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  • You remember back in the days right after the Permian-Triassic

  • extinction event, when that giant flaming asteroid and those

  • methane explosions killed almost all of the organisms on the planet?

  • No, of course you don't because that happened

  • 252 million years ago, and mammals weren't a thing yet.

  • But that's kind of the point of this episode.

  • That asteroid was a...a disturbance to the ecology of the planet.

  • The flora, fauna and soils were largely wiped out,

  • leaving a blank canvas for the organisms that survived, and there

  • really weren't all that many of them, to fill in as they could.

  • What happened after the Permian-Triassic "disturbance"

  • is a dramatic example of ecological succession,

  • how the makeup of a community changes over time, starting from,

  • like, the day after a disturbance.

  • Just, usually, the disturbance is a little less disturbing.

  • The study of how ecological communities change doesn't just

  • look at huge-long periods of time, or the effects of some apocalypse.

  • Succession can easily happen over a season in a park, or in just

  • a few days in a patch of land as small as your garden.

  • And, this might come as a surprise, but disturbances that

  • shake up the status quo within a community actually

  • serve to make that community better in the long run.

  • Because much like life, and the entire universe,

  • succession is all about change.

  • And change is how a universe full of nothing

  • but hydrogen came to include a planet full of life.

  • Disturbances happen in ecosystems all the time, every day:

  • a wildfire, a flood, a windstorm.

  • After these unpredictable events, ecologists kept seeing

  • predictable, even orderly changes in the ecosystem.

  • How life deals with these disturbances is

  • an important key to understanding ecosystems.

  • First, let's note that a tree falling in the forest

  • and a comet falling in the forest, while both disturbances,

  • are different levels of disturbance.

  • Likewise there are a couple of different types of succession.

  • The first type, the one that happens after the asteroid hits

  • or the glacier plows over the landscape or

  • the forest fire-slash-volcano burns the verdant ecosystem

  • into pure desolation, that's called a primary succession:

  • when organisms populate an area for the first time.

  • The jumping off point for primary succession is your basic,

  • lifeless, post- apocalyptic wasteland.

  • You're probably thinking, that place sounds terrible!

  • Who would ever want to live there?

  • Well, actually, there is one tremendous advantage

  • of to desolate wastelands...no competition.

  • A lot of organisms don't mind settling down in the more

  • inhospitable nooks and crannies of the planet.

  • These pioneer species are often prokaryotes or protists,

  • followed by nonvascular plants, then maybe some extra

  • super hardy vascular plants.

  • There are tons of organisms that make their living

  • colonizing dead places. It's their thing.

  • Like before the Permian-Triassic extinction,

  • there were these dense forests of gymnosperms,

  • probably full of species a lot like the conifers, gingkos

  • and cycads we still have today.

  • But after the asteroid hit, the big forests died and were replaced

  • by lycophytes, simpler vascular plants like the now-extinct

  • scale trees and today's club mosses.

  • While they might have had a hard time competing

  • with the more complicated plants during the good times.

  • The rest of the Paleozoic flora barely survived extinction,

  • of all the dozens of species of gingko that were around

  • back then, only one still exists,

  • completely genetically isolated, a living fossil.

  • It's important to remember that when we talk about

  • primary ecological succession, we're talking about plants,

  • pretty much exclusively.

  • Because plants rule the world, remember?

  • Without plants, the animals in a community don't stand a chance,

  • and primary successional species are often plants

  • that have windborne seeds, like lycophytes, or mosses

  • and lichens that have spores that blow in and colonize the area.

  • And the outcome of a primary successional landscape is to build,

  • or rebuild, soils, which develop over time as the mosses,

  • grasses and tiny little plants grow, die and decompose.

  • Once the soils are ready, slightly bigger plants can move in,

  • at which point, we move onto secondary succession.

  • And then it's game on: a whole redwood forest

  • could develop out of that.

  • But primary succession takes a long, long time:

  • like hundreds, maybe thousands of years in some places.

  • In fact, the recovery of these big gymnosperm forests after the

  • Permian-Triassic extinction event took about 4 or 5 million years.

  • Dirt may seem unglamorous to you, but it is alive

  • and beautiful and complicated, and making good soil takes time!

  • Now, secondary succession isn't just the next act after primary

  • succession has made a place livable after some disaster.

  • It's usually the first response after a smaller disturbance

  • like a flood or a little fire has knocked back the plants

  • that have been ruling the roost for a while.

  • Even a disturbance as small as a tree crashing down in the woods

  • can make a tiny patch of forest more like it was 50 years ago,

  • before that one tree got so huge and shady:

  • In that tiny area, there will suddenly be a different

  • microclimate than in the rest of the forest, which might

  • have more sunlight, slightly higher temperatures,

  • less protection from weather, etc.

  • And just like every other ecosystem on earth,

  • this tiny patch of forest will be affected by temperature

  • and precipitation the most, which will

  • be different in different parts of the forest.

  • So, as a result of the fallen tree, the soils will

  • become different, the mix of plants will become different,

  • and different animals will want to do business there because that

  • little niche suits their needs better than other little niches.

  • So the question becomes, when does succession stop

  • and things get back to normal?

  • Never. Because change doesn't end.

  • Change is the only constant people...you know who said that?

  • Heraclitus...in 500 BC. So it's been true since at least then.

  • Consider it a lesson in life. And as ideas in ecology go,

  • it's actually a pretty new way of looking at things.

  • See, back in the early 20th century, ecologists noticed

  • the tendency of communities to morph over time.

  • But they also saw succession in terms of a community changing

  • until it ultimately ended in what they called a climax community,

  • which would have a predictable assemblage of species

  • that would remain stable until the next big disturbance.

  • Well, maybe that's what seemed to be happening, but ecological

  • succession is actually a lot more complicated than that.

  • For starters, there's a little thing called stochasticity

  • or randomness which prevents us from ever knowing exactly what a

  • community is going to look like 100 years after a disturbance.

  • Stochasticity is basically your element of

  • unpredictable variability in anything.

  • So, you can predict with some accuracy what plants are going

  • to take over a glacial moraine after the ice has receded, because

  • the seeds of some colonizer species typically make it there first.

  • But unpredictable things like weather conditions during the early

  • stages of succession can end up favoring another species.

  • The point is, scientists' attempts to predict what a

  • community ends up looking like in 100 years should

  • always be thought of as probabilities, not certainties.

  • Another difficulty with the whole model of a climax community

  • has to do with the idea of an ecosystem eventually stabilizing.

  • That word, "stable"?

  • Whenever it's used in a sentence that also includes the word

  • "ecology", you can pretty much be sure it's being used wrong.

  • Because stability never happens.

  • There are always disturbances happening

  • all the time, in every ecosystem.

  • A small portion of the forest might burn, a windstorm might

  • take out a bunch of trees, some yeehaw might rent himself

  • a backhoe one weekend and clear himself a little patch

  • of heaven on the mountain beside his house because

  • he's got nothing better to do.

  • Who knows! Stuff happens.

  • So instead of ending in some fixed, stable climax community,

  • we now know that an ecosystem is in later successional stages

  • if it has high biodiversity. Lots and lots of biodiversity.

  • The only way biodiversity could be high is if there are tons

  • of little niches for all those species to fit into.

  • And the only way there could be that many niches is if,

  • instead of a single community, an ecosystem was actually

  • made up of thousands of tiny communities, a mosaic of habitats

  • where specific communities of different organisms lived.

  • Such mosaics of niches are created by disturbances over time,

  • with everything always changing here and there.

  • But it's important that these disturbances be

  • of the right kind, and the right scale.

  • Because it turns out that the kind of disturbances

  • that have the greatest effect on biodiversity

  • are the most moderate disturbances.

  • When ecologists figured this out, they decided to call it

  • the Intermediate Disturbance Hypothesis.

  • Because, it hypothesizes that intermediate disturbances,

  • not too big and not too little, are ideal.

  • See, just a little disturbance, like a falling tree or something,

  • isn't enough to really change the game.

  • On the other hand, a really severe disturbance, like getting

  • covered with lava, would take the community all the way

  • back to asteroid wipe-out- level primary succession.

  • But every nice mid-level disturbance creates its own habitat

  • at its own stage of succession with its own unique niches.

  • More niches means more biodiversity, and more biodiversity

  • means more stability and healthier ecosystems.

  • Even if two disturbances happen in two different areas

  • with roughly the same climate at the same time,

  • the stochastic nature of ecosystems mean that the two areas

  • might recover in completely different ways,

  • leading to even more niches and more biodiversity!

  • Now, this does not mean that you should go rent

  • a backhoe tomorrow and cut a swath into the wilderness.

  • It just suggesting that medium-level of disturbance is natural and

  • normal and good for an ecosystem. Keeps everybody on their toes.

  • And, like I said, disturbance happens.

  • And by and large we should let it happen.

  • This, too, is a relatively new idea in ecology.

  • In fact, for most of the history of public land management

  • in the U.S., great swaths of forests were not allowed to burn.

  • People considered the "purpose" of forests to be wood production.

  • And you don't want to burn down some trees that

  • are gonna make you a bunch of money.

  • But because of the lack of intermediate disturbances

  • over a long period of time, we ended up with catastrophic fires

  • like the one that torched Yellowstone National Park back in 1988.

  • A single lightning strike totally annihilated almost 800,000 acres

  • of public forest because the ecosystem hadn't been allowed

  • to indulge in a nice leisurely burn every now and then.

  • But now those forests have undergone more than 20 years

  • of succession, and some parts have even re-burned at

  • a more intermediate level, creating a nice,

  • high-biodiversity mosaic of habitats.

  • And it's gorgeous, you should come visit it sometime.

  • And that is ecological succession for you...

  • how destruction and disturbance lead to beauty and diversity.

  • Just remember what my main man Heraclitus said and

  • you'll be good: the only constant is change.

  • Thank you for watching this episode of Crash Course Ecology.

  • And thank you to everyone who helped us put this episode together.

  • If you want to review any of the concepts we studied today,

  • there's a table of contents over there.

  • And if you have any questions, ideas or comments, we're on

  • Facebook, Twitter and of course, down in the comments below.

  • We'll see you next time.

You remember back in the days right after the Permian-Triassic

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Ecological Succession: Change is Good - Crash Course Ecology #6

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    cyrain posted on 2013/09/25
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