Q.(a) In some cases, measuring population density by counting individuals may not be feasible. Mention two other parameters that can be used and justify their use with suitable examples.
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🔒 Start your 14-day free trial to unlock the full solution →Part (a)Concept understanding — Population Level Properties
Population Level Properties: A First Look
Think about a single person. You can describe them: tall, short, speaks Hindi, works as a teacher, has two children. Now think about a whole village or a whole country. You cannot describe every single person one by one — but you can still say something meaningful about the group as a whole. That is what population level properties are: characteristics that belong to the collection of individuals, not to any one individual.
The Everyday Intuition
Imagine you are looking at a crowd at a railway station. You cannot know each person's name, age, or destination. But you can say: "About 60% of the people here are men" or "The average age seems to be around 30 years." These statements are true of the crowd, not of any single person in it. That is a population level property.
Now take a more personal example. Your family has three generations living together. No single member of your family is "the average" of the family — the grandmother is old, the child is young, the parents are middle-aged. Yet the average age of the family is a real, useful number that describes the family as a whole. That number is a population level property.
The Precise Meaning
In the NCERT textbook for Class 12 Biology (Chapter 11: Organisms and Populations), population level properties are defined as attributes that emerge only when individuals come together to form a population. They do not exist at the level of the individual organism.
Population level properties are emergent properties — they arise from the group, not from any single member. You cannot observe them by studying one individual alone.
The textbook gives a clear example: birth rate. An individual is born or not born — that is a binary event. But a population has a birth rate: the number of births per thousand individuals per year. That rate is a property of the population, not of any one person. Similarly, death rate, sex ratio, and age distribution are all population level properties.
Why This Matters
Understanding population level properties is essential because:
- They allow comparison. You can compare the birth rate of India with that of Japan, even though no two individuals in either country are identical.
- They reveal trends. A population with many young individuals will grow differently from one with many elderly individuals. This tells you about future needs — schools, hospitals, jobs.
- They are the basis of policy. Governments do not make policies for "the average person" — they make policies based on population level data like literacy rate, infant mortality rate, and dependency ratio.
Key Population Level Properties (as per NCERT)
The textbook lists four main properties that every population has:
- Birth rate (natality): The number of new individuals added per unit time per existing individual. It is always a rate, not a count.
- Death rate (mortality): The number of deaths per unit time per existing individual. Again, a rate.
- Sex ratio: The number of females per thousand males (or vice versa). This is a ratio, not a count.
- Age distribution: The proportion of individuals in different age groups — pre-reproductive, reproductive, and post-reproductive. This is often shown as an age pyramid. …
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.
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.
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 …
Part (a)
When counting every individual is not feasible, population density can be estimated by:
- Relative density — an index instead of an absolute count, e.g., the number of fish caught per trap gives relative density of fish in a lake. …
Part (a): When individuals cannot be counted, use relative density (e.g., fish per trap) or indirect signs (pug marks/faecal pellets for tigers); biomass is another option.
Part (b): The Amazon is the most biodiverse region because of constant tropical warmth and solar energy, high productivity, a long undisturbed evolutionary history and abundant niches.
Part (a)
Concept-first idea: Population density is normally 'number per unit area', but for some species direct counting is impossible, so ecologists use substitute parameters that still reflect abundance.
Two parameters (with justification):
- Relative density — instead of an absolute count, an index of abundance is used. For example, the number of fish caught per trap (per unit effort) in a lake gives the relative density of that fish, since counting all fish is impossible.
- Indirect estimation from signs of presence — for elusive animals, presence markers are counted. For example, the tiger population in a forest is estimated from pug marks and faecal pellets, since tigers cannot be counted directly. …
Showing the 12 most recent of 63 on this concept.
- CBSE 2026Set A1 markMCQQ.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 …
- CBSE 2026Set A1 markMCQQ.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 …
- CBSE 2026Set A1 markMCQQ.Which of the following processes decreases population density?(a) Immigration(b) Death(c) Emigration(d) Both (B) and (C)
›Reveal solutionSolution
Population density falls due to mortality (death) and emigration.
Population density changes through four processes. Two increase it — natality (births) and immigration (individuals coming in) — and two decrease it — mortality (deaths) and emigration (individuals leaving the population). Immigration adds individuals, so it …
- CBSE 2026Set ANNUAL1 markQ.Define immigration.
›Reveal solutionSolution
Immigration is one of the four basic factors that regulate population density, adding new individuals from outside into a population.
Population density is regulated by four basic processes: natality (births), mortality (deaths), immigration and emigration. Immigration refers to the number of individuals of the same species that have come into the habitat from elsewhere during a given time period under consideration; it increases th …
- CBSE 2026Set ANNUAL1 markMCQQ.Factors contributing to population growth are(a) natality and immigration(b) natality and emigration(c) mortality and emigration(d) immigration and emigration
›Reveal solutionSolution
Natality and immigration are the two processes that add individuals to a population, thereby driving its growth.
A population's density/size is governed by four processes: natality (births) and immigration (individuals entering the population from elsewhere) both add individuals and therefore contribute to population growth, while mortality (deaths) and emigration (individuals leaving the population) both remove individuals and reduce po …
- CBSE 2026Set ANNUAL1 markQ.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 …
- CBSE 2026Set ANNUAL1 markMCQQ.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 …
- CBSE 2025Set F1 markMCQQ.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 …
- CBSE 2025Set F1 markMCQQ.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 …
- CBSE 2025Set F1 markMCQQ.Which of the following refers to the scientific study of human population?(a) Geography(b) Geology(c) Biology(d) Demography
›Reveal solutionSolution
The study of human populations and their attributes is demography.
Demography is the scientific study of populations, especially human populations — their size, density, distribution, birth and death rates, age and sex structure, and growth. Population attributes analysed in ecology (population growth, age pyramids) draw on demographic data.
…
- CBSE 2025Set ANNUAL1 markMCQQ.In population attributes the ratio of males and females is called -(a) Birth rate(b) Death rate(c) Sex ratio(d) Population density
›Reveal solutionSolution
Sex ratio is one of the key population attributes, expressing the relative number of males and females.
A population is characterised by attributes such as birth rate (natality), death rate (mortality), age distribution, population density and sex ratio. Sex ratio specifically describes the ratio of the number of males to females (or females to males) within the population at a given time, which affects the population's reproductive potential. Birth …
- CBSE 2025Set ANNUAL1 markQ.Write one factor which increases population density.
›Reveal solutionSolution
Population density increases when the rate of addition of new individuals (through births or immigration) exceeds the rate of loss.
A population's density can grow due to natality (the birth rate — the number of new individuals born per unit time) and/or immigration (individuals of the same species entering the habitat from elsewhere). Of the four factors governing population density (natality, mortality, immigration, emigration …
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