Official 36 - lecture 4

listen to part of lecture in a biology class

Back in the 1930's, a biologist named G.F. Gause first proposed what's known as "Gause's hypothesis".

Gause said that whenever you've got two similar species competing for the exact same limited resources, one of them will have some sort of advantage.

however slight that'll eventually enable this species to dominate and ultimately exclude the other one, even cause it to become extinct.

That's why Gause's hypothesis came to be called "The competitive exclusion principle".

Gause did some lab experiments like placing two Paramecium species in the same environment where they would have to compete for the same food.

Paramecium 草履虫

He found that, over time, one species was consistently able to drive out the other, to eliminate it from the habitat, just as his hypothesis predicted.

Now, one of the early criticisms of Gause's hypothesis was that:

sure, it works in simple lab experiments where you have just two competing species in a controlled environment, but the hypothesis falls apart when applied to natural ecosystems where things are more complex。

Now, it's true that in the real world there are lots of examples that seem to contradict the hypothesis.

For example, in the forest of New England, in the northeastern United States, there are some small songbirds called warblers and right in the same area you've got five species of warbler,

New England is a region comprising six states in the Northeastern United States.

all about the same size and all having similar diets of insects, uh, insects that are found on and around trees.

Yet, these five warblers species all managed to coexist.

There is no dominance, no exclusion of one species by another.

How is this possible?

Well, turns out that one warbler species feeds in the uppermost branches, while others favor the middle branches and others feed toward the bottom of the tree.

Also, each warbler species breeds at a different time of year.

breed, bred, breeding: to produce offspring by hatching or gestation

This way the period of peak food requirement, um, when the birds are feeding their chicks, varies from one species to the next.

But does that really contradict Gause's hypothesis?

Because, I mean, are those different warbler species really competing for the same food?

I don't think so.

I think they're more like, you know, almost cooperating so that they don't have to compete.

Excellent! To the casual observer, the warblers do seem to contradict Gause's hypothesis since they all live in the same place and eat the same types of insects.

But if you observe these birds more closely, the warbler species are not really competing with one another for the exact same food at the exact same time, which brings us to a really important concept in ecology: the niche.

Mark, can you tell us what an ecological niche is?

niche:

The place where the plant or animal lives, you know, its habitat.

For example?

Like the polar bear living in the Arctic on the ice sheet.

Arctic 北极 Antarctica 南极

The Arctic is its niche, the habitat it's adapted to survive in.

Okay. That's what most people think of.

But for biologists, the concept of a niche also includes the way an organism functions in its habitat, how it interacts with other plant and animal species, with the soil, the air, the water and so on.

Now let's put it all together.

If you have two similar species competing in the same niche, what's going to happen?

One will dominate the other and eventually eliminate it.

So what could the weaker species do to improve its chances of survival?

Maybe just move to some other area, you know, away from the competitor.

That's one possibility.

But think of the scientific definition of a niche.

Think about the warblers.

Maybe it could find some new way of functioning in its habitat so that it wouldn't have to compete with the dominant species.

Keep the same habitat but not the exact same niche.

Yes, and there are many ways to do that.

The dominant species feeds in one part of the tree and you feed in another.

If the dominant species needs a lot of water you develop the ability to survive on very little water.

You survive on what's left over: water, food, nesting or breeding sites, whatever.