Q1. List the attributes that populations possess but not individuals.
Answer: Populations have certain features that individuals do not. These include:

1. Birth rate: It shows how many new individuals are added to the population over time, while one individual can only give birth.
2. Death rate: It shows how many individuals die in a population, while an individual only dies once.
3. Sex ratio: It tells the proportion of males and females in a population, while one individual is either male or female.
4. Age distribution: It shows how many individuals belong to different age groups, while an individual has only one age.
5. Population density: It tells how many individuals live in a given area, while one individual simply occupies space.

Q2. If a population growing exponentially doubles in size in 3 years, what is the intrinsic rate of increase (r) of the population?
Answer: The integral form of the exponential growth equation is:

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

Given:

$$N_t=2N_0\text{and}t=3\text{years}$$

Substituting the values:

$$2N_0=N_0{e}^{3r}$$

Simplify:

$$2=e^{3r}$$

Take the natural logarithm (ln) of both sides:

$$\ln (2)=3r$$

Since $\ln(2) \approx 0.693$:

$$0.693=3r$$

Solve for $r$:

$$r=\frac{0.693}{3}\approx 0.231$$

Hence, the intrinsic rate of increase (r) of the population is approximately 0.231.

Q3. Name important defence mechanisms in plants against herbivory.
Answer: Plants protect themselves from being eaten by herbivores through morphological and chemical defences.

1. Morphological defences: Some plants develop thorns or spines, such as in Acacia and Cactus, which physically prevent animals from feeding on them.
2. Chemical defences: Many plants produce poisonous or distasteful chemicals. For example, Calotropis produces cardiac glycosides that can make herbivores sick or even kill them. Other plants produce substances like nicotine, caffeine, quinine, strychnine, and opium, which act as natural repellents to herbivores.

Q4. An orchid plant is growing on the branch of a mango tree. How do you describe this interaction between the orchid and the mango tree?
Answer: The relationship between the orchid and the mango tree is an example of commensalism. In this interaction, one organism benefits while the other is neither harmed nor benefited. The orchid grows on the mango tree branch to get better sunlight and space, but it does not take any nutrients from the mango tree. Thus, the orchid benefits, and the mango tree remains unaffected.

Q5. What is the ecological principle behind the biological control method of managing pest insects?
Answer: The ecological principle behind the biological control of pest insects is predation. In this method, natural predators are used to control pest populations. These predators feed on the pests and help keep their numbers in check, maintaining ecological balance. By introducing or encouraging such natural enemies, farmers can manage pests effectively without relying on harmful chemical pesticides.

Q6. Define population and community.
Answer:

• Population: A population is a group of individuals of the same species living in a particular geographical area, sharing or competing for similar resources, interbreeding (or reproducing asexually), and forming a basic unit of evolution and ecological study.
• Community: A community is a collection of different species of plants, animals, and microorganisms living together in a specific habitat, interacting through various relationships such as competition, predation, and symbiosis, thereby maintaining the balance and stability of the ecosystem.

Q7. Define the following terms and give one example for each:
(a) Commensalism
(b) Parasitism
(c) Camouflage
(d) Mutualism
(e) Interspecific competition
Answer: 

(a) Commensalism: This is an interaction in which one species benefits while the other is neither harmed nor benefited.
Example: An orchid growing as an epiphyte on the branch of a mango tree gains space and sunlight, while the mango tree remains unaffected.

(b) Parasitism: In this interaction, one species (the parasite) benefits at the expense of another (the host), which is harmed.
Example: Lice feeding on human blood are ectoparasites that live on the surface of the host.

(c) Camouflage: A defensive adaptation in which the body colour or pattern of an organism allows it to blend with its surroundings and escape detection by predators.
Example: Many insects and frogs use body colours and patterns to merge with leaves or bark.

(d) Mutualism: This is a relationship in which both species involved benefit from the interaction.
Example: Lichens show mutualism between a fungus and a photosynthetic partner (algae or cyanobacteria).

(e) Interspecific Competition: This occurs when individuals of different species compete for the same limited resources such as food or space, reducing the growth or survival of both.
Example: Flamingoes and fish compete for zooplankton in shallow South American lakes.

Q8. With the help of a suitable diagram, explain the logistic population growth curve.
Answer: The logistic growth model describes population growth when resources are limited. It is more realistic than exponential growth because food, space, and nutrients eventually become scarce.

When population density (N) is plotted against time (t), it forms an S-shaped (Sigmoid) curve. This curve shows how population growth slows down as it approaches the carrying capacity (K) of the environment.

Phases of the Curve:

1. Lag Phase: Slow initial growth as the population adjusts to the new environment.
2. Exponential Phase: Rapid growth due to abundant resources.
3. Deceleration Phase: Growth slows down as resources start becoming limited.
4. Stationary Phase: Population size stabilizes at the carrying capacity (K), where birth and death rates balance.

Equation (Verhulst–Pearl Logistic Growth):

Where,

• N = Population size
• r = Intrinsic rate of increase
• K = Carrying capacity

Q9. Select the statement which explains best parasitism.
(a) One organism is benefited.
(b) Both the organisms are benefited.
(c) One organism is benefited, other is not affected.
(d) One organism is benefited, other is affected. 
Answer: (d) One organism is benefited, other is affected.

Q10. List any three important characteristics of a population and explain.
Answer: Three important characteristics of a population are:

1. Birth Rates (Natality): Refers to the number of births during a given period that are added to the initial population density. Expressed as per capita births. Natality increases population density.
2. Death Rates (Mortality): Refers to the number of deaths in the population during a given period. Expressed as per capita deaths. Mortality decreases population density.
3. Age Pyramids (Age Distribution): Refers to the structure showing the percentage of individuals of different age groups in a population. The shape of the pyramid reflects whether the population is growing, stable, or declining.