Organisms and Populations — NCERT Solutions

Population:

A population is defined as a group of individuals of the same species living in a well-defined geographical area, sharing or competing for similar resources, and potentially interbreeding.

• Populations have attributes such as birth rate, death rate, sex ratio, age distribution, and population density.
• Example: All the tigers in the Sundarbans, or all the mango trees in an orchard.

Community:

A community (also called a biotic community) is defined as an assemblage of populations of different species living in the same geographical area at the same time, interacting with one another.

• A community includes all the plants, animals, fungi, and microorganisms in a given habitat.
• Example: All the organisms (plants, animals, microbes) living in a pond or a forest constitute a community.

Key Difference:

(a) Commensalism:

Definition: An interspecific interaction in which one species benefits and the other species is neither harmed nor benefited (unaffected). It is represented as $(+, 0)$.

Example: An orchid (epiphyte) growing on the branch of a mango tree. The orchid benefits by getting support and sunlight; the mango tree is unaffected.

Another example: The barnacles growing on the back of a whale — barnacles benefit (transport, access to food-rich waters) while the whale is unaffected.

(b) Parasitism:

Definition: An interspecific interaction in which one species (the parasite) benefits at the expense of the other species (the host), which is harmed. It is represented as $(+, -)$.

Example: Cuscuta (dodder plant) is a parasitic plant that grows on a host plant, deriving water and nutrients from it, thereby harming the host.

Another example: The human liver fluke (Fasciola hepatica) parasitises the liver of its host.

(c) Camouflage:

Definition: A defence adaptation in which an organism blends with its surrounding environment in terms of colour, shape, or pattern, making it difficult for predators (or prey) to detect it. It is a form of cryptic coloration.

Example: The leaf insect (Phyllium) resembles a green leaf so closely that predators cannot distinguish it from real leaves. Similarly, the stick insect (Carausius) resembles a twig.

Another example: The Kallima butterfly resembles a dry leaf when it sits with its wings closed.

(d) Mutualism:

Definition: An interspecific interaction in which both the interacting species benefit. It is represented as $(+, +)$.

Example: Lichens — a mutualistic association between a fungus and a photosynthetic alga/cyanobacterium. The fungus provides shelter and absorbs minerals; the alga provides food through photosynthesis.

Another example: The relationship between the fig tree (Ficus) and the fig wasp (Agaonidae) — the wasp pollinates the fig flowers and in turn gets a safe site to lay eggs and food for its larvae.

(e) Interspecific Competition:

Definition: An interspecific interaction in which two different species compete for the same limited resources (food, space, light, etc.), and both species are adversely affected. It is represented as $(-, -)$.

Example: Flamingoes and resident fishes in South American lakes compete for the same food resource — zooplankton. Both species are negatively affected by the presence of the other.

Another example: Competition between two grass species for soil nutrients and space in a meadow.

Concept: When resources are limited, a population cannot grow indefinitely. The growth follows a logistic (sigmoid/S-shaped) pattern.

Logistic Growth — Key Points:

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

Where:

• $N$ = Population size at time $t$
• $r$ = Intrinsic rate of natural increase
• $K$ = Carrying capacity of the environment
• $\frac{K-N}{K}$ = Environmental resistance (the unused portion of carrying capacity)

Description of the Logistic Growth Curve (S-shaped / Sigmoid Curve):

The curve has three distinct phases:

1. Lag Phase: Initially, the population is small and growth is slow as individuals establish themselves.

2. Log (Exponential) Phase: The population grows rapidly as resources are still relatively abundant. The curve rises steeply.

3. Deceleration Phase: As population size approaches $K$, resources become limiting. The growth rate slows down.

4. Plateau (Asymptote) Phase: The population stabilises at the carrying capacity ($K$). The growth rate becomes zero ($dN/dt = 0$) because $N = K$.

Diagram:

Population
Size (N)
  |
 K|_ _ _ _ _ _ _ _ _ _ _ _ _ _ (Carrying Capacity)
  |                    /‾‾‾‾‾‾
  |                 /
  |              /
  |           /
  |        /
  |     /
  |  /  (Lag phase)
  |/________________________
                           Time (t)

(The curve is S-shaped/sigmoid, starting slowly, rising steeply, then levelling off at K)

Key Features:

• The population never exceeds $K$ in ideal logistic growth.
• The growth rate is maximum when $N = K/2$.
• This is a more realistic model of population growth in nature compared to exponential growth.
• Also called the Verhulst-Pearl Logistic Growth model.

Conclusion: The logistic growth curve is a sigmoid (S-shaped) curve that shows how a population grows rapidly when small, then slows as it approaches the carrying capacity $K$, ultimately stabilising at $K$.

Correct Answer: (d) One organism is benefited, other is affected.

Justification:
In parasitism, the parasite benefits by deriving nutrition, shelter, or other resources from the host, while the host is harmed (negatively affected) — it loses nutrients, may suffer tissue damage, reduced fitness, or even death. This is represented as a $(+, -)$ interaction.

• Option (a) is incomplete — it does not mention the effect on the other organism.
• Option (b) describes mutualism $(+, +)$.
• Option (c) describes commensalism $(+, 0)$.
• Option (d) correctly and completely describes parasitism $(+, -)$.

Three Important Characteristics of a Population:

1. Birth Rate (Natality):

• Birth rate is defined as the number of births per capita (per individual) in a population per unit time.
• It is expressed as: $$\text{Birth Rate} = \frac{\text{Number of births}}{\text{Population size}}$$
• It contributes to the increase in population size.
• Example: If a pond has 20 water hyacinths and 8 new plants are added in a week, the birth rate = $8/20 = 0.4$ offspring per individual per week.

2. Death Rate (Mortality):

• Death rate is defined as the number of deaths per capita in a population per unit time.
• It is expressed as: $$\text{Death Rate} = \frac{\text{Number of deaths}}{\text{Population size}}$$
• It contributes to the decrease in population size.
• Example: If 4 out of 40 frogs in a pond die in a week, the death rate = $4/40 = 0.1$ individuals per week.
• The difference between birth rate and death rate determines whether a population grows, declines, or remains stable.

3. Age Distribution (Age Pyramid):

• A population is composed of individuals of different ages. The proportion of individuals in different age groups (pre-reproductive, reproductive, and post-reproductive) is called age distribution.
• It is graphically represented as an age pyramid.
• Three types of age pyramids:
• Expanding (triangular): Broad base — high proportion of young individuals — rapidly growing population.
• Stable (bell-shaped): Equal proportion of young and middle-aged individuals — stable/stationary population.
• Declining (urn-shaped): Narrow base — low proportion of young individuals — declining population.
• Example: India's age pyramid has a broad base, indicating a young, rapidly growing population.

Conclusion: Birth rate, death rate, and age distribution are fundamental characteristics of a population that determine its size, growth rate, and future trajectory.