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Principles of Population Growth and Population Pyramid

Grade 9
Jun 7, 2023

Principles of Population Growth


Population: Organisms with similar groups of species in a specific area in which they are able to interbreed are called population.

Population (herd of cows)

Population growth: It is the increase in any group of organisms in a specific area over time.

The capacity of organisms to interbreed increases gradually.


Population growth

When resources are few, exponential growth takes the form of a J-shaped curve.

The best example seen is the growth of bacteria.

If you put 1000 bacteria in a flask with an abundant supply of nutrients, the consumption of nutrients will not deplete quickly but the numbers of bacteria will double by binary fission very quickly. Here there is growth of the population is exponential with respect to time.

Logistic Growth:

Population growth slows as resources become scarce, and it comes to a halt when the environment’s carrying capacity is reached. The logistic growth curve is in the shape of a S.


For example, the growth level of yeast off as the population depletes the nutrients that are necessary for its growth.

Factors Affecting Population Growth are as follows:

Natality: Number of births per unit population per unit time.

Mortality: Number of deaths per unit population per unit time.

Immigration: Addition of organisms from other populations.

Emigration: Movement of an organism

Population Pyramid:

It is exhibited in a pyramidal structure where the population is on the x-axis and age is on the y-axis.

The three main types of population pyramids are expanding pyramid, the constrictive pyramid, and the stable pyramid.

Expansive Pyramid:

It has a triangular (pyramid with a broad base) shape and is expanding.

Expanding age Pyramid

Each age group bar displays shorter than the one preceding it.

It states that more people die as we progress through the pyramid.

Constrictive Pyramid:

It is the opposite of an expansive pyramid. It depicts a lower rate of young people.

Constrictive Pyramid

Stable Pyramid:

Its bell-shaped polygon form indicates that people of all ages are distributed equally. As the growth rate slows and evens out, the pre-reproductive and reproductive age groups become roughly equal in size, with the post-reproductive age group remaining the smallest.

Stable pyramid

Explanation of Different Attributes of Population:

Population Attributes

  • The number of populations changes from time to time.
  • The changing nature of the population is called as dynamics of the population.

Natality: Natality birth rate is the rate at which new individuals are born in a population per unit of time.


Natality is a parameter of the entire population.

Mortality: The mortality rate is the number of individuals of a population loss due to death per unit of time.

Death rate

Mortality decreases the size of the population

Population density: It measures how crowded the population is.

Larger animals generally have lower population densities.

Population density=number of organisms in a region(N)/number of unit area in the region(S),

Therefore PD=N/S.

Age distribution: The population consists of different age groups such as pre-reproductive, reproductive, and post-reproductive stages.

  • In an expanding pyramid, the young population birth rate is high, and the growth of the population is also high.
  • In stable pyramid, the growth rate becomes slow and steady, i.e., the pre-reproductive and reproductive age groups become basically equivalent in size, the post-reproductive group remaining the smallest.
  • Declining suggests a low percentage of young individuals and shows a declining population. When the birth rate is significantly reduced, the pre-reproductive group decreases in proportion to the other two ages of the population, resulting in an un-shaped figure.

Age group Pyramid

Sex Ratio

It is the number of males to females in a population.

For example, if a population has 30% of males to 70% of females among the total individuals of the population, then the sex ratio is 3:7.

Malthusian Theory of Population

  • He was known for his population growth philosophies
  • In 1798 he published his opinions on the effect of population on food supply.
  • His theory has two basic principles
  1. Population grows at geometric rates, i.e., 1, 2, 4, 8, 16, etc.
  2. Food production increases at an arithmetic rate, i.e., 1, 2, 3, 4, etc.

Major Elements of Malthusian Theory

  • Population and Food Supply

Supply of food increases at a slower rate than the population. This means there will be a shortage in food supply in a few years.

A shortage of food supply means the population is increasing.

  • Checks on Population

If the rate of increasing population is greater than the food supply, then disequilibrium will exist. People will die due to lack of food.

Natural calamities such as starvation, epidemics, and wars are named as positive checks by Malthus.

On the contrary, there are man-made checks such as late marriage, self-control, simple living, etc. known as preventive checks.

Principles of Population Growth


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