Science: Natural Environment: Population Growth and Dispersal (For CBSE, ICSE, IAS, NET, NRA 2022)

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Population is a group of similar individuals living in a particular geographical area. Populations of different species of organisms live in the same ecosystem.

Such relationships can be characterized into different types depending on the interaction and the extent to which they associate.

Population is a Group


Mutualism in an interaction between individuals belonging to two different species, that benefit both members. Lichen is a complete entity formed by the association of an alga and a fungus. The main body of the lichen is formed by fungus. The alga manufactures food for itself as well as for the fungus, while the fungus provides water, minerals and shelter to the alga.


Commensalism is an interspecific interaction where one species benefits and the other is unaffected (neither harmed nor benefitted) . Commensal relationships may involve one organism using another for transportation or for housing.


It includes one organism living in or on the body of another living organism from which it derives nourishment and, in the process, harms its host. For example: tapeworm living in the intestine of man.


A close interaction between two or more different organisms of different species living in close physical association. Plants pollinated in this manner produce less pollen than do plants that rely on the wind to transfer pollen. This is an example of symbiotic association.

Population Growth

Population growth is the change in the number of individuals of any species in a population at a given time. The size of the population depends upon the density, natality (birth rate) , mortality (death rate) , population dispersal, age distribution, and environmental resistance that the population has to face.

  • Birth rate or natality: It is defined as the number of live births per thousand per unit time.
  • Mortality rate: The mortality rate of a population is the number of individuals dying per thousand per unit time.

Population Dispersal

It is the movement of individuals or groups of living organisms by which they expand the space or range within which they live . Dispersal operates when organisms leave the space that they have previously occupied, or in which they were born and settle in new areas. It affects the size of the population.

Population dispersal can be of two types:

  • Emigration: It is the permanent outward movement of the organisms from a given population. It decreases the size of the local population.
  • Immigration: It is the permanent inward movement of the organisms from outside into a given population. It increases the size of the local population.

Population growth curves are either J-shaped or S-shaped.

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