CBSE Class 12 Biology Solved Question Paper 2015

Long Answer Type

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(a) List the different attributes that a population has and not an individual organism.

(b) What is population density ? Explain any three different ways the population density can be measured, with the help of an example each.

(a) Following are the attributes that a population has but an individual organism does not have:

  • Birth rate: per capita births. It represents the increase in number with respect to the population.
  • Death rate: per capita deaths. It represents the decrease in number with respect to the population.
  • Sex ratio: Ratio of number of males to females in a given  population.
  • Age distribution: A population at any given time is composed of individuals of different ages.

(b) Population density means number of individuals present per unit area. Population density can be measured by determining the population size. The different methods to study population size are as follows:

(1) Indirect method:  In this method the population density is measured without actually counting or seeing the organism. For examples – the tiger consensus in our national park and tiger reserves are often based on the pug marks and fecal pellets of the tiger.
Example: The number of Parthenium plants in a given area can be measured using the quadrat method.

(2) Direct observation: It involves the counting of organisms in the given area.
Example: In order to determine the number of bacteria growing in a Petri dish, their colonies are counted.

(3)Relative densities: When the population is too large to be counted then relative density is taken into account . For example: The number of fishes caught per trap gives the measure of their total density in a given water body.
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