(a) (i) Write the observations made at the end of Connell's field experiment on barnacles on the rocky sea coa

Question

Q. (a)

(i) Write the observations made at the end of Connell's field experiment on barnacles on the rocky sea coasts of Scotland.

(ii) Name any two categories of organisms that in general are adversely affected by competition.

(OR)

(b) The graphs (X) and (Y) given below depict the diurnal variations in the solar radiations in the month of June (Summer) and in December (Winters) :

(i) Which of the two graphs depicts tropical region and temperate regions respectively ?

(ii) Which of the two regions (X) or (Y) will show high biological diversity and why?

CBSECBSE Class XII Board 2022Subjective· 2mImportance★★★★★

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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.

Note

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.

Important

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 — Biodiversity Explanation

Biodiversity: The Web of Life Around You

Imagine walking into a kitchen garden. You see a few tomato plants, some mint, maybe a rose bush. Now imagine walking into a forest — hundreds of tree species, birds calling from every direction, insects crawling under leaves, fungi on the bark, and a stream with tiny fish. That staggering variety of living things — from the tallest tree to the invisible bacteria in the soil — is what we call biodiversity.

The Simple Intuition

Biodiversity is short for biological diversity. Think of it as the "variety of life" on Earth. Just as a library with only one kind of book is boring and limited, an ecosystem with only one kind of plant or animal is fragile and poor. Biodiversity is nature's library — the more species, the richer and more stable the system.

The Precise Meaning

The NCERT textbook defines biodiversity as:

The variety of living organisms present on Earth, including the different plants, animals, fungi, and microorganisms, their genes, and the ecosystems they form.

This definition has three layers, and each matters:

Genetic diversity — variation within a species. For example, all humans are one species, but we have different eye colours, heights, and resistance to diseases. In crops, genetic diversity means some wheat varieties resist drought while others resist pests.
Species diversity — the number of different species in a region. A tropical rainforest has thousands of species; a cold desert has very few.
Ecological diversity — the variety of habitats, ecosystems, and communities. A forest, a grassland, a coral reef, and a pond are all different ecosystems.

Important

Biodiversity is not just about counting species. It includes the genes inside each species and the ecosystems they create together. Lose one layer, and the others suffer.

Why Does Biodiversity Matter?

For a commerce or humanities student, the question is natural: why should I care about how many beetles live in a forest? The answer is that biodiversity is the foundation of everything we use and consume.

Direct economic value — We depend on biodiversity for food (crops, fish, livestock), medicine (many drugs come from plants and fungi), raw materials (timber, cotton, rubber), and tourism (wildlife sanctuaries, national parks).

Indirect value — This is harder to see but more critical. Forests clean the air, wetlands purify water, bees pollinate crops, and soil microbes recycle nutrients. These are called ecosystem services. If biodiversity collapses, these services stop — and no technology can replace them at scale.

Ethical and cultural value — Every species has a right to exist, regardless of its usefulness to humans. Many cultures, especially in India, revere certain trees, animals, and rivers as sacred. Biodiversity is also a source of beauty, inspiration, and knowledge.

Note

The NCERT textbook emphasises that biodiversity is our life insurance. It gives us resilience — when one species fails due to disease or climate change, another can take its place. A system with low biodiversity (like a wheat monoculture) is one disease away from collapse.

A Quick Look at India's Biodiversity

India is one of the world's mega-diverse countries. Though it has only 2.4% of the world's land area, it contains about 8% of all known species. This includes:

Over 45,000 plant species
More than 90,000 animal species
10 biogeographic zones — from the Himalayas to the Western Ghats to the Andaman Islands

This richness is not accidental. India's varied climate, geography, and ancient landmass have allowed many species to evolve and survive here.

The Threat: Why We Talk About It …

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Part (a): Connell's barnacle experiment showed Balanus competitively excludes Chthamalus from the lower intertidal; competition most affects organisms sharing resources such as herbivores and carnivores.

Part (b): The graph with little seasonal change is the tropical region and the strongly seasonal graph is the temperate region; the tropical region is more diverse because of constant high energy and long evolutionary stability.

Part (a)

Concept-first idea: Interspecific competition occurs when two species need the same limiting resource; the superior competitor can exclude the other from part of its potential range.

(i) Connell's observations. On the rocky Scottish coast, Connell studied two barnacles: Chthamalus (upper shore) and the larger Balanus (lower shore). He found:

Balanus dominates the lower, wetter intertidal and competitively excludes Chthamalus there (by overgrowing/undercutting it).
When Balanus was experimentally removed, Chthamalus was able to colonise the lower zone too.
Thus the lower limit of Chthamalus is set by competition, not by physical tolerance — competition can restrict a species to a smaller "realised niche." …

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Same / Similar Concept — real previous-year questions on the same or a closely similar concept, not this exact question.

Showing the 12 most recent of 45 on this concept.

CBSE 2026Set A1 markMCQ
$Q. How many mega-biodiversity countries are there in the world?$
(a) 25
(b) 34
(c) 14
(d) 17
›Reveal solutionSolution

The world has 17 megadiverse countries that together hold most of the Earth's species; India is one of them.

A small number of countries lying largely within the tropics harbour a very large share of the world's biodiversity. These are termed megadiverse (mega-biodiversity) countries and number 17, including …

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CBSE 2026Set A1 markMCQ
$Q. Which region got comparatively shorter span of time for species evolution and diversification due to frequent glaciation?$
(a) temperate region
(b) tropical region
(c) polar region
(d) glaciation occurred in all regions
›Reveal solutionSolution

Tropics stayed relatively undisturbed and had more evolutionary time; temperate regions faced frequent glaciations, giving less time for diversification.

One explanation for the greater species richness of the tropics is time: tropical latitudes have remained relatively undisturbed for millions of years, providing a long, stable period for species to evolve and diversify. Temperate regions, in contrast, were subj …

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CBSE 2026Set ANNUAL1 mark
$Q. Fill in the blank: When a species become extinct, the plant and animal species associated with an obligatory way also become extinct. It is called ______.$
›Reveal solutionSolution

The linked extinction of dependent species along with the species they rely on is called co-extinction.

Many species are so closely dependent on another species (for food, host or partner) that they cannot survive without it. When such a host or partner species becomes extinct, the plants and animals that were obligately associated with it also die out. This linked loss is called co-extinction, for exa …

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CBSE 2026Set ANNUAL1 markMCQ
$Q. The possible reason for extinction of Steller's sea cow is due to -$
(a) Alien species invasion
(b) Over exploitation
(c) Habitat Loss
(d) Co-extinction
›Reveal solutionSolution

Steller's sea cow became extinct because humans hunted it excessively — an example of over-exploitation.

Over-exploitation is one of the major causes of biodiversity loss ("The Evil Quartet"): when a resource is harvested by humans faster than it can replenish, the species declines to extinction. Steller's sea cow (Hydrodamalis gigas) was a large, docile marine mammal that was kill …

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CBSE 2026Set ANNUAL1 markMCQ
$Q. Two species competing for the same resource can avoid competition by choosing different habits. This phenomenon is called _________ and was supported by _________.$
(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. …
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CBSE 2025Set F1 markMCQ
$Q. Where are woody lianas / climbers found?$
(a) Taiga
(b) Deciduous forest
(c) Tropical rain forest
(d) Shrub (chaparral)
›Reveal solutionSolution

Woody lianas (large climbers) are typical of tropical rain forests.

Lianas are thick, woody climbing plants that root in the ground but use tall trees to reach sunlight in the canopy. They flourish in the warm, humid, densely vegetated tropical rain forest, which has the highest biodiversity and vertical stratification of any biome. Taiga (coniferous), temperate deciduous fore …

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CBSE 2025Set F1 markMCQ
$Q. Which of the following terms is used for the population found in a particular area?$
(a) Natural
(b) Endemic
(c) Alien
(d) Pandemic
›Reveal solutionSolution

Species/populations confined to a particular area are termed endemic.

Endemic refers to a species or population whose natural occurrence is restricted to one particular geographic area and is found nowhere else (for example, many species endemic to the Western Ghats). "Alien" species, by contrast, are introduced from outside a region, and "pandemic" refers …

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CBSE 2025Set ANNUAL1 mark
$Q. Name the type of population interaction seen in the following example: Abingdon tortoise and goat.$
›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 …

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CBSE 2025Set ZOOLOGY1 markMCQ
$Q. Which term is used for the conservation of biodiversity outside the natural habitat?$
(i) In vivo
(ii) Ex situ
(iii) In situ
(iv) In vitro
›Reveal solutionSolution
Ex situ conservation protects biodiversity outside the natural habitat (zoos, botanical gardens, seed/gene banks, etc.).

Conservation strategies are of two main types:

In situ conservation — protecting species within their natural habitat (national parks, sanctuaries, biosphere reserves).

Ex situ conservation — protecting species outside their natural habitat, e.g. zoological parks, botanical gardens, wildlife safari parks, seed banks, gene banks, cryopreservation and tissue culture. …
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CBSE 2024Set E1 markMCQ
$Q. Biodiversity of India is what percentage of the total global species diversity?$
(a) 2.4%
(b) 8.1%
(c) 5%
(d) 9%
›Reveal solutionSolution

India holds 8.1% of global species diversity on just 2.4% of the world's land area — making it a megadiversity country.

Despite occupying only about 2.4% of the world's land area, India accounts for 8.1% of the total global species diversity. This makes India one of the 17 mega-diverse countries of the world. The contrast between th …

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CBSE 2024Set E1 markMCQ
$Q. What was the reason for co-existence of five closely related species of warblers?$
(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:

…

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CBSE 2024Set D1 mark
$Q. Match the following and write the correct pair. Column 'A':$
(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 (−,−).

…

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Q.(a)

Part (a)

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