Q.The diagram below shows the distribution of two barnacle species, Chthamalus and Balanus on a rocky sea shore. When Balanus is experimentally removed, Chthamalus expands its range in lower intertidal zone.
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🔒 Start your 14-day free trial to unlock the full solution →Part (a)Concept understanding — Interspecific Competition
Interspecific Competition: A Simple Introduction
Imagine you and your neighbour both run small bakeries on the same street. You both sell cakes, pastries, and bread. If your neighbour starts selling a new, cheaper cake that your customers love, you will lose some business. You are both competing for the same thing — customers. Now, replace "you and your neighbour" with two different species of plants or animals, and replace "customers" with resources like food, water, space, or sunlight. That is interspecific competition.
The Precise Meaning
In biology, interspecific competition is the struggle between individuals of two different species for a common resource that is limited. The key word is interspecific — it means "between species." (Competition within the same species is called intraspecific competition, but that is a different topic.)
The NCERT textbook (Class 12 Biology, Chapter 13) defines it clearly: "When the populations of two different species compete for the same resource, it is called interspecific competition." The resource could be food, water, nesting sites, light, or even space. The competition is most intense when the two species have very similar needs — that is, when their niches overlap heavily.
A niche is not just a habitat (where an organism lives). It is the role of the species — what it eats, when it is active, how it reproduces, and what resources it uses. Two species with identical niches cannot coexist indefinitely. This is called the Competitive Exclusion Principle (Gause's Principle).
Why Does It Matter?
Interspecific competition is a powerful force in nature. It shapes which species live where, how many individuals survive, and even how species evolve over time. Here are the main consequences:
- One species may be eliminated locally. If one species is a stronger competitor, the weaker one may be pushed out of that area. This is called competitive exclusion.
- Both species may suffer. Even if neither is eliminated, both may have reduced growth, fewer offspring, or lower survival rates because they are spending energy competing.
- Species may evolve to avoid competition. Over many generations, species can develop differences in behaviour, diet, or activity times — a process called resource partitioning. For example, two bird species that eat the same insects might start feeding at different times of day or in different parts of the same tree.
Interspecific competition is not always direct. It can be interference competition (one species actively prevents another from accessing a resource, like a lion chasing away hyenas from a kill) or exploitative competition (both species use the same resource, but one uses it more efficiently, leaving less for the other — like two plant species competing for the same soil nutrients).
A Real-World Example from NCERT …
Part (b)Concept understanding — Interspecific Competition
Interspecific Competition: A Simple Introduction
Imagine you and your neighbour both run small bakeries on the same street. You both sell cakes, pastries, and bread. If your neighbour starts selling a new, cheaper cake that your customers love, you will lose some business. You are both competing for the same thing — customers. Now, replace "you and your neighbour" with two different species of plants or animals, and replace "customers" with resources like food, water, space, or sunlight. That is interspecific competition.
The Precise Meaning
In biology, interspecific competition is the struggle between individuals of two different species for a common resource that is limited. The key word is interspecific — it means "between species." (Competition within the same species is called intraspecific competition, but that is a different topic.)
The NCERT textbook (Class 12 Biology, Chapter 13) defines it clearly: "When the populations of two different species compete for the same resource, it is called interspecific competition." The resource could be food, water, nesting sites, light, or even space. The competition is most intense when the two species have very similar needs — that is, when their niches overlap heavily.
A niche is not just a habitat (where an organism lives). It is the role of the species — what it eats, when it is active, how it reproduces, and what resources it uses. Two species with identical niches cannot coexist indefinitely. This is called the Competitive Exclusion Principle (Gause's Principle).
Why Does It Matter?
Interspecific competition is a powerful force in nature. It shapes which species live where, how many individuals survive, and even how species evolve over time. Here are the main consequences:
- One species may be eliminated locally. If one species is a stronger competitor, the weaker one may be pushed out of that area. This is called competitive exclusion.
- Both species may suffer. Even if neither is eliminated, both may have reduced growth, fewer offspring, or lower survival rates because they are spending energy competing.
- Species may evolve to avoid competition. Over many generations, species can develop differences in behaviour, diet, or activity times — a process called resource partitioning. For example, two bird species that eat the same insects might start feeding at different times of day or in different parts of the same tree.
Interspecific competition is not always direct. It can be interference competition (one species actively prevents another from accessing a resource, like a lion chasing away hyenas from a kill) or exploitative competition (both species use the same resource, but one uses it more efficiently, leaving less for the other — like two plant species competing for the same soil nutrients).
A Real-World Example from NCERT …
Part (a)
Ecological phenomenon shown by the barnacle experiment. When Balanus is removed and Chthamalus then spreads into the lower zone, it shows that Balanus had been keeping Chthamalus out - i.e. interspecific competition (competitive exclusion).
Definition: interspecific competition is the interaction between individuals of two different species that compete for the same limited resource (here, space on the rock), such that one species suppresses or excludes the other. The competitively superior Balanus restricts Chthamalus to the upper zone. …
Part (a): the barnacle experiment demonstrates interspecific competition (competitive exclusion) - Balanus excludes Chthamalus from the lower zone. Part (b): the elimination is explained by Gause's competitive exclusion principle; another example is flamingoes vs fishes competing for zooplankton; species avoid competition by resource partitioning (MacArthur's warblers).
Part (a)
Identifying the phenomenon. In the classic barnacle study, Balanus occupies the lower intertidal zone and Chthamalus the upper. When Balanus is experimentally removed, Chthamalus spreads down into the lower zone as well. This shows that Balanus had been preventing Chthamalus from occupying that zone - a clear demonstration of interspecific competition (specifically, competitive exclusion). …
- CBSE 2026Set ANNUAL1 markMCQQ.Two species competing for the same resource can avoid competition by choosing different habits. This phenomenon is called _________ and was supported by _________.(a) Competitive exclusion, Gause(b) Competitive exclusion, MacArthur(c) Resource partitioning, Gause(d) Resource partitioning, MacArthur
›Reveal solutionSolution
Avoiding competition by using the same resource differently is resource partitioning, shown by MacArthur; Gause's principle is competitive exclusion.
Two close ideas are often confused in the CBSE/NCERT Organisms and Populations chapter:
- Competitive exclusion (Gause's principle): two species competing for the same limiting resource cannot coexist indefinitely; the competitively inferior one is eliminated. …
- CBSE 2025Set ANNUAL1 markQ.Name the type of population interaction seen in the following example: Abingdon tortoise and goat.
›Reveal solutionSolution
Goats introduced to the Galápagos competed with native giant tortoises for the same vegetation, illustrating interspecific competition.
On the Galápagos (Abingdon/Pinta) island, goats were introduced by humans and rapidly multiplied, consuming large quantities of the same vegetation that the native Abingdon giant tortoises depended on for food. Since both species required the same limited food resource, and the presence of one (the fast-breeding, voracious goats) negatively impacted the survival/population of the other (the tortoises, whose numbers declined sharply, contributing to their eventual extinction on that is …
- CBSE 2024Set E1 markMCQQ.What was the reason for co-existence of five closely related species of warblers?(a) Competitive exclusion(b) Resource partitioning(c) Parasitism(d) Commensalism
›Reveal solutionSolution
Five warbler species coexist through RESOURCE PARTITIONING, dividing up the tree so they do not compete directly.
By the competitive exclusion principle, two species competing for exactly the same limiting resource cannot coexist indefinitely. MacArthur's study of five closely related warbler species on the same tree showed how they avoid this:
…
- CBSE 2024Set D1 markQ.Match the following and write the correct pair. Column 'A':(i) Structural genes(ii) T-Lymphocytes(iii) Competition(iv) PBR322(v) Corona radiata. Column 'B':(a) bone marrow(b) vectors(c) ovum(d) detrimental interaction(e) z, y and a(f) RBCs. Find the correct match for item(iii) 'Competition' from Column 'B'.
›Reveal solutionSolution
'Competition' pairs with option (d) 'detrimental interaction' — because in competition, both interacting species are adversely affected.
In population ecology, interspecific interactions are classified by their effect (+, −, or 0) on each of the two interacting species: mutualism (+,+), commensalism (+,0), parasitism/predation (+,−), amensalism (−,0), and competition (−,−).
…
- CBSE 2023Set ANNUAL1 markMCQQ.Allelopathy occurs due to the secretion of which?(a) nectar(b) vitamins(c) toxin(d) pheromone
›Reveal solutionSolution
Allelopathy is due to secretion of toxins (allelochemicals).
Allelopathy is the phenomenon in which one plant releases chemical substances (allelochemicals/toxins) into the environment that inhibit the germination or growth of neighbouring plants of other species, reducing competition. A classic example is the inhibition of nearb …
- CBSE 2017Set ANNUAL1 markQ.What is Resource Partitioning?
›Reveal solutionSolution
Resource partitioning lets ecologically similar species avoid competitive exclusion by dividing up a shared resource.
According to the principle of competitive exclusion, two species competing for the exact same limiting resource cannot coexist indefinitely — one will eventually eliminate the other. Resource partitioning describes how, in nature, species that appear to compete for a resource often avoid actual competition by dividing that resource, for instance by feeding at different times of day, foraging at different heights or locations, or consuming slightly different types/sizes of food. MacArthur's study of five warbler species feeding on the same tree but in differen …
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