Interspecific competition

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)        N = number of individuals;            dN  =  instantaneous rate of change
dt                 K1             r = biotic potential                          dt
t  = time
K= carrying capacity

(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:

• Resources are short supply for competition to take place.
• Competitors must remain genetically unchanged for a sufficiently long period to time for one species to exclude the other.
• Immigrants from areas with different conditions cannot move into the population of the losing species.
• Environmental conditions must remain the same.
• 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.

• 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.