tough and everyone's looking out for themselves in this world.

That's just the way it is, people.

You know how I always say that biology

is ultimately about sex and not dying?

Well, both of those things are more difficult

than we'd like them to be, because of competition.

There's a finite amount of resources on this planet,

so evolution drives us to compete for them so that we can

survive long enough to spread our genes all over the place.

And naturally, competition is a really important part of how

different species interact when their habitats overlap.

These interactions between species are

what define ecological communities.

So it makes sense that community ecology studies these

interactions anywhere they take place, from a tide pool to

the whole ocean, from a rotting log to an entire forest.

But just because inter-species interaction is mostly competitive

doesn't necessarily mean that community ecology is all about big,

bloody, tooth-and-claw scenes like from cable-TV nature shows.

Actually, a lot of it is, but we're not going

to get there until next week.

For now, let's just note that competition, while prevalent

and important, is also pretty dangerous, kind of a hassle,

and can really hurt.

So a lot of inter-species interaction is actually about sidestepping

direct competition and instead finding ways to divvy up resources,

or otherwise let species just get along.

Can you feel the love?

Careful guys! Because right here we're surrounded potentially

lethal interspecific competition going on all over the place.

Since we're animals, we usually think of competition as going

on between animals, but really it happens between almost all

members of the four Kingdoms of life.

Whenever species compete, they're going after the same resources

that they need for their survival and continued population growth.

In this garden, the weeds are competing with the sunflower,

the corn and the dill for nutrients and water in the soil.

So, these resources, because they're finite in this area,

are the limiting factors that we've talked about.

The population can only get as big as these factors will allow.

Now, a particularly nasty weed could, over time,

eliminate the veggies entirely.

Such elimination is known as competitive exclusion, and

it's one of the most fundamental properties in community ecology,

and also, like, life.

Because the fact is, when two species are competing for

the same resources, one of them is eventually

going to be more successful and eliminate the other.

This bitter truth is known as the competitive exclusion principle,

and it was first identified in 1934 by Russian ecologist G.F. Gause

in a study of two closely-related species of microscopic protists.

When he was only 22 years old, Gause made a name for himself

by conducting experiments that pitted one species of protist,

Paramecium aurelia, against another, Paramecium caudatum.

First, Gause grew each species separately with the exact same

resources, and found that each developed rapidly

and established stable populations.

But when he grew them in the same container,

P. caudatum was soon driven to extinction by P. aurelia.

Paramecium aurelia gained a competitive advantage because

its population grew slightly faster than P. caudatum's.

So Gause's experiment showed that, in the absence of another

disturbance, two species that require the same resources

cannot live indefinitely in the same habitat.

The inferior competitor will be eliminated.

Makes sense. But if competitive exclusion is the natural law

of the land, then why isn't all of earth just a crazy crap-circus

of constant competition, predation and ultimately,

extinction of all those losers?

For a couple of reasons: First, not all resources are limiting.

Two species of sharks may compete for water in the ocean,

but the ocean is, you know, gigantic.

So that's not what limits their population growth.

Rather, the amount of food, like a specific fish that they both eat,

could be limiting, while other resources are plentiful.

Second, as the overwhelming diversity of life in almost any

community shows us, most species, even ones that are almost

identical to each other, are adaptable enough to find a way

to survive in the face of competition.

They do this by finding an ecological niche, the sum of all resources,

both biotic and abiotic, that a species uses in its environment.

You can think of an organism's niche as its job in the community

that provides it with a certain lifestyle.

We tend to keep jobs that we can do better than anyone else

in our community, and if we're desperate, we a do a job

that nobody else wants to do.

But no matter what job we have, what it pays

in terms of resources dictates our lifestyle.

So, finding a nice, comfy niche that you have pretty much

to yourself not only provides a steady income of food and other

stuff, it also allows a species to avoid competitive exclusion.

And this, in turn, helps create a more stable ecological community.

It's an elegant and peaceful solution,

I wish we humans could figure out something as good.

But as with anything in life, this relative security

and stability comes at a price.

The bummer is that it prevents some species from living the lifestyle

that they could have if nobody else competed with them at all.

This ideal situation is called

a fundamental niche, and it's just that, an ideal.

Few if any species ever get to live that way.

Instead, because of the need to avoid competitive exclusion

in order to survive, many species end up with

a different job, and hence lifestyle.

It's not necessarily the job that they studied for in college,

but it makes a decent living, and that's called a realized niche.

This, my friends, is how nature does conflict management.

But it sounds kind of unnatural, doesn't it?

I mean, Gause taught us that competition,

and winning the competition, was the natural order of things.

So how can it be that part of the natural order actually

involves letting everyone compete and win just a little bit?

And how did we ever come to discover that things

actually worked this way?

Well, it took a special kind of person, and to tell you

about him, I'm going to need a special kind of chair.

Canadian born ecologist Robert MacArthur was in his late 20's

when he made a discovery that made him one of the most

influential ecologists of the 20th century.

While researching his doctoral thesis at Yale University in 1958,

he was studying five species of warblers that live in coniferous

forests in the northeastern United States.

At the time, because there were so many different species

of warblers that lived, fed and mated in such close quarters,

many ornithologists thought that the birds occupied

the exact same niche and thus, were an exception

to Gause's competitive exclusion principle.

But MacArthur was not convinced.

A mathematician by training, he set out to measure exactly how and

where each kind of warbler did its foraging, nesting and mating.

In order to do this, he studied each tree the birds lived in,

dividing them into zones, 16 zones to be exact,

from bare lichen at the base of the trunk,

to new needles and buds at the tips of the branches.

After many seasons of observing many birds in many trees,

he found that each species of warbler divided its time

differently among the various parts of the tree.

One warbler, called the Cape May, for example,

spent most of its time toward the outside of the tree at the top.

Meanwhile, the Bay Breasted fed mostly around the middle interior.

MacArthur also found that each of the warblers each had

different hunting and foraging habits and even bred at

slightly different times of the year, so that their highest

food requirements didn't overlap.

These differences illustrated how the warblers partitioned

their limiting resources, each finding its realized niche

that allowed it to escape the fate of competitive exclusion.

The phenomenon he observed is now known as resource partitioning, when

similar species settle into separate niches that let them coexist.

Thanks in part to this discovery, MacArthur became known as

a pioneer of modern ecology, encouraging curiosity

and hypothesis driven research, championing the use of genetics

in ecological study, and collaborating with biologists like

E. O. Wilson and Jared Diamond.

Sadly, he died of renal cancer at the age of 42,

but his study of northern warblers remains a classic

example of community ecology that is still taught today.

So, if organisms can do this, if they can behave in ways that

help minimize competition while increasing their odds for survival,

it follows that traits associated with this behavior

would start being selected favorably.

After all, that's what natural selection is for.

When this happens, it's known as character displacement.

To demonstrate, let's go back to some other, famous ecologists

our favorite couple of evolutionary biologists and love birds,

Peter and Rosemary Grant.

I told you before about how they observed the process

of speciation among Darwin's famous Galapagos finches.

Well on the same island, Daphne Major, in 2006,

they witnessed character displacement in action.

For a long time, a small population of finches had the island

to themselves, where they ate a variety of seeds,

including seeds of the feverplant, which were bigger and more

nutritious than the smaller seeds that were available

but were harder for the little finches to open.

Then in 1982, a group of much bigger finches showed up

on the island, and they began to commandeer

the island's abundant supply of feverplant seeds.

Within just 20 years, the Grants found, that the small finches'

beaks shrunk to allow them to specialize in eating only

the smaller, less nutritious seeds, but now the little finches

had those seeds all to themselves.

The traits of the two populations had actually diverged

to help facilitate the partitioning of resources.

See? Competition can be hard on us, but it also can make us

better people, or you know, finches or warblers or kangaroo mice.

But there are also kinds of interspecific interaction in which

species actually join forces in the fight for survival.

This is the ultimate in conflict-avoidance.

In these cases, species in a community actually manage

to avoid competition altogether by forming downright tight

relationships that benefit one, if not both of the parties involved.

You've heard of both of these cases: First, mutualism,

where both species benefit, and commensalism, where one species

benefits and other is kind of like, "Whatever."

Mutualism abounds in nature, and for those who've been paying

attention to CrashCourse,

you've heard me talk about it many, many times before.

A prime example are micorrhizae, the fungal roots that we talked

about a few weeks ago where fungi and plant roots get entangled

and essentially rub each other's backs for nutritious favors.

Others you may have heard about include flowering plants

that produce nectars to attract pollinators, and that bear fruit

to attract animals to help spread the seeds inside.

Oftentimes these relationships become rather needy,

like in the case of termites.

They can't breakdown the cellulose in the wood they eat

without the enzymes produced by microorganisms that live

inside their digestive systems.

Without the little critters, the bigger critters would die.

Such a needy relationship is called obligate mutualism.

By contrast, commensalism is where one species definitely

benefits and the other isn't really hurt or helped.

Such neutrality, of course, is difficult to prove, because even

a seemingly benign interaction probably has some effect.

Barnacles, for example, hitchhike on grey whales, getting a free

ride through swathes of plankton-rich water for feeding.

While clearly a benefit to the barnacles, the relationship

is often considered commensal because the whales probably

don't really care whether the barnacles are there or not.

Or do they? The barnacles might slow down the whale

as it swims through the water, but on the other hand,

they might also serve as a type of camouflage from predators

like orcas, in which case they confer an advantage.

So, it probably comes down to "meh" for the whale,