Gause's Hypothesis

NARRATOR:Listen to part of a lecture in a biology class.

FEMALE PROFESSOR:Okay, back in the 1930s 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'd have to compete for the same food.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, [speaking in critic’s voice, not her own] "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."[returning to her own voice] Now it's true that in the real world, there are lots of examples that seem to contradict the hypothesis.For example, in the forests 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, all about the same size, and all having similar diets of insects-insects that are found on and around trees.Yet, these five warbler species all manage to coexist- there's no dominance, no exclusion of one species by another...How's 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.This way, the period of peak food requirement, when-when the birds are feeding their chicks, varies from one species to the next.[sees hand raised]Yes, Mark.

MALE STUDENT:[formulating as he goes] 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-I think the you're more like, you know, almost cooperating so they don't have to compete.FEMALE PROFESSOR: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?

MALE STUDENT:The place where the plant or animal lives. You know, its habitat?

FEMALE PROFESSOR:[eliciting] For example...

MALE STUDENT:Uh, like the polar bear living in the Arctic, on the ice sheet.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...

Okay, now let's put it all together:if you have two similar species competing in the same niche, what's going to happen?[sees hand raised] Susan?

FEMALE STUDENT:One'll dominate the other, and eventually eliminate it.

FEMALE PROFESSOR:Okay, so what could the weaker species do to improve its chances of survival?

FEMALE STUDENT:[unsure, upspeak] Maybe just move to some other area, you know, away from the competitor?

FEMALE PROFESSOR:That's one possibility. But think of the scientific definition of a niche. Think about the warblers.[sees hand raised] Mark?

MALE STUDENT:Maybe it could find some new way of functioning in its habitat so it wouldn't have to compete with the dominant species... keep the same habitat, but not the same exact niche.

FEMALE PROFESSOR:Yes, an-and there are many ways to do that [begins to give list of examples] - the dominant species feeds in one part of the tree, and you feed in another...

MALE STUDENT:[interrupting politely] If the dominant species needs lots of water, you develop the ability to survive on very little water...

FEMALE PROFESSOR:[summarizing WAMB point] You survive on what's left over. Water, food, nesting or breeding sites,... whatever.