- [Instructor] Let's talk about sustainability.
You've probably come across the word "sustainable"
at some point in your life.
If I decided to continue to talk for the rest of this video
without taking a breath, you might tell me,
"Mia, that's just not sustainable."
In this scenario,
which I obviously would never even dream of attempting,
I'd be depleting the oxygen in my lungs
without replacing it.
Using resources faster than they can be replenished
is not sustainable.
But what does "sustainability" really mean?
Well, the word "sustain" means "to maintain" or "to hold,"
so sustainability is the ability of something
to be maintained over a period of time.
In environmental science, when we talk about sustainability,
we're talking about the ability of the Earth's systems
to survive and adapt to changing environmental conditions
and maintain the health of ecosystems.
It's like a delicate balance
where each process doesn't take too much or produce too much
so that all the other processes can keep on going.
And theoretically, when all these processes
are doing their jobs, this balance could last forever.
But some things that humans do can disrupt this balance.
Let's take a look at an example.
In the early years of the United States,
enslaved and free farm workers in the American South
planted or were forced to plant cotton,
a plant that needs to use the nitrogen
in the soil to survive.
But when they planted cotton
in the same fields year after year,
the cotton plants would deplete the nitrogen levels
in the soil.
This constant depletion of nitrogen was not sustainable
and the cotton crops suffered.
In the early 20th century,
agricultural scientist and inventor George Washington Carver
suggested that farmers could alternate
between growing cotton and growing nitrogen-fixing plants
like peanuts or sweet potatoes.
This is a more sustainable way of farming,
as the nitrogen-fixing plants would replace the nitrogen
that the cotton plants took out of the soil.
Since then, we've discovered a way
of putting nitrogen back into the soil
using chemically-synthesized fertilizers.
But sustainability is more complicated
than just replacing depleted resources.
When farmers use too much fertilizer
or use poor methods of fertilizer application,
all that fertilizer can run off into neighboring waterways.
This excess nitrogen in the water
could cause a lot of algae to grow.
The thing about algae is that when it grows too rapidly,
it also dies rapidly,
and the microbes that decompose the algae
hog the available oxygen in the water,
which makes it difficult for other organisms
living in the water to breathe.
So, when the fertilizer runoff
causes too much algae to grow,
it creates an imbalance in the ecosystem
and harms the other organisms
that rely on the dissolved oxygen in the water,
so the addition of too much fertilizer
is also unsustainable.
But how could a farmer know
whether they're using fertilizer sustainably?
Well, they could look out for environmental indicators.
Environmental indicators are basically
when the Earth tells us,
"Hey, there's something unsustainable in this ecosystem."
But instead of just telling us that straight out,
it tells us in a variety of clues.
For our farmer, environmental indicators can be things
like the amount of dissolved oxygen in the water,
the biological diversity in the area,
or even how much algae goes in nearby water sources.
The series of events triggered by fertilizer pollution
could deplete the amount of oxygen in the water,
so species that live in the water
that require a lot of oxygen to survive
could begin to die off.
These species are called indicator species.
Because they're known to survive
in very specific circumstances,
spotting these species is an indication
that there is a lot of oxygen in the water.
If these species are absent, then that will be an indication
that the pollution has reduced
the amount of dissolved oxygen in the pond.
The farmer could then change their behavior.
In this way, environmental indicators can guide humans
to make sustainable choices.
Another way humans can use environmental indicators
to make decisions about natural resources
is by monitoring populations and estimating their growth.
Imagine that you have a pond in your backyard.
And let's imagine that you have searched
for all of the correct environmental indicators
so you know that the fish in your pond
are not suffering from pollution.
One day, you decide to go fishing.
It might be tempting to fish as much as you can
so that you can invite all of your friends
and have a big happy fish feast.
But if you decided to take all of the fish out of the pond,
then there wouldn't be any left to reproduce,
so your feast next year would be pretty sad.
Even if you just left a couple of fish in the pool,
they might not be able to reproduce enough
to replenish the population.
Other environmental factors
might cause the small fish population
to go down even further.
The fish might catch diseases, run out of food,
or end up being some other creature's feast.
There's some maximum number of fish that you could take
so that there's enough fish left in the pond
to reproduce and replenish the population.
This number is called the maximum sustainable yield.
Basically, you want to catch the most fish
to have the maximum yield while still being sustainable.
If you fish more than the maximum sustainable yield,
then the fish will be captured
more quickly than they can reproduce
and the population will slowly decline.
If you fish less than the maximum sustainable yield,
then the fish population will grow exponentially
until they reach the maximum capacity
that the ecosystem can support.
Some of the fish will eventually die
from density-dependent factors
like lack of food and disease.
On a bigger scale than your imaginary pond,
natural resource managers try to estimate
what the maximum sustainable yield will be
each fishing season.
This is why fishery managers
regulate the amount of certain types of fish
each fisher is allowed to catch per year.
This is to make sure that next year we will still have fish.
So, if we study the Earth's ecosystems
and understand how we impact the environment,
then we can find ways to interact with ecosystems
more sustainably.
And, really, that's what environmental science is all about.