Organisms and Populations - Notes | Class 12 | Part 3: Populations


A population is a group of individuals of same species that live in a given geographical area, share or compete for similar resources and potentially reproduce.

E.g. All the cormorants in a wetland, rats in an abandoned dwelling, teakwood trees in a forest tract, bacteria in a culture plate and lotus plants in a pond etc.

Population ecology is an important area of ecology as it links ecology to population genetics & evolution.

Population Attributes
  • Birth rates: Refer to per capita births. 
E.g. In a pond, there are 20 lotus plants last year and through reproduction 8 new plants are added.
Hence, the current population = 28
The birth rate = 8/20 = 0.4 offspring per lotus per year.
  • Death rates: Refer to per capita deaths.
E.g. 4 individuals in a laboratory population of 40 fruit flies died during a week.
Hence, the death rate = 4/40 = 0.1 individuals per fruit fly per week.
  • Sex ratio: A population has a sex ratio.
E.g. 60% of the population is females and 40% males.
  • Age pyramid: It is the structure obtained when the age distribution (% individuals of a given age or age group) is plotted for the population.
For human population, age pyramids generally show age distribution of males and females in a combined diagram.
Representation of age pyramids for human population 

  • Population size or population density (N): It is the number of individuals of a species per unit area or volume. E.g. population density of Siberian cranes at Bharatpur wetlands in any year is <10. It is millions for Chlamydomonas in a pond.
Population size is also measured in % cover or biomass. E.g. In an area, 200 Parthenium plants and a huge banyan tree are seen. In such cases, measuring % cover or biomass is meaningful to show importance of banyan tree.
Total number is a difficult measure for a huge population. In such cases, relative population density (without knowing absolute population density) is used. E.g. Number of fish caught per trap indicates its total population density in the lake.
In some cases, indirect estimation of population sizes is performed. E.g. Tiger census in national parks & tiger reserves based on pug marks & fecal pellets. 
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