Chapter 11 - Organisms and Populations

Organisms and Populations introduces population ecology—the study of groups of individuals belonging to a species and their interactions with other populations. In the current rationalised NCERT Class 12 Biology textbook, Chapter 11 focuses on population attributes, population growth, life-history variation and interspecific interactions.

Teachoo’s category provides detailed explanations, graph interpretation, NCERT solutions, numerical practice, important questions, MCQs, assertion-reason and case-based questions for CBSE and NEET.

What is a population?

A population is a group of individuals of the same species living in a defined geographical area, sharing or competing for resources and potentially interbreeding. For ecological study, groups produced through asexual reproduction may also be treated as populations.

Population ecology is important because natural selection acts through differences in survival and reproduction within populations. Changes that are difficult to observe at the level of a single individual become measurable at the population level.

Population attributes

Populations possess attributes that individuals do not. An individual can be born or die, but a population has birth and death rates. Important attributes include:

• Population density

• Natality or birth rate

• Mortality or death rate

• Sex ratio

• Age distribution

Population density does not always mean the number of individuals. Depending on the ecological question, density may be estimated through biomass, percentage cover, captures per trap, indirect signs or other measures.

Age distribution can be represented through age pyramids. Pre-reproductive, reproductive and post-reproductive groups produce pyramid shapes associated with expanding, stable or declining populations.

Students should be able to interpret what an age pyramid suggests rather than merely identify its shape.

Changes in population density

Population density changes through four processes:

• Natality adds new individuals through reproduction

• Mortality removes individuals through death

• Immigration adds individuals arriving from elsewhere

• Emigration removes individuals leaving the population

The relationship can be expressed as:

N(t+1) = Nt + [(B + I) – (D + E)]

Births and immigration increase population size, while deaths and emigration decrease it. The relative importance of these processes depends on the population and circumstances. Immigration may be particularly important when a new habitat is being colonised.

Exponential growth

When resources are unlimited, population growth can be exponential. The differential form is:

dN/dt = rN

The integrated form is:

Nt = N₀eʳᵗ

Here, r is the intrinsic rate of natural increase. A graph of population size against time produces a J-shaped curve.

Exponential growth describes potential growth under ideal conditions. It cannot continue indefinitely in a natural environment with finite resources.

Logistic growth

In a habitat with limited resources, growth may follow a logistic pattern. The carrying capacity, K, is the maximum population size that the environment can support under the given conditions.

The logistic equation is:

dN/dt = rN[(K – N)/K]

The graph is sigmoid or S-shaped. Growth begins slowly, accelerates, decelerates and approaches the carrying capacity.

Students should understand the biological meaning of N, r and K and the conditions under which each growth model is appropriate.

Life-history variation

Organisms have evolved different reproductive strategies. Some reproduce once and produce many offspring, while others reproduce repeatedly and invest more in fewer offspring.

Examples such as Pacific salmon, bamboo, oysters, fishes, birds and mammals demonstrate trade-offs in timing, frequency, offspring number and offspring size. These strategies are shaped by selection pressures and environmental constraints.

Population interactions

Different species in a community may benefit, suffer or remain unaffected through interactions. These include:

• Mutualism: both species benefit (+/+)

• Competition: both are negatively affected (−/−)

• Predation: predator benefits and prey is harmed (+/−)

• Parasitism: parasite benefits and host is harmed (+/−)

• Commensalism: one benefits and the other is unaffected (+/0)

• Amensalism: one is harmed and the other is unaffected (−/0)

Students must learn the ecological mechanism and NCERT examples for each interaction.

Predators transfer energy across trophic levels and can regulate prey populations. Prey species may evolve camouflage, chemical defences or other protective traits.

Competition can occur between related or unrelated species and may involve resources that are or become limiting. Competitive exclusion and resource partitioning help explain possible outcomes.

Parasites develop adaptations for obtaining resources and completing transmission, while hosts evolve defensive responses. Brood parasitism is a specialised reproductive interaction.

Mutualistic interactions benefit both partners and may become highly specific through coevolution. Plant–pollinator and plant–fungus relationships provide important examples.

Important graphs, equations and examples

• Expanding, stable and declining age pyramids

• Population-change equation

• J-shaped exponential-growth curve

• S-shaped logistic-growth curve

• Carrying capacity

• Interaction sign table

• NCERT examples of predation, competition, parasitism, commensalism and mutualism

Why is this chapter important?

CBSE tests growth models, interactions, graphs and case studies. NEET frequently asks equations, r and K, age pyramids, life-history examples, interaction signs and NCERT organism pairs.

How should students study the chapter?

Practise every equation with biological meaning, not just symbols. Build an interaction table containing effect on species A, effect on species B, definition and NCERT example. When reading an example, identify which organism benefits and which is harmed.

Frequently Asked Questions

Can population density be measured without counting individuals?

Yes. Biomass, percentage cover, capture rate, faecal pellets, tracks and other indirect measures may be more useful in certain populations.

What is the difference between exponential and logistic growth?

Exponential growth assumes unlimited resources and produces a J-shaped curve. Logistic growth includes resource limitation and carrying capacity and produces an S-shaped curve.

What does carrying capacity mean?

It is the maximum population size that a habitat can support under its current resource and environmental conditions.

Are predation and parasitism represented by the same signs?

Yes. Both are represented as +/− because one species benefits while the other is harmed, although their biological relationships differ.

What is the difference between commensalism and mutualism?

In commensalism, one species benefits and the other is unaffected. In mutualism, both species benefit.

Why is logistic growth considered more realistic?

Natural resources are finite, so population growth is eventually limited by competition and environmental carrying capacity.