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:
•
fundamental = all possible conditions under which population
reproduces itself
•
realized niche = the actual niche exhibited in particular time
& place
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."
There are but two ways in which new species could appear and
survive. In the first, the new variant organism proves to be more
successful in competition and takes someone else's place. In the
second, a place in the economy somehow is standing empty, and the
variant seizes it.
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.