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On the island of Madagascar, there’s a kind
of moth that drinks tears from the eyes of
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sleeping birds.
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When I first heard this, I just sat with that
weird idea: there’s a moth that gets most
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of the nutrients it needs to survive by drinking
bird tears!
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Welcome to the biosphere -- the sphere of
life that extends from the depths of the ocean
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all the way up to 8 kilometers above Earth.
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A lot of incredible things live here, so of
course, as geographers, we want to know why
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bananas and bacteria and tear-drinking moths
show up in some spaces but not others.
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And to do that, we have to zoom out a little.
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For example, that moth gets its nutrients
from birds, while birds rely on seeds and
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berries from the surrounding plants, which
grow with the help of the Sun.
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So the moth and the birds and the plants and
the Sun are all part of an ecosystem -- a
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community of living organisms in an area interacting
with their environment.
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Ecosystems are built on relationships -- even
strange ones that involve tear-theft.
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And the relationship between the amount of
energy a place receives and the movement of
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nutrients is what makes the incredible diversity
of life possible.
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I’m Alizé Carrère and this is Crash Course
Geography.
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INTRO
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The biosphere is a complex web of interconnected
ecosystems.
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And all ecosystems depend on two key things:
the one-way movement of energy and the cyclic
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movement of nutrients.
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Energy flows are the paths energy can take
through an ecosystem.
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Energy generally enters ecosystems from the
Sun but doesn’t return to the Sun -- so
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energy flows are one-way relationships.
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Plants absorb the Sun's energy during photosynthesis,
adding carbon dioxide and water to make carbohydrates
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and grow bigger.
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So the Sun's energy is converted into chemical
energy, which is stored in biomass -- any
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plant or other living thing.
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If a bit of biomass is eaten, it passes on
its chemical energy to continue the energy flow.
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The rate photosynthesis makes energy across
an entire ecosystem, minus the rate that energy
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is used is its net primary production -- or
the amount of stored chemical energy in an
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ecosystem over a certain amount of time.
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For example, on a really small scale, think
of a fish tank ecosystem that you can hold
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in your hands.
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There's water, a fish, soil, rocks, air, light,
food, and one little plant all in a glass bowl.
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In this fish tank ecosystem, the net primary
production is pretty low because only that
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one little plant is absorbing energy from
the Sun (along with any photosynthetic bacteria
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or algae that grows when I forget to clean
the bowl).
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Globally, net primary production on land generally
changes with latitude.
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Productivity is highest between the tropics
and decreases towards higher latitudes and elevations.
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Biogeographers and ecologists who study how
life is distributed on Earth probably figured
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that calling regions of the world "very productive
ecosystem" or "extremely not productive ecosystem"
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would be boring.
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Instead, we classify ecosystems into biomes,
or habitats with similar characteristics,
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including productivity!
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The names are much more descriptive and fun.
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The equator gets the most direct sunlight
and a lot of precipitation, so there’s a
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lot of photosynthesis happening here.
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These highly productive ecosystems are all
tropical rainforest biomes, which are some
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of the most diverse and complex areas of the
planet -- so it's no wonder the tear-drinking
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moth lives here.
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Similar patterns happen on either side of
the equator, but we’re going to turn north
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because there’s more land in the northern
hemisphere.
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There’s also less and less precipitation
as we move out from the equator, and less
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and less productivity because photosynthesis
can't happen without water.
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The biomes gradually shift from tropical rainforests
to tropical savanna to desert.
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Further north, in temperate and high latitudes,
the net primary production varies seasonally.
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Like one biome is the broadleaf deciduous
forests with oak, beech, hickory, maple, elm
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and chestnut trees.
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These trees have increased productivity in
the sunny spring and summer, and shed their
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leaves in the cooler fall and winter seasons.
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Up here in the middle of continents, there
are temperate grassland biomes with rich soils
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that produce the tall grass of prairies and
the shortgrass of steppe climates.
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Further north where there are poorer soils
and colder climates, we meet the boreal forest
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biomes, which have mainly evergreen pine,
spruce, fir and larch trees.
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At even higher latitudes, the decreasing temperatures
give us the icy tundra biome with no trees
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and very little productivity.
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So the amount of energy flow through different
ecosystems varies wildly, which limits which
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type of plants can thrive there.
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And because plants feed more consumers than
any other food source, more plants means more
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biodiversity, or the number of different plants
and animals in an ecosystem.
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And we can't talk about biodiversity without
the other key component of all ecosystems: nutrients.
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Nutrients are chemical elements like carbon,
oxygen, nitrogen, sulfur, and phosphorus -- stored
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in both the living and nonliving parts of
an ecosystem.
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And we actually have technical terms for those
too.
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The living things like plants and animals
and bacteria (or their dead bodies) are the
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biotic parts of an ecosystem.
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And the nonliving things like the soil, atmosphere,
and groundwater are the abiotic parts.
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Unlike how energy flows in one direction,
the paths that nutrients take through the
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ecosystem are nutrient cycles between the
biotic and abiotic parts.
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And unlike energy from the Sun, all the nutrients
we have right now on Earth are all we’ll ever have.
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It's like how nitrogen moves from being a
gas in the atmosphere to a solid in the soil.
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[Instead of a one-way system like...aliens
dropping gift-wrapped boxes of nitrogen from
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space… or at least not that we know of].
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The biotic parts of ecosystems really help
facilitate these nutrient cycles.
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Like, let's look at our fish tank ecosystem
again!
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Producers like our little plant capture nutrients
from the abiotic parts, turning carbon dioxide
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into carbohydrates through photosynthesis
or absorbing nitrogen compounds through its roots.
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Consumers like the fish take nutrients from
other organisms, munching on fish food or
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the plant's leaves.
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And decomposers break down dead plant leaves…
or our fish eventually... and return the nutrients,
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like nitrogen gas, to the abiotic parts of
the tank.
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Ultimately, nutrients cycling through ecosystems
depend on biological, geological, and chemical
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processes operating within the atmosphere,
hydrosphere and lithosphere, and make up Earth’s
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biogeochemical cycles.
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We can compare nutrients across the Earth's
biosphere just like we compared net primary
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production across different latitudes and
biomes.
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Like let's look at three biomes we met before:
the tropical rainforest, deciduous forest,
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and boreal forests.
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We know that there's less and less productivity
as we move up in latitude, so there's less
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and less biomass, and there's also less and
less nutrients.
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Fewer nutrients isn't necessarily a death
sentence for the trees, though.
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It just means that the ecosystem is structured
differently.
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Like boreal forests have a lot of nutrient
filled litter because the cold keeps material
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from decomposing.
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But deciduous forests have a lot of nutrient-rich
soil because it’s warm enough for material
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to decompose, but not warm enough for a lot
of biomass to grow.
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So a tree that’s adapted to life in a cold
boreal forest might not make it in a tropical
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rainforest because of the different energy
availability and nutrient stores.
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Let’s consider the tropical rainforests,
which are the most diverse biomes with lush
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vegetation and a lot of biodiversity.
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But that decadence hides the fragile balance
of all the complex energy flows and nutrient cycles.
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Let's go to the Thought Bubble!
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Within the tropical rainforests, broadleaf
evergreen trees form a canopy at different
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heights, and little or no sunlight reaches
the shady forest floor.
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These huge trees absorb most of the soil nutrients,
which doesn't leave a lot for other organisms.
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And they have a shallow root system to grab
as many of the minerals as possible from biogeochemical
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processes near the surface.
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And as the large amounts of rain filter down
through the soil, the minerals that dissolve
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in water are leached away to inaccessible
deeper levels.
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To survive, the rainforest has to rapidly
cycle nutrients.
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The canopy trees are producers, along with
understory plants that work together to keep
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vital nutrients moving through the ecosystem.
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Herbivores like gorillas and caterpillars
take in those nutrients and move them around
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through their excrement and by being eaten
themselves, like by jaguars or geckos.
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And the warmth and humidity helps decomposers
and their chemical reactions, so any dead
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plants or animals decay quickly.
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Because nutrients get sucked from the soils
so quickly, when those huge trees are cut
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down, the energy flows and nutrient cycles
break.
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Those big producers aren't there to sustain
consumers or shed leaves to recycle nutrients.
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So deforestation, or removing trees to use
the land for something else, can be especially
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destructive in tropical regions if you don't
consider the biogeochemical cycles.
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Thanks, Thought Bubble.
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We have negative associations with the word
"deforestation" for good reason -- a lot of
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tree removal has caused immense damage to
ecosystems.
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But indigenous communities have figured out
a type of calculated clearing that allows
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them to work with the rapid nutrient recycling
of tropical rainforest biomes.
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In parts of Asia, Africa, and South America
with dense tropical forests, many farmers
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have to rely on a kind of subsistence agricultural
practice, which means they only grow enough
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food for their families.
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Staples like rice are grown in southeast Asia,
maize and cassava in South America, and sorghum in Africa.
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Yams, sugarcane, plantains, and vegetables
are also planted to supplement staples and
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to provide fuel and fodder for animals.
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This practice goes by many names, like swidden,
shifting cultivation, and slash-and-burn agriculture.
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The farmers begin by cutting small areas of
tropical forests into slash, or cut vegetation,
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that’s then dried and burned.
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The ash gets mixed with the poor soil to provide
needed minerals and nutrients -- basically
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using all the good stuff stored up in the
vegetation biomass to help new crop plants grow.
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Of course, these crop plants use minerals
and nutrients from the soil as they grow,
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and we eat them to get those minerals and
nutrients in our bodies.
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So after a few years, and before the soil
is completely exhausted, the farmers move
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on to another part of land and repeat the
clearing, burning, and planting process.
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The previous plot is left unplanted, and eventually
the forest will naturally expand to start
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using that soil as part of its carefully balanced
nutrient cycling.
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This land rotation is a key part of why humans
have been able to keep farming like this for
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thousands of years.
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But when widespread clear-cutting happens,
ecosystems can collapse.
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For example, we've seen this destruction in
the Amazon when rice, soy, and corn have been
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commercially cultivated and sold in domestic
and international markets.
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The soil is exhausted after 3-5 years, so
crops can't really grow anymore, and then
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large cattle operations move in.
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As cattle feed and trample the ground, the
soils are exposed to plenty of UV radiation
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from sunlight, as well as cycles of wetting
and drying from precipitation.
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The soils become a brick-like substance called
laterite, which isn’t great for growing…
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so the once-lively rainforest basically becomes
hard, barren, and lifeless.
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Tropical rainforests cover 6 percent of Earth’s
landmass, but contain 50 percent of the world’s
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species, many of which haven’t been described
by science and are critical to the world’s biodiversity.
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But since the 1980s, 1/5 of the Amazon has
been deforested as we build more towns, roads,
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dams, farms, and mines.
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Across the Atlantic, deforestation in Ituri
in the Congo Basin, the world’s second largest
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expanse of equatorial rainforest, is endangering
the mountain gorilla.
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And in southeast Asia clearcutting for palm
oil plantations endanger orangutans, Sumatran
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tigers, and Sumatran elephants.
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When humans disturb the biosphere, we alter
how energy flows and how nutrients cycle,
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which can throw off entire ecosystems in unexpected
ways.
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And if we're not careful, that moth that drinks
the tears of sleeping birds -- or plenty of
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other remarkable living organisms -- may no
longer exist.
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But just like we can be good stewards of our
little fish tank, we can take care of the
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world around us and the much bigger ecosystems
we're a part of.
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Learning where our food comes from and how
it’s produced is a good first step, like
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by supporting farmers who try to grow food
sustainably.
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And we’ll keep talking about our role in
ecosystems, energy, and food and how geography
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and spatial thinking can help address some
of the critical issues we’re facing, like
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how we can have enough food and water to sustain
ourselves and our environment.
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Many maps and borders represent modern geopolitical
divisions that have often been decided without
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the consultation, permission, or recognition
of the land's original inhabitants.
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Many geographical place names also don't reflect
the Indigenous or Aboriginal peoples languages.
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So we at Crash Course want to acknowledge
these peoples’ traditional and ongoing relationship
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with that land and all the physical and human
geographical elements of it.
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We encourage you to learn about the history
of the place you call home through resources
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like native-land.ca and by engaging with your
local Indigenous and Aboriginal nations through
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the websites and resources they provide.
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Thanks for watching this episode of Crash
Course Geography which is filmed at the Team
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Sandoval Pierce Studio and was made with the
help of all these nice people.
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If you want to help keep all Crash Course
free for everyone, forever, you can join our
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community on Patreon.