Sex ratio and age pyramids

Who is in the population?

One person is either a male or a female. But a population has a sex ratio, which is the ratio of males to females. For example, 60 per cent of a population may be females and 40 per cent males. In studies of people the sex ratio is often given as the number of females for every 1000 males.

Three age groups

The individuals of a population are not all of the same age. The share of the individuals of each age is the age distribution. NCERT groups the ages into three.

1. Pre-reproductive: the young ones, who cannot yet breed.
2. Reproductive: the adults, who breed.
3. Post-reproductive: the old ones, who no longer breed.

The age pyramid

A graph of the age distribution is called an age pyramid. The three groups are drawn as bars one above the other. The youngest group is at the bottom. For people, the pyramid has males on one side and females on the other, in one combined diagram.

The shape shows what will happen to the population.

• Expanding: the pre-reproductive group is the widest. The shape is a triangle with a broad base. Many young ones will reach the reproductive age, so the population will grow.
• Stable: the pre-reproductive and reproductive groups are nearly equal. The shape is like a bell. The number of births is about equal to the number of the adults that will die.
• Declining: the pre-reproductive group is small. The shape is like an urn. Fewer young ones will take the place of the adults, so the population will fall.

A rule for this lesson. We compare the share of the pre-reproductive group with the share of the reproductive group. If the first is more than 5 points larger, we call the population expanding. If the two are within 5 points of each other, it is stable. Otherwise, it is declining. This is a teaching rule for three groups. Real studies use many narrow age classes.

An A-Level and IB idea is that a population with many young people goes on growing for years, even if the birth rate falls. This is outside NCERT.

You should find that a large pre-reproductive group gives an expanding population, and a small one gives a declining population. The post-reproductive group is what is left over.

Try it in Python

The program names a population from its age groups, and draws a simple picture with the # sign, one for each per cent.

Practise with new numbers

Common mistakes

• Mixing up the shapes. A broad base is expanding. A bell is stable. An urn is declining.
• Saying that a stable population has no births. It has births. They are equal to the deaths over time.
• Saying that the pyramid shows the present size only. It shows the present shares, and so it gives a view of the future.
• Using the sex ratio for one animal. The sex ratio belongs to the population.

Remember these

• Sex ratio: males to females in the population. Often given as females for every 1000 males.
• Three age groups: pre-reproductive, reproductive and post-reproductive.
• Age pyramid: triangle means expanding, bell means stable, urn means declining.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. Name the three age groups of a population.

2. What shape of age pyramid shows a declining population?

3. A population has 40 per cent pre-reproductive, 40 per cent reproductive and 20 per cent post-reproductive individuals. What kind is it by the rule of this lesson?

4. A town has 500 males and 475 females. How many females are there for every 1000 males?

5. Why is a broad base a sign of a growing population?

References

• NCERT (2023). Biology, Textbook for Class XII, chapter 11, Organisms and Populations, section on population attributes.
• OpenStax Biology 2e (2018), chapter 45, Population and Community Ecology, section on demographics.
• Campbell Biology, 12th edition (2021), Pearson, chapter 53, Population Ecology.