturbance
比如洪水 或是小型的森林火灾 like a flood or a little fire has knocked back the plants
比如洪水 或是小型的森林火灾 that have been ruling the roost for a while.
即便是森林里倒下一棵树这种小干扰 Even a disturbance as small as a tree crashing down in the woods
也能让一片小森林看起来就像是50年前, can make a tiny patch of forest more like it was 50 years ago,
在一棵树长大并且能遮阳之前: before that one tree got so huge and shady:
那个小区域里会出现 In that tiny area, there will suddenly be a different
与其他地方不同的微气候, microclimate than in the rest of the forest, which might
那里可能有更多的阳光 更高的温度 have more sunlight, slightly higher temperatures,
更少的天气保护 等等 less protection from weather, etc.
就像地球上的 其他生态系统 And just like every other ecosystem on earth,
那片地区比起其他地方 被温度和降水影响更大 this tiny patch of forest will be affected by temperature
这些因素在森林里不同的地方 and precipitation the most, which will
都是不同的 be different in different parts of the forest.
倒下的树会让土壤变得不同 So, as a result of the fallen tree, the soils will
死植物的残骸也会让土壤变得不同 become different, the mix of plants will become different,
会导致不同的动物来这里生活 and different animals will want to do business there because that
因为那个小生态位更适合它们 little niche suits their needs better than other little niches.
所以问题就变成了 当演替停止后 So the question becomes, when does succession stop
事情会恢复正常吗? and things get back to normal?
永远不会. 因为变化是不会停止的 Never. Because change doesn't end.
只有变化是永存的.. 你知道是谁说的吗? Change is the only constant people... you know who said that?
赫拉克利特... 在公元前500年说的 Heraclitus... in 500 BC. So it's been true since at least then.
把它当作是生活教训吧 Consider it a lesson in life.
在生态学里 这种看待事物的角度还是蛮新颖的 And as ideas in ecology go it's actually a pretty new way of looking at thing
早在20年代初 生态学家注意到 See, back in the early 20th century, ecologists noticed
群落倾向于 随着时间的推移而改变 the tendency of communities to morph over time.
但他们也注意到 当群落发展到 But they also saw succession in terms of a community changing
"顶极群落"时 演替就停止了 until it ultimately ended in what they called a climax community,
群落里的物种会保持稳定 which would have a predictable assemblage of species
直到下一个大干扰的来临. that would remain stable until the next big disturbance.
也许你以为这是每个群落的最后结果. Well, maybe that's what seemed to be happening, but ecological
但生态演替实际上比那复杂得多 succession is actually a lot more complicated than that.
首先 有个小东西叫做随意性 For starters, there's a little thing called stochasticity
或者说随机性, 因为有它的存在 我们无法知道一个群落受到干扰后 or randomness which prevents us from ever knowing exactly what a
过了100年会看起来怎样 community is going to look like 100 years after a disturbance.
基本上在任何事情上 随机性 Stochasticity is basically your element of
都是个不可预知的重要因素 unpredictable variability in anything.
也许你可以把冰川退去后 So, you can predict with some accuracy what plants are going
占领冰碛的究竟是哪种植物 猜的八九不离十 to take over a glacial moraine after the ice has receded, because
因为有些殖民植物的种子总是最先到达 the seeds of some colonizer species typically make it there first.
但那些不可预知的事情 比如演替早期阶段的天气 But unpredictable things like weather conditions during the early
可能会特别有利于另一个物种 stages of succession can end up favoring another species.
关键是, 如果科学家预测某个群落 The point is, scientists attempts to predict what a
100年后会看起来像什么 community ends up looking like in 100 years should
你应该把这个预测当作一种可能性 它不一定真的会那样的. always be thought of as probabilities, not certainties.
顶级群落模型中的另一个困难 Another difficulty with the whole model of a climax community
是它认为生态系统最终会变得稳定 has to do with the idea of an ecosystem eventually stabilizing.
这个词 "稳定"? That word, "stable"?
每当它被用在句子里, 总是会带上另一个单词 "生态" Whenever it's used in a sentence that also includes the word
你几乎可以确定它被用错了 "ecology", you can pretty much be sure it's being used wrong.
因为稳定从未发生过 Because stability never happens.
总是有干扰在发生 There are always disturbances happening
在每一个生态系统都是. all the time, in every ecosystem.
森林火灾可能会烧掉一小部分森林 A small portion of the forest might burn, a windstorm might
风暴可能吹倒一堆树, take out a bunch of trees, some yeehaw might rent himself
某人可能会在周末借一台挖土机 a backhoe one weekend and clear himself a little patch
清理山上房子旁边的树 of heaven on the mountain beside his house because
因为他闲的只能做这个了 he's got nothing better to do.
谁知道呢! 啥都可能发生 Who knows! Stuff happens.
与其演替成稳定的顶极群落 So instead of ending in some fixed, stable climax community,
我们现在知道了 在生态系统的演替后期 we now know that an ecosystem is in later successional stages
它会有高生物多样性 很多很多的多样性 if it has high biodiversity. Lots and lots of biodiversity.
只有生态位多 才会导致生物多样性高 The only way biodiversity could be high is if there are tons
只有生态位多 才会导致生物多样性高 of little niches for all those species to fit into.
能弄出很多生态位的方法是 And the only way there could be that many niches is if,
与其由一个群落组成 生态系统实际上 instead of a single community, an ecosystem was actually
是由数千个小群落组成的栖息地 made up of thousands of tiny communities, a mosaic of habitats
不同的生物生活在不同的群落里 where specific communities of different organisms lived.
随着时间的推移"干扰"创造了生态位 Such mosaics of niches are created by disturbances over time,
所有的一切都在变化 with everything always changing here and there.
但重要的是, 这些干扰 But it's important that these disturbances be
是正确的类型 正确的规模 of the right kind, and the right scale.
因为事实证明 Because it turns out that the kind of disturbances
这种对生物多样性有巨大影响的干扰 that have the greatest effect on biodiversity
是最温和的干扰 are the most moderate disturbances.
当生态学家想通这个时 他们决定把它称为 When ecologists figured this out, they decided to call it
适度干扰假说. the Intermediate Disturbance Hypothesis.
因为它假设中度的干扰 Because, it hypothesizes that intermediate disturbances,
也就是那些不太大也不太小的, 是理想的. not too big and not too little, are ideal.
小干扰比如像一棵树倒下, See, just a little disturbance, like a falling tree or something,
是不够改变一切的. isn't enough to really change the game.
另一方面, 那些非常严重的干扰, 比如被岩浆覆盖 On the other hand, a really severe disturbance, like getting
会把群落带回 covered with lava, would take the community all the way
初级演替状态. back to asteroid wipe-out-level primary succession.
但每个中级干扰 都在那个演替阶段中 But every nice mid-level disturbance creates its own habitat
创造了栖息地 以及独特的生态位 at its own stage of succession with its own unique niches.
更多生态位 意味着更多生物多样性 而这些生物多样性 More niches means more biodiversity, and more biodiversity
意味着更稳定更健康的生态系统. means more stability and healthier ecosystems.
即便有2个干扰同时发生在 Even if two disturbances happen in two different areas
气候类似的2个不同区域 with roughly the same climate at the same time,
因为生态系统有随机性, 这两个区域 the stochastic nature of ecosystems mean that the two areas
恢复的方式可能完全不同 might recover incompletely different ways,
导致更多生态位 以及更多生物多样性! leading to even more niches and more biodiversity!
当然 这并不意味着你应该租个挖土机 Now, this does not mean that you should go rent
然后到旷野里到处挖. a backhoe tomorrow and cut a swath into the wilderness.
这个假说只是说 中等的干扰是自然的 It just suggesting that medium-level of disturbance is natural and
对生态系统有好处 normal and good for an ecosystem. Keeps everybody on their toes.
就像我说过的 干扰总是会发生 like I said, disturbance happens.
总的来说 我们应该让它发生 And by and large we should let it happen.
这同样也是生态学中的一个相对较新的概念 This, too, is a relatively new idea in ecology.
事实上 美国公共土地管理历史中的大部分时间里 In fact, for most of the history of public land management
大片的森林不允许被烧毁 in the U. S great swaths of forests were not allowed to burn.
人们认为森林的 "目的" 应该是生产木材 People considered the "purpose" of forests to be wood production.
烧掉那些树 And you don't want to burn down some trees that
不会为你赚到钱. are gonna make you a bunch of money.
但由于缺乏中度干扰 But because of the lack of intermediate disturbances
在很长一段时间后 我们会得到灾难性火灾 over a long period of time, we ended up with catastrophic fires
比如1988年 黄石国家公园由雷击所造成的大火 like the one that torched Yellowstone National Park back in 1988 .
烧毁了近80万英亩森林 A single lightning strike totally annihilated almost 800,000 acres
因为那个生态系统 of public forest because the ecosystem hadn't been allowed
不允许被干扰. to indulge in a nice leisurely burn every now and then.
但现在 那些森林经历了超过20年的演替. But now those forests have undergone more than 20 years
这些年里有些地方 又被适度的烧了一次, of succession, and some parts have even re-burned at
创造了一个高生物多样性的栖息地. a more intermediate level, creating a nice,
创造了一个高生物多样性的栖息地. high-biodiversity mosaic of habitats.
那里很漂亮 你应该抽空去看看. And it's gorgeous, you should come visit it sometime.
以上 就是本集的全部内容了. And that is ecological succession for you.
我们学到了破坏和干扰 如何使环境变美和提高多样性. how destruction and disturbance lead to beauty and diversity.
记着 赫拉克利特说过 Just remember what my main man Heraclitus said and
你会好的: 唯一不变的就是变化. you'll be good: the only constant is change.
感谢收看本集生态学速成班 Thank you for watching this episode of Crash Course Ecology.
感谢所有帮忙制作视频的人 And thank you to everyone who helped us put this episode together.
如果你想重看我们今天学的概念 If you want to review any of the concepts we studied today,
那边有张表 there's a table of contents over there.
如果你有任何问题 想法或意见 And if you have any questions, ideas or comments, we're on
我们在 Facebook Twitter 上有帐号. 当然, 你也可以在下面留言. Facebook, Twitter and of course, down in the comments below.
我们下次再见. We'll see you next time.