Chapter 11 Organisms and Populations Class 12 Biology

Question 1

List the attributes that populations possess but not individuals.

A population has several attributes that an individual organism does not:

Birth Rate (Natality): An individual is born, but a population has a birth rate (number of births per unit time).
Death Rate (Mortality): An individual dies, but a population has a death rate (number of deaths per unit time).
Sex Ratio: An individual is either male or female, but a population has a sex ratio (e.g., 60% females, 40% males).
Age Distribution: A population is composed of individuals of different ages (pre-reproductive, reproductive, post-reproductive), which can be represented by age pyramids.
Population Density: The number of individuals per unit area or volume.

Question 2

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

Formula

For exponential growth: \(N_t = N_0 e^{rt}\)
Where:
\(N_t\) = Population size at time t
\(N_0\) = Initial population size
\(r\) = Intrinsic rate of increase
\(t\) = Time period

Calculation

Given: Population doubles in 3 years. So, \(N_t = 2N_0\) and \(t = 3\).

\(2N_0 = N_0 e^{3r}\)
\(2 = e^{3r}\)
Taking natural log (ln) on both sides:
\(\ln(2) = 3r\)
\(0.693 = 3r\)
\(r = 0.693 / 3\)
\(r = 0.231\)

Answer: The intrinsic rate of increase (r) is 0.231.

Question 3

Name important defence mechanisms in plants against herbivory.

Plants have evolved various morphological and chemical defenses against herbivores:

Morphological Defenses

Thorns: e.g., Acacia, Cactus.
Spines: Modified leaves to prevent grazing.
Sticky/Hairy leaves: Discourage insects.

Chemical Defenses

Toxins: e.g., Calotropis produces toxic Cardiac Glycosides. Cattle/goats do not browse on it.
Secondary Metabolites: Nicotine, Caffeine, Quinine, Strychnine, Opium are produced primarily as defenses against grazers and browsers.

Question 4

An orchid plant is growing on the branch of mango tree. How do you describe this interaction between the orchid and the mango tree?

This interaction is called Commensalism.

Orchid (Epiphyte): It benefits by getting physical support to reach sunlight and air. It does not derive nutrition from the mango tree (it has aerial roots for moisture). (Benefited +)
Mango Tree (Host): It provides support but is neither harmed nor benefited by the presence of the orchid. (Neutral 0)

Question 5

What is the ecological principle behind the biological control method of managing with pest insects?

The ecological principle is Predation.

Biological control uses natural predators (or parasites/pathogens) to control the population of pest insects.
Predators act as a natural check on prey populations, preventing them from reaching high densities that cause economic damage.
Example: Use of Gambusia fish to check mosquito larvae proliferation in water bodies.

Question 6

Define population and community.

Population: A group of individuals of the same species living in a specific geographical area at a specific time, which can potentially interbreed. Example: All the lotus plants in a pond.
Community: An assemblage of several populations of different species (plants, animals, bacteria, fungi) living and interacting in a defined area. Example: A pond community includes fish, frogs, algae, lotus plants, and bacteria.

Question 7

Define the following terms and give one example for each: (a) Commensalism (b) Parasitism (c) Camouflage (d) Mutualism (e) Interspecific competition.

Interaction	Definition	Example
(a) Commensalism	Interaction where one species benefits (+) and the other is neither harmed nor benefited (0).	Orchid on Mango tree; Cattle egret and grazing cattle.
(b) Parasitism	Interaction where one species (Parasite) benefits (+) at the expense of the other (Host) which is harmed (-).	Cuscuta on hedge plants; Lice on humans; Ticks on dogs.
(c) Camouflage	Adaptation where an organism blends with its surroundings to avoid detection by predators.	Frogs and insects blending with leaves/bark to escape predators.
(d) Mutualism	Interaction where both participating species benefit (+, +).	Lichens (Algae + Fungi); Mycorrhiza (Fungi + Roots of higher plants).
(e) Interspecific Competition	Competition between two different species for the same limited resources (-, -).	Flamingoes and resident fishes competing for zooplankton in South American lakes.

Question 8

With the help of suitable diagram describe the logistic population growth curve.

[Image of Logistic Growth Curve S-shaped]

Description

Logistic growth occurs when resources (food, space) are limited. The growth curve is Sigmoid (S-shaped).

Lag Phase: Initial slow growth as the population adapts to the environment.
Log (Acceleration) Phase: Rapid growth as individuals reproduce.
Deceleration Phase: Growth rate slows down as resources become limited.
Asymptote: Population density reaches the Carrying Capacity (K) and stabilizes.

Equation

\(\frac{dN}{dt} = rN \left(\frac{K-N}{K}\right)\)

Where \(N\) = Population density, \(r\) = Intrinsic rate of increase, \(K\) = Carrying capacity.

Question 9

Select the statement which explains best parasitism.

(d) One organism is benefited, other is affected.

Explanation: In parasitism, the parasite gets nutrition/shelter (Benefited +) from the host, while the host suffers damage or loss of nutrients (Affected/Harmed -).

Question 10

List any three important characteristics of a population and explain.

Population Density: It is the size of the population in relation to some unit of space. It is expressed as the number of individuals per unit area (for land) or volume (for water). E.g., 200 Parthenium plants per acre.
Natality (Birth Rate): It refers to the number of births during a given period in the population that are added to the initial density. It is expressed as births per 1000 individuals per year.
Mortality (Death Rate): It refers to the number of deaths in the population during a given period. It is expressed as deaths per 1000 individuals per year.

NCERT Solutions