Name: What Are Ecosystems? Crash Course Geography #15（What Are Ecosystems? Crash Course Geography #15）
Uploaded: 2022-06-25T04:28:59.000Z
Duration: 11 min 41 s
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On the island of Madagascar, there's a kind of moth that drinks tears from the eyes of

關係子句

When I first heard this, I just sat with that weird idea: there's a moth that gets most

of the nutrients it needs to survive by drinking bird tears!

Welcome to the biosphere -- the sphere of life that extends from the depths of the ocean

all the way up to 8 kilometers above Earth.

A lot of incredible things live here, so of course, as geographers, we want to know why

bananas and bacteria and tear-drinking moths show up in some spaces but not others.

And to do that, we have to zoom out a little.

For example, that moth gets its nutrients from birds, while birds rely on seeds and

berries from the surrounding plants, which grow with the help of the Sun.

So the moth and the birds and the plants and the Sun are all part of an ecosystem -- a

community of living organisms in an area interacting with their environment.

Ecosystems are built on relationships -- even strange ones that involve tear-theft.

And the relationship between the amount of energy a place receives and the movement of

nutrients is what makes the incredible diversity of life possible.

I'm Alizé Carrère and this is Crash Course Geography.

The biosphere is a complex web of interconnected ecosystems.

And all ecosystems depend on two key things: the one-way movement of energy and the cyclic

Energy flows are the paths energy can take through an ecosystem.

Energy generally enters ecosystems from the Sun but doesn't return to the Sun -- so

Plants absorb the Sun's energy during photosynthesis, adding carbon dioxide and water to make carbohydrates

So the Sun's energy is converted into chemical energy, which is stored in biomass -- any

If a bit of biomass is eaten, it passes on its chemical energy to continue the energy flow.

The rate photosynthesis makes energy across an entire ecosystem, minus the rate that energy

is used is its net primary production -- or the amount of stored chemical energy in an

For example, on a really small scale, think of a fish tank ecosystem that you can hold

There's water, a fish, soil, rocks, air, light, food, and one little plant all in a glass bowl.

In this fish tank ecosystem, the net primary production is pretty low because only that

one little plant is absorbing energy from the Sun (along with any photosynthetic bacteria

or algae that grows when I forget to clean the bowl).

Globally, net primary production on land generally changes with latitude.

Productivity is highest between the tropics and decreases towards higher latitudes and elevations.

Biogeographers and ecologists who study how life is distributed on Earth probably figured

that calling regions of the world "very productive ecosystem" or "extremely not productive ecosystem"

Instead, we classify ecosystems into biomes, or habitats with similar characteristics,

The names are much more descriptive and fun.

The equator gets the most direct sunlight and a lot of precipitation, so there's a

These highly productive ecosystems are all tropical rainforest biomes, which are some

of the most diverse and complex areas of the planet -- so it's no wonder the tear-drinking

Similar patterns happen on either side of the equator, but we're going to turn north

because there's more land in the northern hemisphere.

There's also less and less precipitation as we move out from the equator, and less

and less productivity because photosynthesis can't happen without water.

The biomes gradually shift from tropical rainforests to tropical savanna to desert.

Further north, in temperate and high latitudes, the net primary production varies seasonally.

Like one biome is the broadleaf deciduous forests with oak, beech, hickory, maple, elm

These trees have increased productivity in the sunny spring and summer, and shed their

leaves in the cooler fall and winter seasons.

Up here in the middle of continents, there are temperate grassland biomes with rich soils

that produce the tall grass of prairies and the shortgrass of steppe climates.

Further north where there are poorer soils and colder climates, we meet the boreal forest

biomes, which have mainly evergreen pine, spruce, fir and larch trees.

At even higher latitudes, the decreasing temperatures give us the icy tundra biome with no trees

So the amount of energy flow through different ecosystems varies wildly, which limits which

And because plants feed more consumers than any other food source, more plants means more

biodiversity, or the number of different plants and animals in an ecosystem.

And we can't talk about biodiversity without the other key component of all ecosystems: nutrients.

Nutrients are chemical elements like carbon, oxygen, nitrogen, sulfur, and phosphorus -- stored

in both the living and nonliving parts of an ecosystem.

And we actually have technical terms for those too.

The living things like plants and animals and bacteria (or their dead bodies) are the

And the nonliving things like the soil, atmosphere, and groundwater are the abiotic parts.

Unlike how energy flows in one direction, the paths that nutrients take through the

ecosystem are nutrient cycles between the biotic and abiotic parts.

And unlike energy from the Sun, all the nutrients we have right now on Earth are all we'll ever have.

It's like how nitrogen moves from being a gas in the atmosphere to a solid in the soil.

[Instead of a one-way system like...aliens dropping gift-wrapped boxes of nitrogen from

space… or at least not that we know of].

The biotic parts of ecosystems really help facilitate these nutrient cycles.

Like, let's look at our fish tank ecosystem again!

Producers like our little plant capture nutrients from the abiotic parts, turning carbon dioxide

into carbohydrates through photosynthesis or absorbing nitrogen compounds through its roots.

Consumers like the fish take nutrients from other organisms, munching on fish food or

And decomposers break down dead plant leaves… or our fish eventually... and return the nutrients,

like nitrogen gas, to the abiotic parts of the tank.

Ultimately, nutrients cycling through ecosystems depend on biological, geological, and chemical

processes operating within the atmosphere, hydrosphere and lithosphere, and make up Earth's

We can compare nutrients across the Earth's biosphere just like we compared net primary

production across different latitudes and biomes.

Like let's look at three biomes we met before: the tropical rainforest, deciduous forest,

We know that there's less and less productivity as we move up in latitude, so there's less

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What Are Ecosystems? Crash Course Geography #15（What Are Ecosystems? Crash Course Geography #15）

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