# Biology Class 12 NCERT Solutions: Chapter 13 Organisms and Populations Part 2 (For CBSE, ICSE, IAS, NET, NRA 2022)

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Q. 5 List the attributes that populations but not individuals possess.

**Answer**:

- A population can be defined as a group of individuals of the same species residing in a particular geographical area at a particular time and functioning as a unit.
- For example, all human beings living at a particular place at a particular time constitute the population of humans.
- The main attributes or characteristics of a population residing in a given area are:

- Birth rate (Natality) :
- It is the ratio of live births in an area to the population of an area. It is expressed as the number of individuals added to the population with respect to the members of the population.

- Death rate (Mortality) :
- It is the ratio of deaths in an area to the population of an area. It is expressed as the loss of individuals with respect to the members of the population.
- Sex ratio:
- It is the number of males or females per thousand individuals.
- Age Distribution:
- It is the percentage of individuals of different ages in a given population. At any given time, the population is composed of individuals that are present in various age groups.
- The age distribution pattern is commonly represented through age pyramids.
- Population density:
- It is defined as the number of individuals of a population present per unit area at a given time.

Q. 6 If a population growing exponentially double in size in 3 years, what is the intrinsic rate of increase (r) of the population?

**Answer**:

- A population grows exponentially if sufficient amounts of food resources are available to the individual.
- Its exponential growth can be calculated by the following integral form of the exponential growth equation:

Where,

Population density after time

Population density at time zero

Intrinsic rate of natural increase

Base of natural logarithms

- From the above equation, we can calculate the intrinsic rate of increase (r) of a population.
- Now, as per the question,

Present population density

Then,

Population density after two years

Years

- Substituting these values in the formula, we get:

- Applying log on both sides:

- Hence, the intrinsic rate of increase for the above illustrated population is .