After the principles of food size and pyramid of numbers, the fourth element in Elton's community structure was the "niche," which, ecologically speaking is the outcome of the evolutionary process of differentiation and specialisation. First given its modern name by the California ornithologist Joseph Grinnell, the niche is essentially Darwin's "place" or "office" in the economy of nature.

Niche concept is central to modern ecological theory

In particular, niche is used to describe and analyze

1. ways in which diff. species interact (including competition, resource partitioning, exclusion or coexistence);
2. why some species are rare and others abundant;
3. what determines geographical distribution of a given species;
4. what determines structure and stability of multi- species communities

Term "niche" borrowed from church architecture, but in ecology it is not a spatial term like habitat or environment (though related to these)

What's the difference between the two concepts?

Habitat = "address" (environmental region or community inhabited by a population)

Niche = "profession" (way in which a population utilizes a habitat)

Though realizing that any population's niche is multi- dimensional in very complex ways, ecologists have often simplified analyses by considering one or two dimensions at a time (e.g., food resource niche)

Niche theory distinguishes between fundamental & realized niches:

Realized niche will almost always be a subset of ("smaller than") the fundamental niche, for various reasons; one important determinant of realized niche is presence or absence of different competitors

Recall the definition of ecological competition (competition lecture): presence of N₁ has negative effect on growth rate of N₂ because both populations utilize >1 of same limited (relative to demand) resources

Obviously, the more limited resources two populations have in common (i.e., the more similar their niches are), the greater the impact of competition (all else being equal)

Consider an extreme case: Can 2 populations occupying the same resource niche coexist in the same environment?

If 2 populations occupy same resource niche, then by definition they utilize all the same resources, and in the same manner

Common sense tells us there are 3 possible outcomes to this situation:

1) share resources more or less equally (neither population changes niche)

2) one or both populations alters niche to reduce overlap (niche partitioning)

3) one population loses out completely (competitive exclusion)

Which outcome will occur? Answer from niche theory = 2 or 3, but not 1

This somewhat counterintuitive finding given formal name of competitive exclusion principle (CEP)

CEP states that no two species can permanently occupy the same niche: either the niches will differ, or one will be excluded by the other (note: "excluded" here means replaced by differential population growth, not necessarily by fighting or territoriality)

Of course, 100% niche overlap is unlikely if not impossible; but such an extreme case is not necessary for competitive exclusion or other forms of niche change

Much theory & research in ecology has focused on predicting what actually happens when there is niche overlap & competition: When does exclusion result, when coexistence? How much overlap is possible (a question treated by the "theory of limiting similarity")? How do environmental fluctuations affect this? Why are some species generalists, others specialists?

Both possible responses to niche competition (competitive exclusion, and coexistence via reduction in niche overlap) are commonly observed, and their determinants and features have been studied by three means: lab experiments, field observations, and mathematical models or simulations

Competitive exclusion is commonly observed when a species colonizes a habitat and out- competes indigenous species (probably due to absence of parasites and predators adapted to exploit the colonizer); (e.g., introduced placentals vs. indigenous marsupials in Australia)

Coexistence through niche partitioning is rarely observed directly, but can often be inferred from traces left by "the ghost of competition"

The typical means of doing so is to examine two populations that overlap spatially, but only partially, and then to compare the niche of each population in the area of overlap vs. the area of non- overlap

In such a case, we often observe that in areas where competitors coexist, one or both have narrower niche range (e.g., diet breadth) than in areas where competitor is absent; this is because competition "forces" each competing population to specialize in those resources (or other niche dimensions) in which it has a competitive advantage, and conversely to "give up" on those in which the other population outcompetes it

Thus, in absence of competitors a given species will often utilize a broader array of resources (closer to its fundamental niche) than it will in competitor's presence; this phenomenon is termed "competitive release"

When niche shift involves an evolutionary change in attributes ("characters") of competing populations, it is termed character displacement

This is one of strongest kinds of evidence for role of competition in shaping niches, because it is unlikely to have alternative explanation

A classic example of character displacement is change in length or shape of beaks in ecologically similar bird species that overlap geographically.

The phrase 'place in the economy of nature' generally meant a feeding role or the sources of an organism's food; for instance, the Galapagos iguana that ate seaweed and thus filled "the same place in Nature" held by coastal herbivores in South America.

At other times, Darwin's concept of "place" referred to a more complex and inclusive pattern of behaviour. Sharks and porpoises, for instance, though, have assumed in modern times the habits, and entire manner of living of a place formerly occupied in the Mesozoic era by the ichthyosaurus, a cold-blooded reptile.

Darwin's notion of "place" was fully separable from the organism. He understood that over the long history of the earth a number of very different species might occupy the same place.
In the second step of his logic, Darwin again followed the eighteenth- century naturalists very closely. He believed that the economy of nature is more than a working composite of organisms. Considered abstractly, it is a system of 'places'.Sometimes Darwin preferred to use the word "office," which he borrowed from Linnaeus' "Polity of Nature."

Now and then, however, one could discover a locale where the inventory of living things was not yet complete. The Galapagos Islands presented just such a situation. Their recent emergence, and their isolation meant that only a few creatures thus far had come upon their unexploited resources. Finches were the first small land birds to arrive. The absence of competitors contending for a slot allowed the finches to move into many of the available places, to diversify in a way that would have been impossible in large, open continental areas, where constant migration and strong rivals had always kept them from diversifying.

So, too, in this under populated area could tortoises evolve to fill the role of principal herbivore. Only in such geographically isolated situations- islands or high mountain valleys, for example- could a single group fill so many places and do work that elsewhere others did. But such situations also create a highly vulnerable ecology: In a world where competition is minimal, each organism's defences against new immigrants consequently must be weak.

It was Elton, however, who gave the idea of the niche its prominence in twentieth-century ecology.

He defined it as the "status" or "occupation" of an organism in the community, which defined "what it is doing and not merely what it looks like." In practice, by emphasising the economics of the community, he reduced the niche to a matter of food sources, or what an animal is eating. All communities, he explained, have similar "ground plans" of niche patterns, as they do of food chains. The arctic fox, for example, which lives on guillemot eggs and on the carrion of seals killed by polar bears, occupies the same niche as the African hyena, which eats the eggs of ostriches and the remains of zebras killed by lions. But within any single community, no two species can occupy the same niche; "competitive exclusion" is the unbreakable rule. The pressure of population increase must generate a fierce competition for food, from which only one species can emerge victorious in a niche.

The Russian scientist G. F. Gause, using yeast cells and protozoa in a test-tube environment tested this important notion of exclusion, which was also the essence of Darwin's ecological model, in a laboratory. Gause published his results confirming competition as the law of nature in his book 'The Struggle for Existence' published in 1934, Elton cited these experiments in support of competitive exclusion in subsequent editions of his own work. But when other biologists began to offer examples of several species coexisting in the same food niche- thus denying the universality of competitive exclusion- the niche concept had to be stretched to include once more all the activities of the organism. For example, all warblers in a spruce woods may eat the same food, but each species builds its nest at a unique, characteristic level in the trees. Thus the niches, considered as patterns of behaviour rather than food sources, remained distinctive.

But this compromise also made the whole idea so general as to be almost useless as a measurable entity. It was even tautological because the niche is the species, and the species is the niche. Since all species are by definition different, and since behaviour as much as bone structure defines a species, it would seem obvious that niches must differ without necessarily being the outcome of competitive exclusion. Historically, the more important problem with the niche or place idea has been whether to regard niches as preordained, pre- existing slots to be filled in nature, or simply as post hoc descriptions of what an organism does with its environment. On this matter Elton was silent, as his students have been. However, the niche remains today a favourite principle, especially among competition-minded researchers.