Class 12 Biology Notes Chapter 13 (Organisms and Populations) – Examplar Problems Book

Alright class, let's delve into Chapter 13: Organisms and Populations from your NCERT Exemplar. This chapter forms the bedrock of Ecology and is crucial for understanding how life sustains itself at various levels. Pay close attention, as these concepts frequently appear in competitive government exams.

Chapter 13: Organisms and Populations - Detailed Notes

1. Ecology: The Study of Interactions

• Definition: Ecology is the branch of biology that studies the interactions among organisms and between organisms and their physical (abiotic) environment.
• Levels of Biological Organisation relevant to Ecology:
  • Organism: The basic unit of ecological study; how an individual adapts to its environment.
  • Population: A group of individuals of the same species living in a given geographical area at a particular time, sharing or competing for similar resources, and potentially interbreeding.
  • Community: An assemblage of different populations (plants, animals, microbes) interacting in a specific area.
  • Ecosystem: The community of organisms interacting with each other and with their physical environment. (Includes biotic and abiotic components).
  • Biome: Large geographical areas characterized by specific climate zones and predominant plant and animal communities (e.g., Desert, Rainforest, Tundra).
  • Biosphere: All the ecosystems of the Earth combined; the sum of all life.

2. Organism and Its Environment

• Environment: Includes all biotic (living) and abiotic (non-living) factors that surround and influence an organism.

• Habitat: The specific place or locality where an organism lives; its 'address'.

• Niche (Ecological Niche): The functional role an organism plays in its ecosystem, encompassing its resource utilization (food, shelter), tolerance range to environmental conditions, and interactions with other species. It's the organism's 'profession' or 'role'.

  • Key Point: No two species can occupy the exact same niche indefinitely in the same habitat (Gause's Competitive Exclusion Principle).

• Major Abiotic Factors: These significantly influence the life of organisms.

  • Temperature:
    • Most ecologically relevant factor. Affects enzyme kinetics, metabolic activity, and physiological functions.
    • Eurythermal: Organisms that can tolerate a wide range of temperatures (e.g., most mammals, birds).
    • Stenothermal: Organisms restricted to a narrow range of temperatures (e.g., polar bears, coral reefs, many reptiles/amphibians).
    • Adaptations: Hibernation (winter sleep), Aestivation (summer sleep), Allen's Rule (mammals from colder climates generally have shorter ears and limbs to minimize heat loss), Bergmann's Rule (animals in colder regions tend to have larger body size).
  • Water:
    • Essential for all life; availability dictates distribution.
    • Desert organisms have special adaptations for water conservation (e.g., Kangaroo rat internal fat oxidation, thick cuticle in plants, CAM pathway).
    • Aquatic organisms face issues related to water quality (pH, salinity).
    • Euryhaline: Organisms that can tolerate a wide range of salinities (e.g., salmon, eels).
    • Stenohaline: Organisms restricted to a narrow range of salinities (e.g., most freshwater fish, marine invertebrates like starfish).
  • Light:
    • Primary source of energy (photosynthesis).
    • Important for photoperiodism (affects flowering in plants, reproduction/migration in animals).
    • Light quality and intensity vary (e.g., different strata in forests, depth in aquatic ecosystems). UV radiation can be harmful.
    • Adaptations: Sciophytes (shade plants), Heliophytes (sun plants), adaptations of deep-sea organisms (bioluminescence).
  • Soil:
    • Nature and properties depend on climate, weathering process, and soil development.
    • Key properties: Soil composition (mineral particles, humus), grain size (determines texture - sand, silt, clay), aggregation (determines structure), pH, mineral composition, topography, water holding capacity, percolation.
    • Determines the type of vegetation and supports diverse animal life.

• Responses to Abiotic Stresses: Organisms evolve strategies to cope with stressful environmental conditions.

  • Regulate: Maintain constant internal environment (homeostasis) despite external changes (e.g., mammals, birds - thermoregulation, osmoregulation). Energetically expensive.
  • Conform: Internal environment changes with the external environment (e.g., most plants, invertebrates, fish, amphibians, reptiles). Less energy-intensive but restricts activity range.
  • Migrate: Temporary movement from a stressful habitat to a more hospitable area and return when conditions improve (e.g., Siberian cranes to Keoladeo National Park, Arctic tern).
  • Suspend: Reduce metabolic activity and enter a state of dormancy during unfavorable conditions.
    • Bacteria, Fungi, Lower Plants: Thick-walled spores.
    • Higher Plants: Seeds, vegetative propagules.
    • Animals: Hibernation (winter dormancy, e.g., bears, squirrels), Aestivation (summer dormancy, e.g., snails, fish), Diapause (suspended development in zooplankton/insects).

3. Adaptations

• Definition: Any attribute of the organism (morphological, physiological, behavioural) that enables it to survive and reproduce in its habitat. Adaptations evolve over long periods through natural selection.
• Examples:
  • Morphological: Spines (modified leaves) and photosynthetic stems in Opuntia; thick cuticle on leaves; blubber in seals; mimicry in insects.
  • Physiological: Kangaroo rat's internal water production; altitude sickness acclimatization (increased RBC production, breathing rate); antifreeze proteins in Antarctic fish.
  • Behavioural: Desert lizards basking in the sun (to raise body temp) and moving to shade (to cool down); migration; nocturnal activity in desert animals.

4. Populations

• Definition: Group of individuals of the same species in a defined area.

• Population Attributes: Characteristics of the group, not the individual.

  • Birth Rate (Natality): Number of births per unit population per unit time (e.g., births per 1000 individuals per year).
  • Death Rate (Mortality): Number of deaths per unit population per unit time.
  • Sex Ratio: Ratio of males to females in a population.
  • Age Distribution: Proportion of individuals in different age groups (pre-reproductive, reproductive, post-reproductive). Represented by Age Pyramids:
    • Expanding: Pyramid shape; high proportion of young individuals, high birth rate (e.g., India).
    • Stable: Bell shape; relatively even distribution across age groups.
    • Declining: Urn shape; lower proportion of young individuals, low birth rate (e.g., Japan).
  • Population Density (N): Number of individuals per unit area or volume. Measured by:
    • Total count (census).
    • Sampling methods (e.g., quadrats for plants, traps for animals).
    • Indirect methods (e.g., pug marks, fecal pellets for tigers). Relative density is often used when absolute numbers are hard to get.

• Population Growth: Change in population size over time. Influenced by:

  • Natality (B): Increases population size.
  • Mortality (D): Decreases population size.
  • Immigration (I): Individuals coming into the habitat. Increases size.
  • Emigration (E): Individuals leaving the habitat. Decreases size.
  • Equation: Nt+1 = Nt + [(B + I) – (D + E)]
    • Where Nt = Population density at time t, Nt+1 = Population density at time t+1.

• Growth Models: Describe how populations grow under different conditions.

  • Exponential Growth (Geometric Growth):
    • Occurs when resources (food, space) are unlimited.
    • Results in a J-shaped curve when N is plotted against time (t).
    • Equation: dN/dt = rN
      • dN/dt = Rate of change in population size
      • r = Intrinsic rate of natural increase (b - d)
      • N = Population size
    • Integral form: Nt = N0 * e^(rt)
      • Nt = Population density after time t
      • N0 = Population density at time zero
      • e = Base of natural logarithms (2.71828)
    • 'r' is an important parameter assessing impacts of biotic/abiotic factors on population growth.
  • Logistic Growth (Verhulst-Pearl Logistic Growth):
    • More realistic model as resources are usually limited.
    • Population growth slows down as it approaches the carrying capacity.
    • Results in a Sigmoid or S-shaped curve.
    • Carrying Capacity (K): The maximum population size that an environment can sustain indefinitely given the available resources.
    • Equation: dN/dt = rN [(K-N)/K]
      • (K-N)/K represents environmental resistance – the effect of limiting factors.
    • When N is small compared to K, growth is almost exponential.
    • When N approaches K, growth rate slows down.
    • When N = K, growth rate becomes zero.

5. Population Interactions

• Interactions between different species (interspecific interactions). Can be beneficial (+), detrimental (-), or neutral (0).
  • Mutualism (+/+): Both interacting species benefit.
    • Examples: Lichens (fungi + algae/cyanobacteria); Mycorrhizae (fungi + roots of higher plants); Pollination (plants & pollinators like bees, birds); Fig and Wasp (obligatory relationship); Rhizobium in root nodules. Co-evolution is often seen.
  • Competition (-/-): Both interacting species are negatively affected. Occurs when resources are limited.
    • Intraspecific: Competition among individuals of the same species.
    • Interspecific: Competition between individuals of different species.
    • Gause's Competitive Exclusion Principle: Two closely related species competing for the same limited resources cannot coexist indefinitely; the competitively inferior one will eventually be eliminated. (Exception: Resource partitioning).
    • Resource Partitioning: Species facing competition may evolve mechanisms to avoid it by choosing different times for feeding, different foraging patterns, or different resources (e.g., MacArthur's warblers on the same tree).
    • Connell's field experiment: Demonstrated competitive release with barnacles (Balanus and Chthamalus) on Scottish rocky coasts.
  • Predation (+/-): One species (predator) kills and consumes the other (prey).
    • Importance: Transfers energy across trophic levels; keeps prey populations under control; maintains species diversity by reducing competition among prey.
    • Adaptations: Predators (speed, camouflage, sharp teeth/claws); Prey (camouflage/crypsis, chemical defenses - e.g., Monarch butterfly acquiring toxins from milkweed, thorns/spines in plants, mimicry - Batesian: harmless mimics harmful; Mullerian: two or more unpalatable species resemble each other).
  • Parasitism (+/-): One species (parasite) depends on another species (host) for food and shelter, harming the host.
    • Types: Ectoparasites (live on the external surface of the host, e.g., lice on humans, ticks on dogs, Cuscuta on hedge plants); Endoparasites (live inside the host body, e.g., tapeworm, Plasmodium, liver fluke).
    • Adaptations of Parasites: Loss of unnecessary sense organs, presence of adhesive organs (suckers, hooks), loss of digestive system (tapeworm), high reproductive capacity. Complex life cycles often involving intermediate hosts (e.g., liver fluke - snail and fish).
    • Brood Parasitism: Parasite lays eggs in the nest of the host, letting the host incubate and raise them (e.g., Cuckoo lays eggs in Crow's nest). Co-evolution of egg resemblance.
  • Commensalism (+/0): One species benefits, and the other is neither harmed nor helped.
    • Examples: Orchid growing as an epiphyte on a mango tree (orchid gets shelter, mango is unaffected); Barnacles growing on the back of a whale; Cattle egret foraging near grazing cattle (stirs up insects); Clownfish living among stinging tentacles of sea anemone (gets protection).
  • Amensalism (-/0): One species is harmed, while the other is unaffected.
    • Examples: Penicillium secreting penicillin, which kills bacteria; Allelopathy (secretion of chemicals by one plant that inhibit the growth of other plants, e.g., black walnut).

Multiple Choice Questions (MCQs)

1. Which of the following represents the correct sequence of ecological hierarchy?
   a) Population -> Organism -> Community -> Ecosystem
   b) Organism -> Community -> Population -> Ecosystem
   c) Organism -> Population -> Community -> Ecosystem
   d) Ecosystem -> Community -> Population -> Organism

2. Organisms that can tolerate a wide range of temperatures are called:
   a) Stenothermal
   b) Eurythermal
   c) Stenohaline
   d) Euryhaline

3. The functional role played by an organism in its environment, including resource utilization and interactions, is termed its:
   a) Habitat
   b) Biome
   c) Niche
   d) Territory

4. The logistic population growth model is represented by the equation dN/dt = rN[(K-N)/K]. What does 'K' represent in this equation?
   a) Intrinsic rate of natural increase
   b) Population density at time zero
   c) Carrying capacity
   d) Environmental resistance factor

5. Which type of population interaction is represented by the relationship between a Cuckoo and a Crow, where the Cuckoo lays eggs in the Crow's nest?
   a) Mutualism
   b) Competition
   c) Brood Parasitism
   d) Commensalism

6. Gause's Competitive Exclusion Principle states that two species competing for the same limited resources cannot coexist indefinitely if:
   a) They practice resource partitioning
   b) Their ecological niches are identical
   c) One species is a predator
   d) They show commensalism

7. Which of the following is an example of a physiological adaptation to environmental stress?
   a) Desert lizards basking in the sun
   b) Migration of Siberian cranes
   c) Increased RBC production at high altitudes
   d) Spines on a cactus plant

8. An orchid growing as an epiphyte on a mango tree is an example of:
   a) Parasitism (+/-)
   b) Mutualism (+/+)
   c) Commensalism (+/0)
   d) Amensalism (-/0)

9. A population with a large proportion of pre-reproductive individuals and a smaller proportion of reproductive and post-reproductive individuals will likely show:
   a) Declining growth
   b) Stable growth
   c) Exponential growth initially, followed by a crash
   d) Expanding growth

10. The interaction where one species is harmed and the other is unaffected is known as:
    a) Predation
    b) Competition
    c) Amensalism
    d) Parasitism

Answer Key for MCQs:

1. c
2. b
3. c
4. c
5. c
6. b
7. c
8. c
9. d
10. c

Study these notes thoroughly. Focus on definitions, examples, the different types of interactions with their signs (+, -, 0), and the population growth models. Understanding the underlying concepts is key for tackling application-based questions in your exams. Good luck!