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  • At this point, people have been studying the impacts that

  • humans have had on the world around us for a solid 50 years.

  • And while it's hard to get a handle on exactly how

  • the choices we make every day affect the environment,

  • there's no question that our lifestyles,

  • our cars, our need for more farmland,

  • and our love of all kinds of plastic stuff,

  • are putting the hurt on ecosystems all over the world.

  • Human activity all by itself, just people doing what they do,

  • could be responsible for the extinction of nearly a thousand

  • plant and animal species to date, most of them over the last century.

  • And even if you don't particularly care about the Barbary lion

  • or the Saint Helena olive, or the passenger pigeon,

  • or anything else we've driven into extinction, the thing is,

  • we need these other organisms.

  • The ecosystems of the world are working very hard for us, every day:

  • filtering water, sucking carbon dioxide out of the air,

  • producing all the food we eat,

  • all very important ecosystem services,

  • benefits that the natural world provides us for free.

  • So, having ecosystems and keeping them intact is important,

  • not only for the organisms who live in them, but also for us,

  • the animals who rely on them for thousands and thousands

  • of things that we could never do for ourselves.

  • Over the next two episodes,

  • we're going to look at these systems and how our actions are

  • affecting the ecosystems that we need for our survival.

  • Basically, we're messing up the environment 6 ways from Sunday.

  • But to make it easy on ourselves, let's start with the top 5.

  • We often hear about all the different ways

  • that our behavior is affecting the biosphere.

  • Extinctions, climate change, deforestation, acid rain,

  • desertification, pollution and more.

  • But you're asking, "Well, why are all those things bad?

  • What's going on?

  • How is this stuff turning the earth into sausage?

  • I don't understand."

  • Well I do understand, which is why I'm qualified to make this video.

  • So, let me lay it on you.

  • The services that ecosystems provide for us, all the dirty work

  • they do, can be broken up into four different categories.

  • They're things that we could never, ever, ever duplicate or work around,

  • no matter what kind of smarty-pants technology we come up with:

  • First, healthy ecosystems provide support services that create

  • and replenish the foundation of the earth's biological systems.

  • These services include recycling all of the compounds that are

  • necessary for life, through the carbon, water,

  • nitrogen and phosphorous cycles.

  • They also include other processes we've talked about before,

  • like forming new soils and producing atmospheric oxygen.

  • Some ecosystems contribute more to these services than others, but

  • none of them can get these basic jobs done unless they are intact.

  • Two: Ecosystems also perform provisioning services,

  • giving us the raw materials we need to live.

  • Like, the ocean provides food in the form of fish sticks and stuff.

  • And rivers, aquifers and other freshwater sources give us water.

  • Plants and animals also yield all kinds of fiber

  • that we use for clothing and shelter.

  • And all around us we find sources of fuel, whether it's biomass

  • in the form of grasses or wood, hydropower in the form of flowing

  • water, or the carbon locked in millions-of-years-old trees

  • that we're now re-releasing into the atmosphere.

  • But I'm getting ahead of myself.

  • Ecosystems also perform super-important regulating services,

  • moderating many of the earth's systems that

  • can get dangerous if they get out of whack.

  • Like as we learned in Biology: fungi and other organisms take on

  • the task of decomposing dead things and poop.

  • Meanwhile, plants help filter the water you drink

  • and the air you breathe, and provide flood control.

  • And they also absorb all that carbon you exhale and that your

  • car belches out, which in turn, helps regulate the climate.

  • And finally, number four, ecosystems are just kind of awesome.

  • It's nice to be surrounded by happy plants and critters,

  • doing their business.

  • Nice, robust ecosystems give us places to play,

  • scenes to inspire us, and things to just discover and learn about.

  • These are their less tangible, but still important cultural services.

  • An interesting thing about ecosystem services is that

  • economists actually can, and do, calculate the monetary value

  • they provide for humanity.

  • If, for example, we had to do all of the things that ecosystems

  • do for us, it would cost us 46 trillion dollars per year.

  • Which is a lot, considering that the output of the global economy

  • is 66 trillion dollars per year.

  • So, yeah, we should be happy that we don't have to pay for all that.

  • But you'll notice that I keep saying that ecosystems

  • can only serve up all this awesome sauce if they are "intact."

  • By that I mean they specifically have to have

  • their biodiversity intact, because ecosystems are just

  • a bunch of living and nonliving things working together,

  • so unless their living parts are healthy,

  • they're basically just rocks and weather.

  • The main reason biodiversity is so important is that

  • it makes ecosystems more resilient to that never-ending change

  • we talked about a few weeks ago.

  • Ecosystems with high biodiversity are way more resilient

  • to disturbances than those with low biodiversity.

  • In a high-biodiversity system, if you take one species out

  • of the mix, it's less likely that the ecosystem will collapse.

  • Take a hectare of Amazonian rainforest: in that little patch

  • of land, there are more different species of plants and animals

  • than there are in all of Europe.

  • So if a species of insect goes extinct, there's less risk

  • that the whole house of cards will fall than, say,

  • in the Sonoran Desert, where there are very few organisms, so the

  • disappearance of one species could affect the entire ecosystem.

  • So the best way to understand our impacts on the environment

  • is through how we affect biodiversity.

  • Unfortunately, it turns out that we've been doing a really

  • bang-up job of endangering some of the highest biodiversity

  • ecosystems on the planet.

  • In some cases, we're having impacts on the organisms

  • themselves directly, in other cases, we're affecting biodiversity

  • indirectly, by creating one or two changes in that ecosystem

  • that cascade into all kinds of problems for living things.

  • First, let's look at that hectare of Amazonian rainforest again,

  • because even though it's one of those super-resilient ecosystems,

  • we're having a serious impact on it. How?

  • Well, first by removing a lot of what makes a forest a forest: trees.

  • According to some estimates, we're clear-cutting around

  • 8 thousand hectares of trees a day to provide land to graze cattle on,

  • and to harvest wood to make coffee tables or whatever.

  • When you cut down a hectare of rainforest, suddenly a place

  • where a few thousand species used to live turns into a place

  • where just a handful of species live: some grass, some weeds,

  • maybe some rats or mice, some insects and, you know, some cows.

  • Because man, we love cows.

  • And when you take out so many of the living things

  • on that hectare of land, a bunch of things happen.

  • For starters, you're not just affecting that ecosystem,

  • but neighboring ecosystems as well.

  • For instance, all those trees that were cut down provided

  • the service of regulating the flow of all that rain

  • that rainforests get, not only by absorbing some of it

  • but also by slowing down runoff, letting the water seep

  • into the soil, before slowly making its way into streams

  • and rivers and ultimately the ocean.

  • But when those trees are gone, the water hits the land

  • and shoots off into the nearest stream, causing erosion

  • and washing minerals and chemicals all the way to the sea,

  • where it affects marine ecosystems.

  • And when I say "affect," I don't mean in a good way.

  • This, my friends, is what's called a cascade effect,

  • in this case caused by deforestation,

  • one of the most obvious, observable human impacts.

  • In addition to causing more flooding and changes in water quality,

  • deforestation on a large scale can lead to another impact:

  • desertification, or the spread of dry, unproductive landscapes.

  • But cutting down trees doesn't automatically turn a forest

  • into a desert, desertification is driven along by

  • additional factors, like overgrazing by cattle, and over-irrigation.

  • So how can over-watering something make it turn into a desert?

  • Well, when we use groundwater to irrigate crops,

  • the natural salts in the groundwater build up in the soil,

  • eventually making it so salty that nothing wants to live there.

  • Over time, fertile land near desert ecosystems becomes overtaxed,

  • and the desert spreads.

  • And this is exactly what has happened in China over the past century,

  • where overgrazing and the cities' unquenchable thirst for water have

  • caused the Gobi Desert to grow by 3,600 square kilometers every year.

  • Now, these two impacts by themselves clearly limit

  • the biodiversity of otherwise lush ecosystems.

  • But because they also result in fewer trees that provide

  • the all-important services of releasing oxygen and absorbing CO2,

  • you know what domino's gonna fall next: the climate.

  • Carbon dioxide: the principle greenhouse gas.

  • It insulates the Earth.

  • So it stands to reason that the more CO2 there is in the atmosphere,

  • the warmer the earth will be.

  • And the thing is, we're reducing the size of forests

  • at the same time as we're unleashing all kinds of

  • greenhouse gases by burning fossil fuels.

  • This double-whammy is much of what's driving global warming.

  • As a result, we're seeing decreases in the levels of polar sea ice,

  • which means less habit for polar bears, seals and sea birds.

  • More temperate animals are moving closer to the poles, and hotter,

  • drier conditions are causing more grass fires and forest fires.

  • And while the climate has changed many times in the past,

  • those changes usually took place over centuries or even millennia,

  • giving organisms time to adapt or move,

  • these changes are taking place within our lifetimes.

  • And it's kind of a huge deal. And it's complicated.

  • It'd take me at least, like, 10 minutes and 52 seconds

  • to explain it all in detail.

  • Which is why I did that in another video.

  • By now hopefully you can see how one human impact

  • can lead to another, and how, even indirectly,

  • they can end up reducing biodiversity.

  • But it's hard to overlook the more immediate impacts

  • we can have on ecosystems.

  • One of the more in-your-face ways we affect biodiversity

  • is by introducing nonnative species,

  • either intentionally or unintentionally.

  • Again, there are so many examples of this

  • that you can learn more about it in another video I did.

  • But suffice it to say: whether it's kudzu in North America,

  • or cane toads in Australia, invasive species have a knack

  • for out-competing or outright eating native species

  • to the point that it rocks the world of an entire ecosystem.

  • And finally, probably the most direct impact we have

  • on biodiversity is simply over-harvesting certain organisms.

  • We're overfishing the oceans to meet growing demand

  • for popular fish species, like tuna,

  • while on land we're exterminating important predators,

  • like wolves, to protect livestock...those cows again.

  • And the less diverse those ecosystems are,

  • the more vulnerable they become to disturbances,

  • including those other 4 impacts I just mentioned.

  • And the fact is, there's a bunch more where those came from.

  • Because there's a whole separate set of effects that humanity

  • has on the biosphere that stems simply from us putting the wrong

  • amounts of certain stuff in the wrong place at the wrong time.

  • That's what we call pollution, so tune in next time

  • when we'll explore what it really is, where exactly

  • it's coming from, and what we can do about it.

  • Thank you for watching another kind of depressing

  • episode of Crash Course Ecology.

  • And thanks to everyone who helped us put it together.

  • There's a table of contents over there

  • if you want to click to review anything.

  • Or the links are down below in the description.

  • And if you have any questions or comments or ideas for us,

  • please leave them on Facebook or Twitter or of course,

  • down in the comments below.

At this point, people have been studying the impacts that

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5 人間が環境に与える影響クラッシュコース エコロジー#10 (5 Human Impacts on the Environment: Crash Course Ecology #10)

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    Chi-feng Liu に公開 2021 年 01 月 14 日
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