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Population Interaction: Competition, Practice problems and FAQs

How is the leader of an organisation selected? There would definitely be some opponents for the winner. So from a group of people a single person is selected through some kind of competition. Right? The type of competition varies depending on the scenarios. You might have also engaged in one or the other school competitions and won a prize. Likewise competition exists among different species in a habitat for survival. What do you think they compete for?? Just like you and your sibling fight for that perfect spot on the couch, that last slice of pizza, the bigger room, etc., the species in a habitat compete over resources such as food, space, light, etc. Interesting right?? Let us learn some more interesting facts about how competition works in an ecosystem.

Competition

Competition is a type of population interaction in which different species compete over resources that they share. 

image

Amongst the competing species, fitness of one species is significantly lower in the presence of another species. Fitness here refers to the reproductive fitness of an organism which indicates its ability to reproduce and give birth to healthy and fertile offspring. The fitter and more superior species survive and are selected by nature whereas the less fit and inferior species are eliminated from the competition.

Reproductive fitness is measured in terms of ‘r’, that is, the intrinsic rate of increase in population size. 

Intrinsic rate of natural increase is the difference between birth per capita and death per capita. 

Equation for intrinsic rate

Graph showing the fitness of species in competition

Charles Darwin had described the mechanism of natural selection as survival of the fittest. He said that the survival of a species did not depend on how strong or intelligent it was but rather on how responsive it was to change. So the species which survives is naturally selected. 

Let’s try to understand this more. 

Types of Competition

Intraspecific Competition

It is the competition between individuals within a particular species. An example is the Silver gorilla. The alpha or dominant male mates with the female gorillas within a group. If another male wants to mate the female, they fight it out. 

gorillas

Fig: Gorillas competing with each other for dominance

Interspecific Competition

It is the competition between individuals of different species for a common resource. 

For example, in the South American lakes, both the visiting flamingoes and the resident fishes feed on zooplankton. In spite of being totally unrelated, these two species compete for the common food resource. So it proves that totally unrelated species could also compete for the same resource.

Flamingos and fishes feeding on the zooplankton

During spring time, birds such as thrushes, warblers, and finches sing elaborate songs not only to attract mates but also to defend their territories against rivals. In this type of competition, the birds are competing even though resources, i.e mates, are abundant. 

Interspecific competition between birds for territory

Interference Competition

The feeding efficiency of one species might be reduced due to the interfering and inhibitory presence of another species, even if resources like food, shelter, space are abundant. This is known as interference competition and shows that the competing species do not always fight over limiting resources. Competition can occur even when resources are abundant.

For instance, the Abingdon tortoise in Galapagos Islands became extinct within a decade after goats were introduced on the island, apparently due to the greater browsing efficiency of the goats. Hence, the feeding efficiency of the Abingdon tortoise was reduced due to the interfering and inhibitory presence of the goats even though resources i.e grazing land was abundant. 

Galapagos island

Experiments on Competition

Gause’s Exclusion Principle

In the 1930s, a Russian biologist, Georgy F. Gause, proposed his take on the interspecific competition in nature based on his experiments with different species of Paramoecium. Paramoecium is an aquatic single-celled ciliated protozoan that survives on a diet of bacteria, yeast, algae and other small protozoa.  

Georgy F. Gause

He cultured P. Caudatum and P. Aurelia in petri plates, one each with P. Caudatum and P. Aurelia and one combined culture. The same amount of nutrients was added to each petri plate.  

Culture plates on the first day of experiment

He counted the number of organisms in each plate everyday for 3 weeks. He observed that the petri plates with separate cultures grew exponentially. But in the petri plate with the combined culture, the number of P. Caudatum decreased while P. Aurelia increased. Hence, P. Aurelia outcompeted P. Caudatum for resources. He called this competitive exclusion

The petri dishes after three weeks

So Gause's competitive exclusion principle states that two closely related species competing for the same resources cannot coexist indefinitely and the competitively inferior one will be eliminated eventually. But this principle only holds true if the resources are present in a specific amount.

Competitive Release

The barnacle Balanus dominates the rocky intertidal regions of Scotland, and excludes the barnacle Chathamalus from that zone. 

In the 1950s, Joseph Connell conducted a classic set of removal experiments to test for competitive interactions between these two species of barnacle.

Barnacle population before removing Balanus  

He found that when he removed the superior Balanus from the area, there was an increase in the growth of the Chathamalus barnacle as the competition decreased. This phenomenon is called competitive release. 

Barnacle population after removing the Balanus

So Competitive release occurs when a species, whose distribution is restricted to a small geographical area due to the presence of a competitively superior species, expands its range of distribution dramatically upon removal of the competing superior species.

Resource Partitioning

What if the organisms do not compete with each but rather co-exist?

In another set of experiments that Gause performed, he cultured P. Caudatum and P. Bursaria with the same amount of nutrients.  

Culture plates on day 1 of the experiment

He saw that they both grew and neither reached a point of extinction i.e they co-existed. P. Caudatum feeds on the bacteria while P. Bursaria feeds on the yeast in the nutrient medium. But their growth was lesser than when they were grown separately, indicating that they still were in competition with each other. This type of interaction is called resource partitioning. 

Culture plates after some weeks of the experiment

Resource partitioning is the phenomenon where, if two species compete for the same resource, they could avoid competition by choosing different times for feeding, different foraging patterns, etc.

MacArthur’s Theory of Resource Partitioning

From his experiments, MacArthur found that different species of warblers, inhabiting a tree, divided their time of feeding differently among various parts of the tree. He also found that they had different foraging habits too.

For example, the Cape May warbler eats flying insects and hence remains on the outer side of the tree. The other warblers feed on insects in the more interior parts of the tree. The warblers also feed at different times of the day as they have somewhat different sleeping times. Thus the times of their peak food requirements are not the same. They are partitioning a limited resource, that is, their supply of insects.  

Different warbler species and their foraging habitats

Practice Problems of Population Interaction

Ques:- Which of the following statements regarding the competitive exclusion principle is incorrect?

A. It was proposed by Georgy Gause.
B. Two closely related species competing for the same resources cannot coexist indefinitely.
C. It holds good even when the resources are not limited.
C. Competitively superior species will eventually eliminate the competitively inferior ones.

Answer: Competitive exclusion principle states that two closely related species competing for the same resources cannot coexist indefinitely and the competitively inferior one will be eliminated eventually. This principle may be applicable only if the resources are limited and not otherwise. 

Hence the correct option is c.

Ques:- Assertion: Five closely related species of warblers living on the same tree were able to avoid competition and could co-exist. 

Reason: They showed behavioural differences in their foraging activities.

A. Both the assertion and reason are true. Reason is the correct explanation of assertion
B. Both the assertion and reason are true. Reason is not the correct explanation of assertion
C. Assertion is true but the reason is false
D. Both assertion and reason are false

Answer MacArthur found that five closely related species of warblers living on the same tree were able to coexist by avoiding competition. Not only did they have different feeding times for different parts of the tree, they also had different foraging habits. Due to differences in their sleeping times, the peak hour of food requirement also varied between these species of warblers. Thus, they were able to coexist by reducing the intensity of competition for the same resource i.e supply of insects. 

Hence the correct option is a.

Ques:- Which of the following population interactions is likely to be seen in an area that has a high population density of rabbits?

A. Predation
B. Interspecific competition
C. Intraspecific competition
D. Mutualism

Answer: Intraspecific competition exists between members of a particular species.

Since the population density of rabbits is high, they will eventually compete among themselves for the available resources (food, shelter, etc.) Hence, it will lead to intraspecific competition. 

Hence the correct option is c.

Ques:- A species which is restricted to a small geographical area due to the presence of another species that is competitively superior, is found to expand its distribution when the competing species is removed. This phenomenon can be termed as

A. resource partitioning
B. competitive release
C. Parasitism
D. predation

Answer A species which is distributed across a small geographical area on account of a competitively superior species, often expands its distributional range dramatically when the competing species is removed. This phenomenon is referred to as competitive release.

Hence the correct option is b.

FAQs of Population Interaction

Ques:-What are the two major types of competitions?

Answer: Two major types of competitions are intraspecific and interspecific competitions. 

Intraspecific competition is the competition that exists between members of a particular species.

Interspecific competition is the competition between individuals of different species for a common resource. 

Ques:- What is interference competition?

Answer The feeding efficiency of one species might be reduced due to the interfering and inhibitory presence of another species, even if resources like food, shelter, space are abundant. It is known as interference competition.

Ques:- Explain Gause’s exclusion principle.

Answer Gause's competitive exclusion principle states that two closely related species competing for the same resources cannot coexist indefinitely and the competitively inferior one will be eliminated eventually.

Ques:- What is resource partitioning?

Answer Resource partitioning is the phenomenon where, if two species compete for the same resource, they could avoid competition by choosing partitioning the resources by opting for different times for feeding, different foraging patterns, etc.

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