Population attributes: birth rate, death rate, sex ratio and age pyramids
A population has features no single organism has: birth rate, death rate, sex ratio and age distribution, shown as an age pyramid whose shape tells its growth status.
Attributes of a population
An individual is born and dies, but it cannot have a birth rate. A rate compares a number of events with the size of a group, so only a population can have one. Ecologists give birth and death rates per capita, which means per individual present. The main population attributes are birth rate (natality rate), death rate (mortality rate), sex ratio and age distribution.
Worked example: birth rate and death rate
A village pond had 40 water lily plants last year. Reproduction added 10 new plants this year. In a laboratory, 9 of 75 fruit flies died in one week. Find the birth rate of the lilies and the death rate of the flies.
- Birth rate = number of births in the period ÷ number at the start.
- Lilies: 10 new plants ÷ 40 plants = 0.25 offspring per plant per year.
- Death rate = number of deaths in the period ÷ number at the start.
- Flies: 9 deaths ÷ 75 flies = 0.12 deaths per fly per week.
Birth rate = 0.25 per plant per year; death rate = 0.12 per fly per week. Always state the time period.
Sex ratio
Sex ratio is the proportion of females to males in a population, for example 60 per cent females and 40 per cent males. It matters because the number of females largely decides how many young can be born. India's census reports sex ratio as the number of females per 1,000 males.
Age pyramids: what the shape tells you
| Shape | What it shows | Growth status |
|---|---|---|
| Broad base, narrow top (triangular) | Many pre-reproductive young; few old individuals | Expanding: the population is growing |
| Pre-reproductive and reproductive bands nearly equal (bell-shaped) | Young individuals just replace the adults | Stable: size stays about the same |
| Narrow base, wider middle (urn-shaped) | Fewer young than reproductive adults | Declining: the population is shrinking |
How to draw age pyramids
- Draw a vertical line down the middle. Put males on the left and females on the right.
- Stack horizontal bars for the age groups, the youngest at the bottom and the oldest at the top.
- Mark three bands from the bottom: pre-reproductive, reproductive and post-reproductive.
- Draw three figures side by side. (a) A wide base narrowing to a point: label it Expanding. (b) Nearly straight sides, narrowing only near the top: label it Stable. (c) A narrow base with a bulging middle: label it Declining.
- Write the title: Age pyramids of a human population.
India has a large share of young people, so its age pyramid has a broad base that is slowly narrowing. Planning for schools, jobs and care of the elderly depends on reading this pyramid.
1 marks: “Why can a single organism not have a birth rate?” A rate compares births with the size of a group, so only a population can have a birth rate.
2 marks: “A pond had 30 fish at the start of a year. During the year, 12 young were born. Calculate the per capita birth rate.” Birth rate = births ÷ starting number = 12 ÷ 30 = 0.4 offspring per fish per year.
3 marks: “Draw age pyramids of expanding, stable and declining populations. What does each show?” Three figures with pre-reproductive, reproductive and post-reproductive bands, males on the left, females on the right. Triangular (broad base): growing. Bell-shaped: stable. Urn-shaped (narrow base): declining.
Triangle grows, Bell stays, Urn goes: expanding, stable, declining.