Diversity is Not Only a Characteristic of Living Organisms But Also of Populations - Class 12 NCERT Biology

Understanding Populations: More Than Just Living Organisms

In Class 12 NCERT Biology, ecology studies the interactions among organisms and their environment. Populations are central to this study. A population is a group of individuals of the same species living in a specific geographical area, sharing resources and potentially interbreeding.

Examples include:

• All cormorants in a wetland
• Rats in an abandoned building
• Teakwood trees in a forest
• Bacteria in a culture plate
• Lotus plants in a pond

Populations differ from individuals because they possess unique attributes such as birth rates, death rates, sex ratio, and age distribution. These attributes help ecologists understand how populations grow, survive, and evolve over time.

Key Attributes of Populations Explained

Populations have several important characteristics that help describe their status and dynamics:

• Population Size (N): The total number of individuals in the population.
• Population Density: Number of individuals per unit area or volume. Can be absolute or relative.
• Birth Rate: Number of births per individual per unit time.
• Death Rate: Number of deaths per individual per unit time.
• Sex Ratio: Proportion of males to females, e.g., 60% females and 40% males.
• Age Distribution: Proportion of individuals of different ages, often shown as an age pyramid.

For example, if 8 new lotus plants appear in a population of 20 in one year, the birth rate is $\frac{8}{20} = 0.4$ offspring per plant per year.

Similarly, if 4 fruit flies die in a population of 40 in one week, the death rate is $\frac{4}{40} = 0.1$ deaths per fruit fly per week.

Population Density and Measurement Techniques

Population density is a crucial measure in ecology. It tells us how crowded a population is in its habitat.

• Absolute Density: Total count of individuals, e.g., 1000 algae in a pond.
• Relative Density: Used when absolute counts are difficult, e.g., number of fish caught per trap.
• Biomass or Percentage Cover: Useful for comparing species of different sizes, such as carrot grass vs. banyan tree.

Indirect methods are also used for elusive animals, like counting tiger pug marks or fecal pellets.

Understanding population density helps in conservation and management of species.

Age Distribution and Population Growth Patterns

Age distribution shows how many individuals of different age groups exist in a population. It is often depicted as an age pyramid.

• Growing Population: Wide base, many young individuals.
• Stable Population: Uniform distribution across ages.
• Declining Population: Narrow base, fewer young individuals.

For humans, age pyramids show male and female distribution in each age group.

Age structure influences population growth and helps predict future trends. For example, a population with many young individuals is likely to grow rapidly.

Worked Example:

If a population doubles in size in 3 years, what is the intrinsic rate of increase $r$?

Using exponential growth formula:

$$ N = N_0 e^{rt} $$

When population doubles, $\frac{N}{N_0} = 2$, and $t = 3$ years.

Taking natural logarithm:

$$ \ln 2 = rt $$

$$ r = \frac{\ln 2}{3} = \frac{0.693}{3} \approx 0.231 \text{ per year} $$

This means the population grows at about 23.1% per year.

Significance of Studying Populations in Ecology and Evolution

Studying populations is vital because evolutionary changes occur at this level. While individuals experience environmental changes, natural selection acts on populations to favour traits that improve survival and reproduction.

Population ecology helps us understand:

• How populations grow or decline
• Interactions within and between species
• Effects of environmental factors on population dynamics

For example, birth and death rates influence population size, while sex ratio affects reproduction potential.

Understanding these concepts is essential for conservation biology, pest management, and sustainable resource use.

This knowledge builds the foundation for advanced topics like population growth models, community interactions, and ecosystem dynamics in Class 12 NCERT Biology.