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Biology · Class 12 Science

Ch 11Organisms and Populations — Class 12 Biology, concept-first.

The environment of any organism is the sum total of everything that surrounds it and can affect its growth, survival, reproduction, and behaviour. Ecologists conventionally split the environment into two broad, interacting components.

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Key concepts

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Environment and Ecology

The environment of an organism is the sum of everything external to it that can influence its survival, growth, and reproduction, split into two broad components.

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Chapter contents

The NCERT structure, section by section. Open a section to see its questions, then read the concept-first solution.

11.1

Environment: Meaning and Scope

The environment of any organism is the sum total of everything that surrounds it and can affect its growth, survival, reproduction, and behaviour.

11.2

Habitat and Niche

The habitat of an organism is simply its address -- the actual physical place, characterised by a specific set of abiotic and biotic conditions, where that organism naturally lives.

11.3

Population Interactions: An Overview

A population is defined in ecology as a group of individuals of the same species, living within a defined geographical area, sharing or competing for similar resources, and potentially able to interbr…

11.4

Mutualism

Mutualism is an interspecific interaction in which both species involved derive a benefit from the association -- represented as (+, +) in the plus/minus notation.

11.5

Competition

Competition is the interaction in which two or more species (or two individuals of the same species) are harmed because they depend on a resource that is in short supply relative to demand -- represen…

11.6

Predation

Predation is the interaction in which one species, the predator, captures, kills, and feeds upon another species, the prey -- represented as (+, −), since the predator clearly benefits while the prey…

11.7

Parasitism

Parasitism is an interspecific interaction in which one species, the parasite, obtains its nourishment by living on or within the body of another species, the host, at the host's expense -- represente…

11.8

Population Attributes: Size, Density and Sex Ratio

A single organism has an age, a sex, and (while alive) a state of health, but it does not have a "birth rate" or a "density." These are emergent properties that appear only once individuals are consid…

11.9

Natality and Mortality: Birth Rate and Death Rate

Natality, commonly called the birth rate, refers to the number of births occurring within a population during a given time period, usually expressed as a per capita rate -- the number of births per in…

11.10

Age Distribution and Age Pyramids

Age distribution refers to the proportion of individuals in a population that belong to each of its different age classes, conventionally grouped into three broad categories: the pre-reproductive age…

11.11

Population Growth: Exponential and Logistic Growth Curves

When resources such as food, space, and nutrients are effectively unlimited, a population is free to grow at the fastest rate its biology allows, a pattern called exponential growth.

Summary

This chapter surveyed how organisms relate to their surroundings and to one another, and the measurable attributes of the populations they form, as laid out in the WBCHSE syllabus for Chapter 11.

Very Short Answer Questions

+Show 8 questions8 questions
  1. Q1Define "environment" in the ecological sense, naming its two broad components.Free
  2. Q2Distinguish between habitat and niche in one sentence each.Free
  3. Q3Define mutualism and give one example of a mutualistic pair of organisms.Free
  4. Q4State Gause's Competitive Exclusion Principle in one sentence.Preview
  5. Q5Define parasitism and name one example of an ectoparasite found on humans.Preview
  6. Q6What is meant by population density? Give the general way in which it is expressed.Preview
  7. Q7Define natality (birth rate) as it is used in population ecology.Preview
  8. Q8What is an "expanding" age pyramid, and what shape does it show?Preview

Short Answer Questions

+Show 10 questions10 questions
  1. Q9Differentiate between predation and parasitism, giving one example of each.Free
  2. Q10Explain why the realised niche of a species is usually narrower than its fundamental niche.Free
  3. Q11Describe the lichen as an example of an obligate mutualistic association, naming the two partners involved.Free
  4. Q12Explain Gause's Competitive Exclusion Principle with reference to two species competing for an identical, limiting resource.Preview
  5. Q13List any four adaptations shown by predators that make them more efficient hunters.Preview
  6. Q14List any four adaptations shown by prey species to reduce the chance of being caught by a predator.Preview
  7. Q15Distinguish between ectoparasites and endoparasites, giving one example of each.Preview
  8. Q16What is brood parasitism? Explain with the example of the koel (Asian koel) and the crow.Preview
  9. Q17Distinguish between natality and mortality, and between immigration and emigration, as the four factors that change population size.Preview
  10. Q18Draw a distinction between a "stable" and a "declining" age pyramid in terms of the proportion of individuals in the pre-reproductive age gr…Preview

Long Answer Questions

+Show 8 questions8 questions
  1. Q19With the help of a suitably described graph, explain the difference between exponential (J-shaped) and logistic (S-shaped) population growth…Free
  2. Q20Describe the four major types of population interactions -- mutualism, competition, predation, and parasitism -- giving one real example of…Free
  3. Q21Discuss "resource partitioning" as a mechanism that allows ecologically similar species to coexist without one competitively excluding the o…Free
  4. Q22Explain how population attributes such as birth rate, death rate, sex ratio, and age distribution together determine whether a population is…Preview
  5. Q23Describe the various adaptations shown by parasites for a parasitic mode of life.Preview
  6. Q24With the help of the Verhulst-Pearl logistic growth equation, explain how the rate of population growth slows as population size approaches…Preview
  7. Q25Discuss the ecological and evolutionary significance of predation, including its role in maintaining species diversity within a community.Preview
  8. Q26Explain the concept of ecological niche, distinguishing between the fundamental niche and the realised niche, and discuss why no two species…Preview

Sample & Board Papers

Sample papers and previous-year board questions for this subject.

+Show 9 questions9 questions
  1. Q1Describe xerophytic adaptations in plants. Presence of patagia represents which type of adaptation? **OR** Mention the sources of air pollut…Preview
  2. Q2The overall growth curve for the world human population is (a) an inverted V-shaped curve (b) a straight line (c) a J-shaped curve (d) none…Preview
  3. Q3Define autecology. **OR** Define biome.Preview
  4. Q4Warm and humid nature is characteristic of which one of the following? (a) Temperate forest (b) Tropical rain forest (c) Coniferous forest (…Preview
  5. Q5What do you mean by parasitism? Mention one parasitic adaptation. (1+1) **OR** Mention how ozone layer is depleted in the atmosphere.Preview
  6. Q6Characteristics of Xerophytic plants are (a) Thick cuticle (b) Sunken stomata (c) Both (a) and (b) (d) None of thesePreview
  7. Q7What are the differences between habitat and niche? What is mycorrhiza? (2+1) **OR** What are the differences between S and J shaped growth?…Preview
  8. Q8Characteristic of xerophyte is (a) thick cuticle (b) sunken stomata (c) presence of water storing tissue (d) all of thesePreview
  9. Q9Define Synecology.Preview

More questions

+Show 8 questions8 questions
  1. Q27The sum total of all the physical, chemical, and biological factors that surround an organism, along with the specific role that organism pl…Free
  2. Q28In the association seen in a lichen (a fungus and an alga/cyanobacterium living together), the interaction is best classified as: (A) Commen…Free
  3. Q29The phenomenon by which a bird such as the koel lays its eggs in the nest of another species (such as the crow), which then rears the koel's…Free
  4. Q30Which growth equation is represented by dN/dt = rN(K − N)/K ? (A) Exponential growth (B) Logistic growth (C) Linear growth (D) None of thesePreview
  5. Q31An age pyramid with a broad base and a narrow apex, showing a large proportion of pre-reproductive individuals, represents a population that…Preview
  6. Q32Which of the following is a classic laboratory example of interspecific competition? (A) Two species of Paramecium (P. aurelia and P. caudat…Preview
  7. Q33The number of individuals of a species present per unit area or per unit volume at a given time is called: (A) Natality (B) Population densi…Preview
  8. Q34Which of the following is an ectoparasite commonly found on humans? (A) Ascaris (roundworm) (B) Tapeworm (C) Head louse (Pediculus) (D) Plas…Preview