What Are Ecosystems? Crash Course Geography #15

On the island of Madagascar, there’s a kind
of moth that drinks tears from the eyes of

sleeping birds.

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.

INTRO

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

movement of nutrients.

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

energy flows are one-way relationships.

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

and grow bigger.

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

plant or other living thing.

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

ecosystem over a certain amount of time.

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

in your hands.

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"

would be boring.

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

including productivity!

The names are much more descriptive and fun.

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

lot of photosynthesis happening here.

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

moth lives here.

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

and chestnut trees.

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

and very little productivity.

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

type of plants can thrive there.

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

biotic parts of an ecosystem.

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

the plant's leaves.

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

biogeochemical cycles.

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,

and boreal forests.

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

and less biomass, and there's also less and
less nutrients.

Fewer nutrients isn't necessarily a death
sentence for the trees, though.

It just means that the ecosystem is structured
differently.

Like boreal forests have a lot of nutrient
filled litter because the cold keeps material

from decomposing.

But deciduous forests have a lot of nutrient-rich
soil because it’s warm enough for material

to decompose, but not warm enough for a lot
of biomass to grow.

So a tree that’s adapted to life in a cold
boreal forest might not make it in a tropical

rainforest because of the different energy
availability and nutrient stores.

Let’s consider the tropical rainforests,
which are the most diverse biomes with lush

vegetation and a lot of biodiversity.

But that decadence hides the fragile balance
of all the complex energy flows and nutrient cycles.

Let's go to the Thought Bubble!

Within the tropical rainforests, broadleaf
evergreen trees form a canopy at different

heights, and little or no sunlight reaches
the shady forest floor.

These huge trees absorb most of the soil nutrients,
which doesn't leave a lot for other organisms.

And they have a shallow root system to grab
as many of the minerals as possible from biogeochemical

processes near the surface.

And as the large amounts of rain filter down
through the soil, the minerals that dissolve

in water are leached away to inaccessible
deeper levels.

To survive, the rainforest has to rapidly
cycle nutrients.

The canopy trees are producers, along with
understory plants that work together to keep

vital nutrients moving through the ecosystem.

Herbivores like gorillas and caterpillars
take in those nutrients and move them around

through their excrement and by being eaten
themselves, like by jaguars or geckos.

And the warmth and humidity helps decomposers
and their chemical reactions, so any dead

plants or animals decay quickly.

Because nutrients get sucked from the soils
so quickly, when those huge trees are cut

down, the energy flows and nutrient cycles
break.

Those big producers aren't there to sustain
consumers or shed leaves to recycle nutrients.

So deforestation, or removing trees to use
the land for something else, can be especially

destructive in tropical regions if you don't
consider the biogeochemical cycles.

Thanks, Thought Bubble.

We have negative associations with the word
"deforestation" for good reason -- a lot of

tree removal has caused immense damage to
ecosystems.

But indigenous communities have figured out
a type of calculated clearing that allows

them to work with the rapid nutrient recycling
of tropical rainforest biomes.

In parts of Asia, Africa, and South America
with dense tropical forests, many farmers

have to rely on a kind of subsistence agricultural
practice, which means they only grow enough

food for their families.

Staples like rice are grown in southeast Asia,
maize and cassava in South America, and sorghum in Africa.

Yams, sugarcane, plantains, and vegetables
are also planted to supplement staples and

to provide fuel and fodder for animals.

This practice goes by many names, like swidden,
shifting cultivation, and slash-and-burn agriculture.

The farmers begin by cutting small areas of
tropical forests into slash, or cut vegetation,

that’s then dried and burned.

The ash gets mixed with the poor soil to provide
needed minerals and nutrients -- basically

using all the good stuff stored up in the
vegetation biomass to help new crop plants grow.

Of course, these crop plants use minerals
and nutrients from the soil as they grow,

and we eat them to get those minerals and
nutrients in our bodies.

So after a few years, and before the soil
is completely exhausted, the farmers move

on to another part of land and repeat the
clearing, burning, and planting process.

The previous plot is left unplanted, and eventually
the forest will naturally expand to start

using that soil as part of its carefully balanced
nutrient cycling.

This land rotation is a key part of why humans
have been able to keep farming like this for

thousands of years.

But when widespread clear-cutting happens,
ecosystems can collapse.

For example, we've seen this destruction in
the Amazon when rice, soy, and corn have been

commercially cultivated and sold in domestic
and international markets.

The soil is exhausted after 3-5 years, so
crops can't really grow anymore, and then

large cattle operations move in.

As cattle feed and trample the ground, the
soils are exposed to plenty of UV radiation

from sunlight, as well as cycles of wetting
and drying from precipitation.

The soils become a brick-like substance called
laterite, which isn’t great for growing…

so the once-lively rainforest basically becomes
hard, barren, and lifeless.

Tropical rainforests cover 6 percent of Earth’s
landmass, but contain 50 percent of the world’s

species, many of which haven’t been described
by science and are critical to the world’s biodiversity.

But since the 1980s, 1/5 of the Amazon has
been deforested as we build more towns, roads,

dams, farms, and mines.

Across the Atlantic, deforestation in Ituri
in the Congo Basin, the world’s second largest

expanse of equatorial rainforest, is endangering
the mountain gorilla.

And in southeast Asia clearcutting for palm
oil plantations endanger orangutans, Sumatran

tigers, and Sumatran elephants.

When humans disturb the biosphere, we alter
how energy flows and how nutrients cycle,

which can throw off entire ecosystems in unexpected
ways.

And if we're not careful, that moth that drinks
the tears of sleeping birds -- or plenty of

other remarkable living organisms -- may no
longer exist.

But just like we can be good stewards of our
little fish tank, we can take care of the

world around us and the much bigger ecosystems
we're a part of.

Learning where our food comes from and how
it’s produced is a good first step, like

by supporting farmers who try to grow food
sustainably.

And we’ll keep talking about our role in
ecosystems, energy, and food and how geography

and spatial thinking can help address some
of the critical issues we’re facing, like

how we can have enough food and water to sustain
ourselves and our environment.

Many maps and borders represent modern geopolitical
divisions that have often been decided without

the consultation, permission, or recognition
of the land's original inhabitants.

Many geographical place names also don't reflect
the Indigenous or Aboriginal peoples languages.

So we at Crash Course want to acknowledge
these peoples’ traditional and ongoing relationship

with that land and all the physical and human
geographical elements of it.

We encourage you to learn about the history
of the place you call home through resources

like native-land.ca and by engaging with your
local Indigenous and Aboriginal nations through

the websites and resources they provide.

Thanks for watching this episode of Crash
Course Geography which is filmed at the Team

Sandoval Pierce Studio and was made with the
help of all these nice people.

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