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You remember back in the days right after the Permian-Triassic
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extinction event, when that giant flaming asteroid and those
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methane explosions killed almost all of the organisms on the planet?
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No, of course you don't because that happened
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252 million years ago, and mammals weren't a thing yet.
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But that's kind of the point of this episode.
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That asteroid was a...a disturbance to the ecology of the planet.
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The flora, fauna and soils were largely wiped out,
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leaving a blank canvas for the organisms that survived, and there
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really weren't all that many of them, to fill in as they could.
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What happened after the Permian-Triassic "disturbance"
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is a dramatic example of ecological succession,
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how the makeup of a community changes over time, starting from,
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like, the day after a disturbance.
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Just, usually, the disturbance is a little less disturbing.
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The study of how ecological communities change doesn't just
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look at huge-long periods of time, or the effects of some apocalypse.
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Succession can easily happen over a season in a park, or in just
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a few days in a patch of land as small as your garden.
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And, this might come as a surprise, but disturbances that
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shake up the status quo within a community actually
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serve to make that community better in the long run.
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Because much like life, and the entire universe,
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succession is all about change.
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And change is how a universe full of nothing
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but hydrogen came to include a planet full of life.
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Disturbances happen in ecosystems all the time, every day:
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a wildfire, a flood, a windstorm.
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After these unpredictable events, ecologists kept seeing
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predictable, even orderly changes in the ecosystem.
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How life deals with these disturbances is
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an important key to understanding ecosystems.
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First, let's note that a tree falling in the forest
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and a comet falling in the forest, while both disturbances,
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are different levels of disturbance.
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Likewise there are a couple of different types of succession.
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The first type, the one that happens after the asteroid hits
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or the glacier plows over the landscape or
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the forest fire-slash-volcano burns the verdant ecosystem
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into pure desolation, that's called a primary succession:
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when organisms populate an area for the first time.
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The jumping off point for primary succession is your basic,
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lifeless, post- apocalyptic wasteland.
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You're probably thinking, that place sounds terrible!
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Who would ever want to live there?
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Well, actually, there is one tremendous advantage
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of to desolate wastelands...no competition.
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A lot of organisms don't mind settling down in the more
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inhospitable nooks and crannies of the planet.
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These pioneer species are often prokaryotes or protists,
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followed by nonvascular plants, then maybe some extra
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super hardy vascular plants.
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There are tons of organisms that make their living
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colonizing dead places. It's their thing.
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Like before the Permian-Triassic extinction,
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there were these dense forests of gymnosperms,
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probably full of species a lot like the conifers, gingkos
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and cycads we still have today.
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But after the asteroid hit, the big forests died and were replaced
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by lycophytes, simpler vascular plants like the now-extinct
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scale trees and today's club mosses.
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While they might have had a hard time competing
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with the more complicated plants during the good times.
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The rest of the Paleozoic flora barely survived extinction,
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of all the dozens of species of gingko that were around
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back then, only one still exists,
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completely genetically isolated, a living fossil.
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It's important to remember that when we talk about
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primary ecological succession, we're talking about plants,
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pretty much exclusively.
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Because plants rule the world, remember?
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Without plants, the animals in a community don't stand a chance,
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and primary successional species are often plants
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that have windborne seeds, like lycophytes, or mosses
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and lichens that have spores that blow in and colonize the area.
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And the outcome of a primary successional landscape is to build,
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or rebuild, soils, which develop over time as the mosses,
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grasses and tiny little plants grow, die and decompose.
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Once the soils are ready, slightly bigger plants can move in,
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at which point, we move onto secondary succession.
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And then it's game on: a whole redwood forest
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could develop out of that.
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But primary succession takes a long, long time:
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like hundreds, maybe thousands of years in some places.
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In fact, the recovery of these big gymnosperm forests after the
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Permian-Triassic extinction event took about 4 or 5 million years.
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Dirt may seem unglamorous to you, but it is alive
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and beautiful and complicated, and making good soil takes time!
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Now, secondary succession isn't just the next act after primary
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succession has made a place livable after some disaster.
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It's usually the first response after a smaller disturbance
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like a flood or a little fire has knocked back the plants
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that have been ruling the roost for a while.
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Even a disturbance as small as a tree crashing down in the woods
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can make a tiny patch of forest more like it was 50 years ago,
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before that one tree got so huge and shady:
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In that tiny area, there will suddenly be a different
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microclimate than in the rest of the forest, which might
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have more sunlight, slightly higher temperatures,
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less protection from weather, etc.
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And just like every other ecosystem on earth,
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this tiny patch of forest will be affected by temperature
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and precipitation the most, which will
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be different in different parts of the forest.
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So, as a result of the fallen tree, the soils will
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become different, the mix of plants will become different,
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and different animals will want to do business there because that
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little niche suits their needs better than other little niches.
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So the question becomes, when does succession stop
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and things get back to normal?
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Never. Because change doesn't end.
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Change is the only constant people...you know who said that?
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Heraclitus...in 500 BC. So it's been true since at least then.
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Consider it a lesson in life. And as ideas in ecology go,
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it's actually a pretty new way of looking at things.
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See, back in the early 20th century, ecologists noticed
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the tendency of communities to morph over time.
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But they also saw succession in terms of a community changing
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until it ultimately ended in what they called a climax community,
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which would have a predictable assemblage of species
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that would remain stable until the next big disturbance.
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Well, maybe that's what seemed to be happening, but ecological
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succession is actually a lot more complicated than that.
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For starters, there's a little thing called stochasticity
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or randomness which prevents us from ever knowing exactly what a
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community is going to look like 100 years after a disturbance.
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Stochasticity is basically your element of
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unpredictable variability in anything.
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So, you can predict with some accuracy what plants are going
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to take over a glacial moraine after the ice has receded, because
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the seeds of some colonizer species typically make it there first.
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But unpredictable things like weather conditions during the early
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stages of succession can end up favoring another species.
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The point is, scientists' attempts to predict what a
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community ends up looking like in 100 years should
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always be thought of as probabilities, not certainties.
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Another difficulty with the whole model of a climax community
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has to do with the idea of an ecosystem eventually stabilizing.
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That word, "stable"?
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Whenever it's used in a sentence that also includes the word
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"ecology", you can pretty much be sure it's being used wrong.
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Because stability never happens.
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There are always disturbances happening
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all the time, in every ecosystem.
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A small portion of the forest might burn, a windstorm might
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take out a bunch of trees, some yeehaw might rent himself
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a backhoe one weekend and clear himself a little patch
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of heaven on the mountain beside his house because
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he's got nothing better to do.
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Who knows! Stuff happens.
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So instead of ending in some fixed, stable climax community,
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we now know that an ecosystem is in later successional stages
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if it has high biodiversity. Lots and lots of biodiversity.
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The only way biodiversity could be high is if there are tons
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of little niches for all those species to fit into.
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And the only way there could be that many niches is if,
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instead of a single community, an ecosystem was actually
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made up of thousands of tiny communities, a mosaic of habitats
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where specific communities of different organisms lived.
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Such mosaics of niches are created by disturbances over time,
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with everything always changing here and there.
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But it's important that these disturbances be
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of the right kind, and the right scale.
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Because it turns out that the kind of disturbances
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that have the greatest effect on biodiversity
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are the most moderate disturbances.
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When ecologists figured this out, they decided to call it
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the Intermediate Disturbance Hypothesis.
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Because, it hypothesizes that intermediate disturbances,
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not too big and not too little, are ideal.
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See, just a little disturbance, like a falling tree or something,
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isn't enough to really change the game.
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On the other hand, a really severe disturbance, like getting
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covered with lava, would take the community all the way
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back to asteroid wipe-out- level primary succession.
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But every nice mid-level disturbance creates its own habitat
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at its own stage of succession with its own unique niches.
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More niches means more biodiversity, and more biodiversity
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means more stability and healthier ecosystems.
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Even if two disturbances happen in two different areas
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with roughly the same climate at the same time,
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the stochastic nature of ecosystems mean that the two areas
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might recover in completely different ways,
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leading to even more niches and more biodiversity!
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Now, this does not mean that you should go rent
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a backhoe tomorrow and cut a swath into the wilderness.
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It just suggesting that medium-level of disturbance is natural and
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normal and good for an ecosystem. Keeps everybody on their toes.
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And, like I said, disturbance happens.
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And by and large we should let it happen.
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This, too, is a relatively new idea in ecology.
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In fact, for most of the history of public land management
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in the U.S., great swaths of forests were not allowed to burn.
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People considered the "purpose" of forests to be wood production.
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And you don't want to burn down some trees that
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are gonna make you a bunch of money.
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But because of the lack of intermediate disturbances
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over a long period of time, we ended up with catastrophic fires
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like the one that torched Yellowstone National Park back in 1988.
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A single lightning strike totally annihilated almost 800,000 acres
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of public forest because the ecosystem hadn't been allowed
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to indulge in a nice leisurely burn every now and then.
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But now those forests have undergone more than 20 years
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of succession, and some parts have even re-burned at
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a more intermediate level, creating a nice,
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high-biodiversity mosaic of habitats.
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And it's gorgeous, you should come visit it sometime.
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And that is ecological succession for you...
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how destruction and disturbance lead to beauty and diversity.
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Just remember what my main man Heraclitus said and
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you'll be good: the only constant is change.
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Thank you for watching this episode of Crash Course Ecology.
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And thank you to everyone who helped us put this episode together.
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If you want to review any of the concepts we studied today,
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there's a table of contents over there.
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And if you have any questions, ideas or comments, we're on
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Facebook, Twitter and of course, down in the comments below.
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We'll see you next time.