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Chapter 14
INTERSPECIFIC COMPETITION
Individuals of one species interact with individuals of another species.
The effects of those interactions on population growth can be neutral, positive or negative.
1. Neutral interactions have no effect on the growth of the population. This is called
neutralism.
2. Positive interactions benefit both organisms. They are called mutualism.
3. Commensalism occurs when on species benefits and the other is not affected in any
way.
4. Amensalism is the coaction in which one species is adversely affected by other
species, which remains unaffected.
5. Predation is the killing and consumption of a prey.
6. Parasitism is in an interaction in which one organism, usually small, lives in or on
another from which it obtains food.
There is adverse effect on the two species or individuals that compete for a resource that is in
limited supply. This interaction between species is interspecific competition.
Interspecific competition may be exploitative or interference.
Exploitation means that the individual takes as much of the resource as it can but does not
interact with other members of the population, e.g. each pig at a trough has a place and eats as
much as it could.

The focus of competition is the food.
Interference is the type of intraspecific competition that involves interaction between
individuals, e. g. lions eating a zebra in which the non-dominant lions cannot get a place to feed
and have to wait until the more dominant animals have finished.
Place becomes the focus of competition rather than the food.
CLASSIC COMPETITION THEORY
THE LOTKA-VOLTERRA MODEL.
The Lotka-Volterra predatory model assumes that the number of predators depend on the prey
population.
Lotka and Volterra arrived independently to mathematical expressions that describe the
interaction between two species using the same resource.
Both began with the logistic equation for population growth:

for species 1 and species 2
dN1 = r1N1 (K1 – N1)
dt
K1
N = number of individuals;
r = biotic potential
t = time
K= carrying capacity
dN = instantaneous rate of change
dt
(K - N) it is a measure of the environmental resistance or the effect of
K
crowding. This represents the opportunity for further population
growth.
dN2 = r2 N2 (K2 – N2)
dt
K2
they then added to the logistic equation for each species a coefficient to account for the
competitive effect of one species on the population growth of another.
For species 1, αN2 is the coefficient that gives the competitive effect of species 2. N2 is the
number of species 2 individuals and α is the competitive impact per individual of species 2 on
species 1.
This constant converts the number of members of one species population, 2, into an equivalent
number of members of the other , 1.
For species 2, the coefficient is βN1.
The paired equations, which now consider both intraspecific and interspecific competition are…
dN1 = r1N1 (K1 – N1 – αN2)
dt
K1
dN2 = r2 N2 (K2 – N2 – βN1)
dt
K2
In the logistic equation, as the number of individuals in each population (N) increases toward its
carrying capacity (K), the growth of the population (dN/dt) approaches zero.
The addition of each new individual has an inhibitory effect on its further population growth:
intraspecific competition. This inhibitory effect of species 1 on itself is 1/K1, an of specie 2 on
itself is 1/K2.
The inhibitory of effect of each new individual of species 2 on species 1 is α/K1, and each new
individual of species 1 on species 2 is β/K2.
Any one species will stop growing when its carrying capacity has been reached by the
combination of its own numbers plus the individuals of the other species multiplied by the
appropriate competition coefficient
Species 1 will stop growing when N1 + αN2 = K1
Species 2 will stop growing when N2 + βN1 = K2
When N2 is 0, then N1 = K1; when N1 is 0, then N2 = K1/α.
When N1 is 0, then N2 = K2; when N2 is 0, then N1 = K2/β
The presence of species 1 decreases the carrying capacity of species 2, and vice versa.
The two species will reach equilibrium when dN1 = dN2 = 0
dt
dt
SEE GRAPHS ON PAGE 245. MAKE SURE YOU UNDERSTAND THESE GRAPHS AND CAN
INTERPRET THEM.
The Lotka-Volterra model produces a cyclic population curve showing the population fluctuation
of prey and predator.
It is too simple to fit any real interacting populations of prey and predator.
COMPETITIVE EXCLUSION
Gause's rule or the competitive exclusion principle
Two species with identical ecological requirements cannot occupy the same environment.


Two species cannot occupy the same ecological niche.
Complete competitors cannot coexist.
Conditions for necessary for competitive exclusion to take place:
1. Resources are short supply for competition to take place.
2. Competitors must remain genetically unchanged for a sufficiently long period to time for
one species to exclude the other.
3. Immigrants from areas with different conditions cannot move into the population of the
losing species.
4. Environmental conditions must remain the same.
5. Competition must continue long enough for equilibrium to be reached.
In the absence of one of these conditions species usually coexist.
STUDIES OF COMPETITION
Coexistence of competing species.
The potentiality of the natural world is so vast that it is difficult to find two species that have
exactly the same diet and nesting requirement.
Complete competitors, if they exist, must be very rare.
The question is how strong competition has to be before coexistence is impossible.
When intraspecific competition (effects of crowding) is more important that interspecific
competition.
Reduction of interspecific competition is achieved by small difference in the use of the resource
that is potentially limiting.
Habitat partitioning reduces interspecific competition.
These differences probably evolved as a result of the unfavorable effects of interspecific
competition.
Natural selection reduces competition by reducing the niche overlap.
e. g. Darwin’s medium ground finch and cactus ground finch on the Isle of Daphne,
Galapagos Islands.
Competitive exclusion
Competitive exclusion (replacement) has been observed. It often involves the introduction of
a competing species that is more aggressive.
e. g. replacement of black duck in Ontario western Quebec, and the mid-Atlantic and
great Lakes region by mallards
Undisturbed areas with high biodiversity of species are highly resistant to invasion by new
species.
Competition may be the factor that limits the geographical range of a species. Convincing cases
are scarce.
Competitive exclusion is most evident when exotic species successively invade new habitats
and out-compete native species for space, nutrients, or other resources, eventually displacing
them.
Allelopathy
Allelopathy is a form of amensalism.
Some organisms release toxic chemicals into the environment with unfavorable effect on other
species.
Allelopathy is most common in plants, e. g. black walnut, bracken fern. It has been found in
fungi and sponges.
Most allelopathic compounds known in plants are terpenoids and phenols; others are acids and
bases.
Chemicals could be released by plants as volatile compounds from leaves, exudates of the root
system or by leaching from leaves and litter.
Allelopathy plays a role in reducing success or survival of competing plants, and interfering with
herbivory in various ways.
In sponges, potentially competing invertebrates are reduced, and in fungi, nearby bacteria
growth is inhibited.
Diffused competition.
In nature, species rarely interact as pairs.
Competition is spread among species over a number of resources.
The combined effect of minimal competition of many species can result in the equivalent of a
strong competition from one competing species for one resource.
One competitor, specie 1, may maintain a population down of species 2 that in turn is affecting a
third species. If the first competitor is removed, species two increases and causes the decline of
species 3
The combined effect of many species may have a strong effect on one species.
RESOURCE PARTITIONING AND UTILIZATION
Many species coexist in the same habitat by utilizing different resources.
o
o
Animals eat different seed sizes or different parts of the grasses.
Plants occupy different position on a soil moisture gradient, require different proportions
nutrients, or have different tolerances for light and shade.
Each species exploits a portion of the resources that becomes unavailable or unusable to other
species.
As populations use resources they are also depleting the resource base, which may or may not
be renewed.
The rate of consumption versus the rate of renewal is important in interspecific competition.
DIFFERENTIAL RESOURCE UTILIZATION
Plants compete for several resources at the same time: light, nutrients, moisture.
A model for plant competition has been presented by Tilman (1980, 1982, 1986) which takes
into consideration population growth and resource utilization. This model still needs testing in
the field.
Other sources of information:
http://io.uwinnipeg.ca/~simmons/1116/16comeco.htm
http://ecology.botany.ufl.edu/ecologyf02/Competition.html
http://www.marietta.edu/~biol/biomes/competition.htm
THE NICHE
Niche is the place and role a species plays in the ecosystem.
It is a somewhat nebulous concept interpreted in various ways as habitat, functional roles, food
habits, and morphological traits.
An organism's niche consists of many physical and environmental variables.
An organism's niche consists of many physical and environmental variables.
Guild is a group of species that exploits the same class of environmental resource in a similar
way, e.g. ants and rodents that feed on seeds of desert plants; frugivorous monkeys and birds
in the tropical rain forest.

Their niches overlap to a large extent.
The fundamental niche is the niche a species could occupy in the absence of competitors and
other interacting species.
Realized niche is the niche it actually occupies in the presence of competitors and other
interacting species.