Population Attributes and Age Pyramids: ISC Class 12 Biology

Understand Population Attributes and Age Pyramids

Population Attributes and Age Pyramids: key facts

Natality

$b=\frac{\text{number of births}}{\text{initial population}}$

Per individual per unit time; as a decimal or %.

Mortality

$d=\frac{\text{number of deaths}}{\text{initial population}}$

Same time unit as natality.

Sex ratio

$\%\text{ females}=\frac{F}{F+M}\times 100$

Or as females per 1000 males.

Mark-recapture estimate

$N\approx \frac{M\times C}{R}$

M marked first, C caught second time, R marked among them.

How to answer Population Attributes and Age Pyramids questions

1. 1Write the formula first, then substitute; examiners give marks for both.
2. 2Births = final − initial only if there were no deaths or migration; read the question.
3. 3Divide by the initial population and state the unit (per individual per year) or give a percentage.
4. 4For pyramids, compare the pre-reproductive base with the reproductive middle to decide expanding, stable or declining.
5. 5For density, choose the measure that fits: numbers, per cent cover or biomass, relative density, or indirect signs.
6. 6In mark-recapture sums, check that the proportion marked in the second catch equals M/N.

Population Attributes and Age Pyramids in real life

The idea, met in an everyday situation. Answer the question in your own words, then compare with the model answer. The one-line hook is what to remember.

Tricks to remember Population Attributes and Age Pyramids

Short memory aids. Say each one aloud once, then use it on a question the same day.

• T-B-U = Grow, Stay, Go

  Triangle grows, Bell stays, Urn goes down.

  Use it when: Pyramid status.

• Divide by the start

  Per capita rates always divide by the starting number.

  Use it when: Natality and mortality sums.

• Count, Cover, Catch, Clues

  Numbers, per cent cover/biomass, catch per trap, pug marks and pellets.

  Use it when: Measuring density.

Population Attributes and Age Pyramids: solved examples

Ordered from board level to HOTS. Try each one before you open the solution.

Where students lose marks in Population Attributes and Age Pyramids

The slips examiners see most often, and what to write instead.

• ✗Mistake: Giving only the difference in numbers

  In 2024 examiners reported that candidates only subtracted the original from the final population and did not express natality as an annual rate per individual.

  ✓Do this: Natality = births ÷ initial population; give a per capita rate (such as 0.3 per individual per year, or 30%), not the bare difference in numbers.

  Usually costs: 1 mark.

• ✗Mistake: Not writing the formula

  Examiners (2024) advised writing the formula first, then substituting values.

  ✓Do this: Formula, substitution, answer with unit.

  Usually costs: Method mark.

• ✗Mistake: Wrong pyramid for the status

  Calling a bell-shaped pyramid 'expanding' is a common slip.

  ✓Do this: Base broad = expanding; base equal = stable; base narrow = declining.

  Usually costs: 1 mark.

• ✗Mistake: Dividing by the final population

  Per capita rates use the starting population.

  ✓Do this: Always divide by the initial N.

  Usually costs: 1 mark.

Practice questions on Population Attributes and Age Pyramids

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

1. Q1 · 1 mark · MCQBoard level

   An urn-shaped age pyramid indicates a population that is:
   (a) expanding (b) stable (c) declining (d) not reproducing

   Hint

   Narrow base.

   Answer

   (c) declining.

2. Q2 · 2 marks · Short answerBoard level

   Define natality and mortality. Why are they called attributes of a population and not of an individual?

   Hint

   Per capita rates.

   Answer

   Natality is the number of births in a population during a given period; mortality is the number of deaths in that period (both usually expressed per capita). An individual is born and dies once, but only a population has rates of birth and death.

3. Q3 · 1 mark · Assertion-reasonHarder board level

   Assertion (A): For a banyan tree in a field of carrot grass, per cent cover is a better measure of population size than number.
   Reason (R): A single banyan can have a far larger role in the community than many small plants.
   (a) Both A and R are true and R is the correct explanation of A.
   (b) Both A and R are true but R is not the correct explanation of A.
   (c) A is true but R is false.
   (d) A is false but R is true.

   Hint

   Size matters.

   Answer

   (a).

4. Q4 · 2 marks · Short answerHarder board level

   A town of 1500 people records 120 births and 45 deaths in a year. Find the birth rate, death rate and per capita growth rate.

   Hint

   Divide by 1500.

   Answer

   b = 120/1500 = 0.08; d = 45/1500 = 0.03; r = 0.08 − 0.03 = 0.05 per person per year.

5. Q5 · 2 marks · Short answerHarder board level

   Eight 1 m² quadrats in a 500 m² meadow contain 96 daisy plants in all. Estimate the density and total number of daisies.

   Hint

   Mean per quadrat.

   Answer

   Density = 96/8 = 12 per m²; total ≈ 12 × 500 = 6000 daisies.

6. Q6 · 2 marks · Short answerHarder board level

   How is the tiger census in Indian reserves usually carried out, and why is a direct count not used?

   Hint

   Signs left behind.

   Answer

   It is based on indirect evidence such as pug marks and faecal pellets. Tigers are elusive, range widely and are dangerous to approach, so direct counting is impractical.

7. Q7 · 3 marks · Short answerHOTS / competency

   Suppose two countries have equal populations today. Country X has a triangular age pyramid and country Y an urn-shaped one. Predict their populations after 30 years and give two different social challenges each will face.

   Hint

   Young versus ageing.

   Answer

   X will grow because a large young group will enter the reproductive age; Y will shrink and age because few young replace adults. X: need for more schools, jobs, food and housing; pressure on resources. Y: shortage of working-age people; higher costs of care and pensions for the elderly.

8. Q8 · 3 marks · Case-basedHOTS / competency

   Read and answer.
   Suppose the number of water hyacinth plants in a lake is 80 at the start of a year and 104 at the end; no plants died or were removed.
   (a) Calculate the natality. (1)
   (b) Express it as a percentage. (1)
   (c) Why should the formula be written before substituting values? (1)

   Hint

   Births ÷ initial.

   Answer

   (a) Births = 104 − 80 = 24; natality = 24/80 = 0.3 per plant per year. (b) 30%. (c) Examiners award marks for the correct formula and method; writing it first also prevents the common error of giving only the difference (24) instead of a per capita rate.

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1. 1.Shape of a stable population's age pyramid?Show answer

   Bell-shaped.

2. 2.What does natality mean?Show answer

   Number of births in a population during a given period.

3. 3.How is tiger population estimated?Show answer

   From pug marks and faecal pellets.

4. 4.What is relative density?Show answer

   A comparative measure such as fish caught per trap, used when absolute numbers are not needed.

5. 5.Three age groups in an age pyramid?Show answer

   Pre-reproductive, reproductive and post-reproductive.

6. 6.30 deaths among 250 beetles in a month: death rate?Show answer

   0.12 per beetle per month.

Population Attributes and Age Pyramids: frequently asked questions