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Organisms and Populations Class 12 Notes

Ecology is the branch of biology which mainly involves the study about the interactions among organisms and their environment. Basically, there are four levels of biological organization.

  • Organisms.
  • Populations.
  • Communities.
  • Biomes.

Let us discuss the first two levels of organizations, organisms and population.


An organism refers to a contiguous living system that lives in an environment and has the ability to adapt and retain certain structure and behaviour. It includes fungi, bacteria, plants, animals, and humans. An organism collectively forms a population. The population forms a community which operates the ecosystem. The ecosystem consists of both biotic and abiotic factors.

Major abiotic factors which lead to variation in the physical and chemical conditions of different habitat are temperature, water, light, and soil.

Organisms and Environment

All organisms possess the ability to adapt to different environmental conditions due to genetic variations. This increases their chances of survival. The polar bear has different characteristic traits that help them to adapt to extremely cold climate of Antarctica. The dense fur coats protect them from cold and predators. The waxy coat repels water and keeps the body warm. These features help them to adapt to the cold.


Population is defined as a group of individuals or organisms of any species living in a well–defined geographical area, at a specific time with the capability of interbreeding. For example, population of deer in a forest.

Population Attributes

  • Birth rate- Total number of individuals born in a given period of time.
  • Death rates- Total number of deaths in a period of time.
  • Sex Ratio- Total number of females and males per 1000 individuals.
  • Age pyramid: A plot of age distribution.

Population Growth

Population growth refers to the increase in the number of individuals in a population. This depends on various factors such as weather, food availability, predator pressure, etc. The population density changes due to the following factors:

  • Natality: The number of births in a population in a given time period.
  • Mortality: The number of deaths in a population in a given time period.
  • Emigration: The number of individuals who moved to some other habitat in a certain time period.
  • Immigration: The number of individuals who have come into the habitat from elsewhere in a certain period of time.

Population Growth Models

The population growth models include:

  1. Exponential Growth: In the limited supply of food, the population follows an exponential growth.
  2. Logistic Growth: When the resources are finite and become limited sooner or later, the population growth is said to be logistic.

Population Interactions

This refers to the interaction between different populations. There are various modes of population interaction. These include:


This is a type of interaction in which an organism kills and feeds on another organism. The one who kills is known as the predator and the one who is killed is the prey.


This is the type of biological interaction between different animals or species in which both are harmed.


Parasitism is a type of interaction between species in which the parasite lives inside the body of another organism and cause harm to it.


In this type of interaction one organism benefits while the other is neither benefitted nor harmed.


In this type of interaction, both the species or organisms are benefitted from each other.

Learn more about organisms and populations by registering at BYJU’S website.

Few Important Questions

  • What is adaptation?
  • What is hibernation?
  • List out the Importance of light to plants?
  • Distinguish between Hibernation and Aestivation?
  • What are Biotic and Abiotic Factors of Living Organisms?

Further Reading

Frequently asked Questions on CBSE Class 12 Biology Notes Chapter 13: Organisms and Populations

Name some of the recently extinct species in India.

1. Asian straight-tusked elephant 2. Stegodon.3. Bharattherium.4. Sivatherium.5. Bramatherium.6. Megalochelys atlas.

What are the main reasons for extinction of species?

1. Demographic and genetic phenomena.2. Destruction of wild habitats.3. Introduction of invasive species.4. Climate change.5. Hunting and illegal trafficking.

What is meant by ‘Population variation’?

Population variance gives an idea of how data points in a specific population are spread out.

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