we've talked about how to make an organism.

And you know what I've learned in those 38 weeks?

Putting a living thing together is hard!

There are molecules that make up organelles that run cells,

which come together to form tissues,

which make up organs that make up systems.

And knowing this stuff is incredibly important,

because it shows us the ground rules for being a living thing,

on this particular planet anyway.

But still, there's so much more to biology than that!

I mean, understanding how an organism goes about its internal

business is great, but it doesn't tell us much

about its place in our world.

For that, we need ecology,

the study of the rules of engagement for all of us Earthlings.

Ecology seeks to explain why the world looks and acts the way it does.

Why the South Pole looks different from the Congo,

and why there are mosquitos all over the place

while black rhinos are practically extinct.

The short answer to this question is:

because the world is crammed with things, both animate and not,

that have been interacting with each other all the time, every day,

since life on this planet began.

The even shorter answer is that all life and all of these things

interacting with each other depend on just two things.

Try to guess what they are.

In the meantime, get ready, because Crash Course Biology is taking

its final voyage outside the body and into the entire world!

In a way, you can think of all living things, great white sharks,

pond scum, potato plants, as molecules that react with each other.

Each one of us organisms is pretty piddling in the scheme of things,

just like a single oxygen molecule, which we need to make ATP

to fuel our bodies.

But it can't get much done by itself.

But if you get a million oxygen molecules together with

some other types of molecules, suddenly they're unleashing

a googlejillion megawatts of ATP power

to animate the bag of meat that is you.

This same principle applies to organisms:

As you put individual organisms together,

they can interact with each other and their environments,

to create something larger than the sum of its parts.

And just as every organism has a hierarchy of biological systems,

from molecules to organelles, cells, tissues and organs,

so too does Earth have tiers of ecological order.

Like, when a bunch of members of a species are together in a certain

area, and they interact pretty often, you've got a population.

Population ecologists study why populations grow or shrink over time,

depending on where they are.

When two or more populations of different species live together,

we call that a community.

Think of an ecological community as Mr. Roger's Neighborhood,

but with the people in the neighborhood

eating each other sometimes.

Because that's what species do when they

live together, they interact.

Sometimes that means predation, sometimes cooperation, and sometimes

competition for resources like food, water and living space.

So, a community ecologist studies how the interactions between

community members and their environment affect how many of each

species there are within a community.

One level up from communities are ecosytems,

which are made up of groups of organisms in a specific area

and the nonliving parts of their environment,

like soil, water, and air.

If you take a bunch of living things and plop them down in one place

that has a specific mix of climate, soil chemistry, and topography,

that's going to make up one kind of ecosystem.

But if you put them down in a completely different place,

they're going to work in completely different ways to

form a completely different ecosystem.

Ecosystem ecology specifically explores how energy and materials

flow through an ecosystem, and how the physical environment

impacts the stuff living there.

Now, a lot of people get ecosystems confused

with the next step up: which is biomes.

A biome, however, is where organisms have evolved similar

techniques to adapt to a general set of conditions.

For example, a grassland is a kind of biome, there are scores

of different grassland ecosystems all over the globe,

but the organisms in each one have made similar evolutionary

concessions to all the conditions that grasslands share,

like hot summers, cold winters, and not too much rain

but more rain than you'd find in a desert biome.

Other biomes include tropical rainforest,

tundra, deserts, and oceans.

The only level above the biome is the biosphere,

which includes the atmosphere, the whole earth and everything

that gets used by anything that's alive.

So, why do all of these many levels of ecological activity

look the way that they do?

Like, why do some organisms like to live in one place but not another?

And what makes Earth's various populations, communities,

ecosystems and biomes different from each other?

Well, factors that determine what a place is gonna look like

fall into two different categories: biotic, or living,

and abiotic, not living.

Biotic factors include stuff like predators,

as well as animals or plants that provide either competition

or some benefit, like food or shelter.

Abiotic factors, on the other hand, include temperature,

moisture, sunlight, elevation,

elements that have nothing to do with organisms in the ecosystem,

but which influence them just as much as other living things do.

Now, from these two categories, the most influential factors

are the ones that living things are most particular about.

That is, the things they need most, but only at certain levels.

And these preferences all come down to chemistry.

For example, almost all chemical reactions that happen inside

living things are governed by enzymes.

They're the catalysts for pretty much

all the action going on inside you.

And these enzymes are most effective within a set of temperatures:

Chemical reactions within the body slow way down when it's really

cold, and very high temperatures change the shape of enzymes,

making them less effective.

So temperature is one of the major factors that determines

why animals live in certain places.

And if you look at the places on the earth with the most

biodiversity, or different kinds of living things,

you'll find that it's in the places where the temperature's

within the ideal range for enzyme function.

What else? Well, everybody's got to eat, at least if you're

an animal or a fungus or some other kind of heterotroph,

so you'd think that food would also be way up on the list.

But actually, it's plants and other autotrophs like cyanobacteria

and protists that are the base of nearly every food chain,

and they have to be fed, too.

So again, it comes down to chemistry.

The key ingredient plants need for photosynthesis is water,

which is also what we need to burn ATP,

maintain homeostasis in our bodies, and all that jazz.

So the quest for food ultimately comes down for a need for water.

So, yeah: surprise! Water and temperature are the two things

that organisms care about the most.

Ergo, they're what ecologists focus on when determining

why certain organisms hang out in one place over another.

Together, these two factors define every biome on the planet.

For instance, a Saguaro cactus has evolved to live in the

Sonoran Desert of North America, which is super hot

and gets very little precipitation.

So, the Sonoran Desert is full of animals and plants that can,

just like the Saguaro, take the heat and also the extreme,

face-crumbling dryness.

But if you put these animals in the Amazon rainforest,

even though it's hot enough for them, it's just too wet.

So, yeah, the things that live in a biome are ultimately

determined by how much water is there and the temperature.

And in turn, these inhabitants determine

how the biome looks, called its physiognomy.

So now, we are going to take a look at all the different

types of biomes out there.

There are places on the planet that get lots and lots of rain,

around 300 centimeters a year, and are pretty warm,

around 25-30 degrees C on average, which is Speedo-wearin' weather,

as far as I'm concerned.

These biomes are the tropical rainforests, which generally

hug the equator and have unbelievably high biodiversity because

everybody's wanting to get a piece of that sweet tropical action.

And then on the complete opposite side of that scale,

we have the tundra, most of which is above the Arctic Circle,

in Antarctica, or way up at the top of some mountains.

Tundra gets little precipitation and some well-below-zero temperatures.

And what lives there? Not much. A couple of mosses and liverworts,

maybe a few species of grasses, some birds and a handful of mammals.

The same goes for the desert biome, where there's very little

rainfall and very high temperatures.

Like the tundra, without much water, there can't be very many large plants.

And where there aren't a lot of plants, there aren't a lot