Download BIO 300 Ecology

BIO 300
Ecology
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LECTURE 15
COMPETITION
Chapter 9. We will not cover the Lotka-Volterra graphical model. We will also not cover Measurement of the Competition
Coefficient. Do read the section on Empirical Examples of the Model’s Predictions. This covers how abiotic conditions
may affect the outcome of competition. This will not be covered in lecture.
I. DEFINITION OF COMPETITION AND A REVIEW OF EXAMPLES
A. Definition1. Short- Competition results in mutually adverse effects to organisms that utilize a common resource in short
supply.
2. Longer - Competition is an interaction between individuals, brought about by a shared requirement for a resource
in limited supply, and leading to a reduction in survivorship, growth and/or reproduction of the competing
individuals.
II. TYPES OF COMPETITON
A. Intraspecific competition - competition between individuals of the same species leads to density-dependent birth and
death rates.
1. density-dependent the effects of intraspecific competition on any individual is greater the more competitors there
are. The following are consequences of intraspecific competition.
B. Interspecific competition- competition between individuals of different species, but birth and death rates of one
species vary with the population density of the others species, and vice versa.
C. Major distinction between intraspecific and interspecific competition.
- An important distinction between intraspecific and interspecific competition is the difference in their population
consequences.
1. Intraspecific competition leads to the stable regulation of population
2. Interspecific competition may cause the extinction of one of the competing populations.
D. Exploitation or resource competition (scramble)1. Exploitation competition occurs when an individual is affected by the amount of resource that remains after it
has been exploited by the other, neighboring individuals.
2. All individuals are equally affected; there are no “winners,” or “losers.”
3. Each individual is affected by the amount of shared resource remaining.
a. Individuals do not necessarily interact to each other, but to the level of the resource available.
4. Exploitation can only occur if the resource in question is in limited supply.
E. Interference competition (contest) 1. The deleterious effects of limited resources are confined to a fraction of the population.
a. Some individuals acquire resources at the expense of others. There are “winners,” and “losers.”
2. Individuals interact directly with each other - One individual will actually prevent another from occupying a
portion of the habitat and therefore, prevent it from exploiting the resources in it.
3. Algal succession example. Green alga, Ulva is early, mid-successional red algae and late successional red alga,
Gigartina.
a. Early and mid-successional algae interfere with colonization by Gigartina.
b. Ulva subject to mortality by desiccation and crab herbivory. Mid-successional red algae overgrown by
epiphytes.
c. Gigartina eventually prevails.
4. Individuals do not have to physically interact, but one organism may still interfere with the another’s acquisition
of resources. See allelopathy in III. B. 7. , below.
F. Diffuse competition – competition from many species. Seedlings in the understory of a forest.
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III. COMPETITIVE EXCLUSION AND COEXISTENCE
A. What are the consequences of exploitation competition?
1. Lotka and Volterra –Two species with similar requirements cannot coexist in the same community. The more
efficient species should drive the other to extinction.
2. Gause used Paramecium data as key evidence for the competitive exclusion principle –‘two species cannot
coexist on the same limiting resource’ in a stable environment.
3. Elton’s defintion of niche – “The niche of an animal means its place in the biotic environment, its relations to food
and endemies.”
4. Eugene Odum defined it more clearly, “If an organisms habitat is its address, the niche is its occupation.” How an
organism makes a living.
5. Hutchinson’s niche – n-dimensional hypervolume.
6. Niche breadth and niche separation in book.
B. Competition doesn’t always result in competitive exclusion – Interference competition.
1. If two species have niches that overlap to a lesser extent, both may survive. This can happen if each species can
get by relying on that set of resources the other species doesn’t use, or reducing the range of resources so that it
relies on that part of the niche where it can out-compete the other.
2. Barnacles - Chthamalus grows on the upper portions of the rocks, Balanus grows on the lower portions.
3. Balanus fails to survive far above the low tide line due to desiccation. Its fundamental niche is the same as its
realized niche.
4. When Balanus is removed, Chthamalus is able to spread to lower strata. This indicates that it has a large
fundamental niche, but that only part of that niche is realized because of Balanus.
5. Consequences are often not the same for both species – the outcome is often highly asymmetric.
6. The extreme case of asymmetry is amensalism.
7. Allelopathy is one form of amensalism. (Tree-of-heaven, sassafras, sycamore and walnut are allelopathic trees.)
C. Resource Partitioning 1. Gause also stated that if two competing species are to coexist in a stable environment, then they do so as a result
of niche differentiation. If there is no such differentiation, or if it is precluded by the habitat, then one competing
species will eliminate or exclude the other.
2. R.H.MacArthur- “If complete competitors cannot coexist, how different do two species have to be in order to
coexist in the same habitat?”
a. Five species of migratory warbler appeared to be foraging for the same prey in the same spruce tree. How
could this be?
3. What is not always clear is, does this have anything to do with competition?
a. Some ecologists think so. David Lack has used the phrase ‘the ghost of competition past.’
b. He believes that the species coexist as a result of evolutionary responses to interspecific competition.
4. However, there is no proof of this scenario. And we cannot go back in time to check whether the species
competed more in the past than they do now.
5. So, we must consider 5 plausible, alternate explanations.
a. The five species of warblers are competitors now and coexist as a result of the observed niche differentiation.
b. The species competed in the past but do not do so now – but their niche differentiation, which allows their
coexistence, evolved in response to that past competition (evolutionary force).
c. Competition in the past eliminated a number of other species, leaving behind only those that were different in
their utilization of the habitat. Thus, we can still see the hand of the ghost of competition past – but acting as
an ecological force (eliminating species) rather than an evolutionary one (changing them).
d. The species have, in the course of their evolution, responded to natural selection in different but entirely
independent ways. They are distinct species, and they have distinctive niches. But they do not compete now,
nor have they ever competed; they simply happen to be different. The coexistence of the warblers, and the
structure of the community of which they are part, have nothing to do with competition (The null hypothesis).
e. Finally, although the niches of the species are distinct, they are not different enough to allow all five to
coexist in an unvarying environment where competition runs its course. But the environment does vary and
competition does not run its course, and the species therefore owe their coexistence to their responses, overall,
to a patchy and ever-changing world.
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IV. DISTURBANCE, HETEROGENEITY AND COEXISTENCE.
A. Unpredictable gaps and the revenge of the wimps.
1. Gaps or any area of vegetation that is disturbed or cleared is considered a gap, or a patch of resources.
2. Think of black cherry, which is shade intolerant. However, large individuals do occur in the forest.
3. The presence of gaps allows 'fugitive' species to coexist with highly competitive species - in this case, more shade
tolerant species such as oak and maple.
4. Other species that are slower to establish themselves will eventually out-compete, or perhaps outlive the fugitive
species.
B. Unpredictable gaps - the pre-emption of space. First come, best served.
1. When two species compete on equal terms, the results are usually predictable. But the colonization of unoccupied
space is rarely evenhanded.
2. Competition experiment between two annual grasses that grow together in California: Bromus rigidus and Bromus
madritensis.
a. When sown together, B. rigidus contributes overwhelmingly to the biomass of the mixed population ( ≈ 79%).
b. Delaying the sowing of B. rigidus into the mixture tips the balance in favor of B. madritensis. (a delay of 22
days and B. madritensis comprises over 95% of the mixtures biomass.
3. The outcome of competition is not necessarily based on the inherent competitive abilities of the competing
species. Even supposedly inferior species can exclude superior species if given a head start. In a changing and
unpredictable environment, both species may coexist.
V. CHARACTER DISPLACEMENT
A. Another result of competition is character displacement.
1. When two species share a characteristic (that is, it overlaps) live together in the same environment, the
characteristics tend to diverge.
2. Evidence for competition in the past is demonstrated by species whose populations are sometimes sympatric and
sometimes allopatric.
3. Good example are finches of the Galapagos which display character displacement in beaks sizes and hence, size of
seeds they eat.
B. Resource utilization curve - Generalized model of how overlap in a characteristic, or one aspect of the niche, by two
species results in the divergence of that characteristic so that both species overlap as little as possible.