together and learning how it works on many levels,

how populations of organisms interact, how communities thrive

and ecosystems change, and how humans are wrecking the nice,

perfectly functioning systems Earth has been using

for hundreds of thousands of years.

And now it's graduation day!

This here is like the commencement speech,

where I talk to you about the future and our role in it,

and how what we're doing to the planet is totally awful,

but we're taking steps to undo some of the damage that we've done.

So what better way to wrap up our series on ecology than by taking

a look at the growing fields of conservation biology

and restoration ecology.

These disciplines use all the kung fu moves that we've learned

about in the past 11 weeks and apply them to

protecting ecosystems and cleaning up messes that we've already made.

And one of the main things they teach us is that doing these things

is difficult, like, in the way that uncooking bacon is difficult.

So let's look at what we're doing, and try to uncook this

unbelievably large pile of bacon we've made!

Just outside of Missoula, Montana, where I live,

we've got a Superfund site. Not Superfun...Superfund.

A hazardous waste site that

the government is in charge of cleaning up.

The mess here was made more than a hundred years ago, when there was

a dam in the Clark Fork River behind me called the Milltown Dam.

This part of Montana has a long history of copper mining,

and back in 1908, there was a humongous flood that washed

about 4.5 million cubic meters of mine tailings chock full of arsenic

and toxic heavy metals into the Clark Fork River.

And most of it washed into the reservoir

created by the Milltown dam.

I mean, actually it was lucky that the dam was there,

it had only been completed six months before,

or the whole river system, all the way to the Pacific Ocean,

would have been a toxic mess.

As it happened, though, only about 160 kilometers of the river

was all toxic-messed-up.

A lot of it recuperated over time, but all that nasty

hazardous waste was still sitting behind Milltown Dam,

and some of it leached into the groundwater

that started polluting nearby resident's wells.

So scientists spent decades studying the extent

of the damage caused by the waste and coming up with ways to fix it.

And from 2006 to 2010, engineers carefully removed

all the toxic sediment as well as the dam itself.

Now, this stretch of the Clark Fork River runs unimpeded

for the first time in over a century, and the restored area

where the dam used to be is being turned into a state park.

Efforts like this show us conservation biology

and restoration ecology in action.

Conservation biology involves measuring the biodiversity

of an ecosystem and determining how to protect it.

In this case, it was used to size up the health

of fish populations in the Clark Fork River,

which were severely affected by the waste behind the dam,

and the dam blocking their access to spawning grounds upstream,

and figuring out how to protect them during the dams removal.

Restoration ecology, meanwhile, is the science of restoring

broken ecosystems, like taking an interrupted, polluted river

and turning it into what you see taking shape here.

These do-gooder, fix-it-up sciences are practical rather

than theoretical, by which I mean, in order to fix something

that's broken, you've got to have a good idea of what's

making it work to begin with.

If something was wrong with the expansion of the Universe,

we wouldn't be able to fix it because we have no idea, at all,

what's making all that happen.

So in order to fix a failing ecosystem, you have to figure out

what was holding it together in the first place.

And the glue that holds every ecosystem together is biodiversity.

But then of course, biodiversity can mean many different things.

So far we've generally used it to mean species diversity,

or the variety of species in an ecosystem.

But there are also other ways of thinking about biodiversity

that help conservation biologists and restoration ecologists

figure out how to save species and repair ecosystems.

In addition to the diversity of species, ecologists look at

genetic diversity within a species as a whole and between populations.

Genetic diversity is important because it makes evolution possible

by allowing a species to adapt to new situations

like disease and climate change.

And then another level of biodiversity has to do with

ecosystem diversity, or the variety of different ecosystems

within an area.

A big ol' forest, for example, can host several kinds of ecosystems,

like wetland, alpine, and aquatic ones.

Just like we talked about when we covered ecological succession,

the more little pockets you've got performing different functions,

the more resilient the region will be as a whole.

So, yeah, understanding all of this is really important

to figuring out how to repair an ecosystem that is in shambles.

But how do conservation biologists take the information about what

makes an ecosystem tick and use it to save the place from going under?

Well, there's more than one way to approach this problem.

One way is called small-population conservation.

This approach focuses on identifying species and populations

that are really small, and tries to help boost their numbers

and genetic diversity.

Low population and low genetic diversity are kind of the death knell

for a species.

They actually feed off each other,

one problem making the other problem worse,

ultimately causing a species to spiral into extinction.

See, when a tiny little population suffers from inbreeding

or genetic drift, that is, a shift in its overall genetic makeup,

this leads to even less diversity, which in turn

causes lower reproduction rates and higher mortality rates,

which makes the population smaller still.

This terrible little dynamic is known

by the awesome term extinction vortex.

The next step is to figure out

how small a population is too small.

Ecologists do this by calculating what's called

the minimum viable population, which is the smallest size

at which a population can survive and sustain itself.

To get at this number, you have to know the real

breeding population of, say, grizzly bears

in Yellowstone National Park, and then you figure out everything

you can about a grizzly's life history:

how long they live, who gets to breed the most,

how often they can have babies, that kind of thing.

After all that information is collected, ecologists can run

the numbers and figure out that for the grizzlies in Yellowstone,

a population of, say hypothetically, 90 bears would have about

a 95% chance of surviving for 100 years,

but if there were a population of 100 bears,

the population would likely be able to survive for 200 years.

Something to note: ecology involves a lot of math.

So if you're interested in this, that's just the way it is.

So, that's the small-population approach to conservation.

Another way of preserving biodiversity focuses on populations

whose numbers are in decline,

no matter how large the original population was.

This is known as declining population conservation,

and it involves answering a series of related questions that get

at the root of what's causing an organism's numbers to nosedive.

First, you have to determine whether the population

is actually declining.

Then, you have to figure out how big the population

historically was and what its requirements were.

And finally, you have to get at what's causing the decline

and figure out how to address it.

Milltown Dam actually gives us a good example of this process.

In the winter of 1996, authorities had to release some of the water

behind the dam as an emergency measure, because of a big ice flow

in the river that was threatening to break the dam.

But when they released the water, a bunch of toxic sediment

went with it, which raised the copper concentrations downriver

to almost 43 times what state standards allowed.

As a result, it's estimated about half of the fish downstream died.

Half the fish! Dead!

And researchers have been monitoring the decline in populations ever since.

This information was really helpful in determining what

to do with the dam.

Because we knew what the fish population was like before

and after the release of the sediment, it was decided that

it would be best to get the dam out as soon as possible,

rather than risk another 1996 scenario.

Which brings me to the place where conservation biology

and restoration ecology intersect.

Restoration ecology is kind of where the rubber meets the road

in conservation biology.

It comes up with possible solutions for ecological problems.

Now, short of a time machine, which I'm working on, you can't

really get a natural environment exactly the way it used to be.

But you can at least get rid of whatever is causing the problem

and help re-create some of the elements that

the ecosystem needs to function properly.

All this involves a whole suite of strategies.

For instance, what's happening in Milltown is an example of

structural restoration, basically the removal and cleanup

of whatever human impact was causing the problem.

In this case, the dam and the toxic sediments behind it.

And then the rebuilding of the historical natural structure,

here the meanders of the river channel and the vegetation.

Another strategy is bioremediation,

which recruits organisms temporarily to help remove toxins,

like bacteria that eat wastes or plants that

leach out metals from tainted soils.

Some kinds of fungi and bacteria are even being explored

as ways to bio-remediate oil spills.

Yet another, somewhat more invasive restoration method

is biological augmentation.

Rather than removing harmful substances, this involves adding

organisms to the ecosystem to restore materials that are gone.

Plants that help fix nitrogen like beans, acacia trees

and lupine are often used to replenish nitrogen in soils

that have been damaged by things like mining or overfarming.

And ecologists sometimes add mycorrhizal fungi to help

new plantings like native grass take hold.

But of course, we're just humans, and we're not as smart

as millions of years of evolution.

Sometimes we get things wrong.

For example, when you bring an

invasive species into a place

to eradicate an invasive species, sometimes you just end up with

two invasive species on your hands,

which collapses the ecosystem even more rapidly.

The introduction of cane toads to Australia in the 1930s

to control beetles is a particularly infamous example.

Not only are they everywhere now but because they're toxic they're

poisoning native species like dingos that try to eat them. Nice.

So you know what? I have an idea.

After spending the past couple of weeks talking about

ecological problems, I've come to the conclusion that it's just

easier to protect ecosystems rather than trying to fix them.

Because we know a lot about what makes ecosystems tick,

so if we spend more time trying to save them from us and our stuff,

we'll spend less time cleaning up after ourselves

and running the risks of getting it wrong.

Because as we all know, the sad fact is:

uncooking bacon is impossible. But we can eat it.

Thank you for joining me on this quick three-month jaunt

through the natural world,

I hope it made you smarter not just in terms of passing your exams

but also in terms of being a Homo-sapien

that inhabits this planet more wisely.

And thank you to everyone who helped us put these episodes together:

our technical director Nick Jenkins,

our editor Caitlin Hoffmeister,

our writers Blake DePastino, Jesslyn Shields and myself,

our sound designer Michael Aranda,

and our animators and designers Peter Winkler and Amber Bushnell.

And the good news is: there's more Crash Course coming at you soon.

If you have any questions or comments or ideas, we're on Facebook

and Twitter, and of course, down in the comments below.

We'll see you next time.