class 12 notes

Class 12 Biology Notes Chapter 13 (Organisms and populations) – Biology Book

Philoid

Philoid

— 7 min read

Biology
Alright students, let's focus on Chapter 13, 'Organisms and Populations'. This chapter forms the bedrock of Ecology and is crucial for understanding the interactions between living beings and their environment. Pay close attention, as concepts from here frequently appear in various government examinations.

Chapter 13: Organisms and Populations - Detailed Notes

1. Introduction to Ecology

  • Ecology: The study of 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 organism adapts and survives in 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.
    • Biome: Large geographical areas characterized by specific climate conditions (temperature, precipitation) and dominant plant and animal communities (e.g., Desert, Rainforest, Tundra, Grassland).
    • Biosphere: All the biomes together; the sum of all ecosystems on Earth.

2. Organism and Its Environment

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

  • Habitat: The specific place where an organism lives, characterized by its physical features and biotic components.

  • Niche: The functional role an organism plays in its ecosystem, including its resource utilization, tolerance range, and interactions with other organisms. It represents the 'profession' of the organism.

  • 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).
    • Water:
      • Life originated in water and it's essential for all living organisms.
      • Availability dictates productivity and distribution, especially in terrestrial habitats (deserts vs. rainforests).
      • For aquatic organisms, water quality (pH, salinity, chemical composition) is critical.
      • Euryhaline: Organisms tolerant of a wide range of salinities (e.g., salmon).
      • Stenohaline: Organisms tolerant of only a narrow range of salinities (e.g., most freshwater fish, sharks).
    • Light:
      • Essential for photosynthesis (autotrophs).
      • Affects foraging, reproductive, and migratory activities of many animals (photoperiodism).
      • Light intensity and quality vary (e.g., deep sea, forest canopy vs. floor). UV component can be harmful.
    • Soil:
      • Nature and properties depend on climate, weathering process, and soil development method.
      • Key properties: Soil composition (particle size), aggregation (determines water holding capacity and percolation), pH, mineral composition, topography (affects erosion, water drainage). These determine vegetation type and supported animal life.

  • Responses to Abiotic Factors: Organisms evolve strategies to cope with environmental stress.

    • (i) Regulate: Maintain constant internal environment (homeostasis) despite external changes (e.g., birds, mammals maintain constant body temperature - thermoregulation; osmoregulation). Energetically expensive, especially for small animals (due to larger surface area to volume ratio).
    • (ii) Conform: Internal environment changes with the ambient conditions (e.g., most plants, invertebrates, fish). Body temperature or osmotic concentration changes with surroundings. Energetically less expensive. 99% of animals and nearly all plants are conformers.
    • (iii) Migrate: Temporary movement from a stressful habitat to a more hospitable area and return when conditions improve (e.g., Siberian cranes migrating to Keoladeo National Park, Bharatpur).
    • (iv) Suspend: Reduce metabolic activity and enter a dormant state during unfavourable conditions.
      • Hibernation: Winter sleep (e.g., bears, squirrels).
      • Aestivation: Summer sleep to avoid heat and desiccation (e.g., snails, fish).
      • Diapause: Stage of suspended development in zooplankton under unfavourable conditions.
      • Spores/Cysts: Thick-walled structures in bacteria, fungi, lower plants.
      • Seed Dormancy: In higher plants to tide over stress, germinate under favourable conditions.

3. Adaptations

  • Any attribute of the organism (morphological, physiological, behavioural) that enables it to survive and reproduce in its habitat. Adaptations evolve over long evolutionary time through natural selection.
  • Examples:
    • Kangaroo Rat (Desert): Meets water needs through internal fat oxidation (water is a byproduct), excretes highly concentrated urine.
    • Desert Plants (Xerophytes): Thick cuticle, sunken stomata (reduce transpiration), CAM pathway (stomata open at night), reduced leaves (spines), extensive root systems.
    • Mammals in Cold Climates: Shorter ears and limbs to minimize heat loss (Allen's Rule). Blubber (thick fat layer) for insulation (e.g., seals).
    • Altitude Sickness Adaptation: At high altitudes (low O2), humans initially experience nausea, fatigue. Acclimatisation involves increased RBC production, decreased haemoglobin binding affinity, increased breathing rate.
    • Desert Lizards: Behavioural adaptation - bask in the sun to absorb heat when cool, move into shade when hot (thermoregulation).
    • Deep Sea Organisms: Biochemical adaptations to withstand immense pressure.

4. Populations

  • Population Attributes: Characteristics unique to a population, not an individual.

    • Birth Rate (Natality): Per capita births (e.g., births per 1000 individuals per year).
    • Death Rate (Mortality): Per capita deaths.
    • Sex Ratio: Ratio of males to females in a population.
    • Age Distribution: Proportion of individuals in different age groups (pre-reproductive, reproductive, post-reproductive). Plotted as an Age Pyramid.
      • Expanding Pyramid: High proportion of young individuals (growing population). Triangular shape.
      • Stable Pyramid: Even distribution across age groups (stable population). Bell-shaped.
      • Declining Pyramid: Higher proportion of older individuals (declining population). Urn-shaped.
    • Population Density (N): Number of individuals per unit area or volume. Can be measured by total count, sampling (quadrats, traps), or indirect methods (pugmarks, fecal pellets). Relative density is often used when absolute density is hard to measure.

  • Population Growth: Size of a population changes over time depending on:

    • Natality (B): Number of births during a given period.
    • Mortality (D): Number of deaths during a given period.
    • Immigration (I): Number of individuals of the same species that have come into the habitat from elsewhere.
    • Emigration (E): Number of individuals of the population who left the habitat.
    • Equation: N(t+1) = N(t) + [(B + I) – (D + E)]
      • (B+I) increases population density.
      • (D+E) decreases population density.

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

    • (i) Exponential Growth:
      • Occurs when resources (food, space) are unlimited.
      • Equation: dN/dt = rN
        • dN/dt = rate of change in population size
        • N = population size
        • r = intrinsic rate of natural increase (b-d, birth rate minus death rate)
      • Integral form: N(t) = N(0)e^(rt)
        • N(t) = population density after time t
        • N(0) = population density at time zero
        • e = base of natural logarithms (2.71828)
      • Results in a J-shaped curve when N is plotted against time. Characteristic of populations colonizing a new habitat or recovering after a crash. Not sustainable long-term in nature.
    • (ii) Logistic Growth:
      • Occurs when resources are limited. More realistic model.
      • Introduces Carrying Capacity (K): The maximum population size that a given environment can sustain indefinitely.
      • Equation: dN/dt = rN [(K – N) / K]
        • (K – N) / K represents environmental resistance. As N approaches K, growth rate slows down.
      • Results in a Sigmoid or S-shaped curve when N is plotted against time. Shows phases: lag phase, acceleration phase, deceleration phase, and asymptote (when N=K).

5. Population Interactions

  • Interactions between different species in a community. Can be beneficial (+), detrimental (-), or neutral (0).
    • Mutualism (+/+): Both interacting species benefit.
      • Examples: Lichens (fungus + alga/cyanobacterium), Mycorrhizae (fungi + roots of higher plants), Pollination (plants + pollinators), Fig and Wasp (specific pollination).
    • Competition (-/-): Both species are negatively affected. Occurs when resources are limited.
      • Interference Competition: Direct fighting or preventing access to resources.
      • Exploitative Competition: Indirectly competing by consuming shared limited resources.
      • Gause's Competitive Exclusion Principle: Two species competing for the same limiting resources cannot coexist indefinitely if other ecological factors are constant; the competitively inferior one will be eliminated eventually.
      • Resource Partitioning: Species facing competition may evolve mechanisms to co-exist by using different resources or using the same resource at different times or places (e.g., MacArthur's warblers foraging in different parts of the same tree).
    • Predation (+/-): One species (predator) kills and consumes the other (prey).
      • Role: Transfers energy across trophic levels, keeps prey populations under control.
      • Prey Defenses: Camouflage (cryptic coloration), chemical defenses (monarch butterfly), toxins, thorns (plants), mimicry.
      • Predator Adaptations: Speed, stealth, camouflage, sharp claws/teeth.
    • Parasitism (+/-): One species (parasite) depends on the other (host) for food and shelter, harming the host.
      • 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, liver fluke, Plasmodium). Often have complex life cycles and reduced sensory/locomotory organs.
      • Brood Parasitism: Parasitic bird lays eggs in the nest of a host bird, letting the host incubate and raise them (e.g., Cuckoo lays eggs in Crow's nest).
    • Commensalism (+/0): One species benefits, the other is unaffected.
      • Examples: Orchid growing as an epiphyte on a mango tree, Barnacles growing on the back of a whale, Cattle egret foraging near grazing cattle (insects flushed out), Clownfish living among tentacles of sea anemone.
    • Amensalism (-/0): One species is harmed, the other is unaffected.
      • Examples: Penicillium secreting penicillin which kills bacteria (often considered antibiosis/competition), Allelopathy (plants releasing chemicals that inhibit growth of nearby plants).

Multiple Choice Questions (MCQs)

  1. Which of the following represents the correct sequence of ecological organisation from lower to higher level?
    a) Population -> Organism -> Community -> Biome
    b) Organism -> Community -> Population -> Biome
    c) Organism -> Population -> Community -> Biome
    d) Biome -> Community -> Population -> Organism

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

  3. Aestivation is a mechanism used by some animals primarily to cope with:
    a) Extreme cold and food scarcity
    b) High temperatures and desiccation
    c) Low oxygen levels at high altitudes
    d) High water salinity

  4. Allen's Rule states that mammals living in colder climates tend to have:
    a) Longer limbs and ears
    b) Shorter limbs and ears
    c) Thicker fur only
    d) Higher metabolic rate only

  5. An age pyramid with a broad base, tapering towards the top, indicates:
    a) A declining population
    b) A stable population
    c) An expanding population
    d) A population with high emigration

  6. Under which condition does a population typically exhibit exponential growth (J-shaped curve)?
    a) When resources are limited
    b) When the carrying capacity (K) is reached
    c) When resources are unlimited
    d) When predation pressure is high

  7. The term 'K' in the logistic growth equation dN/dt = rN[(K-N)/K] represents:
    a) Intrinsic rate of natural increase
    b) Population density at time zero
    c) Carrying capacity of the environment
    d) Environmental resistance factor

  8. The interaction between fungi and algae/cyanobacteria in lichens is an example of:
    a) Parasitism
    b) Commensalism
    c) Mutualism
    d) Competition

  9. Gause's principle of competitive exclusion applies when:
    a) Two species cooperate for mutual benefit
    b) Two species compete for different limiting resources
    c) Two species compete for the same limiting resources
    d) One species preys upon another

  10. The relationship between cattle egrets and grazing cattle, where egrets benefit from insects flushed out by cattle movement and cattle are unaffected, is an example of:
    a) Mutualism
    b) Predation
    c) Parasitism
    d) Commensalism

Answer Key:

  1. c) Organism -> Population -> Community -> Biome
  2. b) Eurythermal
  3. b) High temperatures and desiccation
  4. b) Shorter limbs and ears
  5. c) An expanding population
  6. c) When resources are unlimited
  7. c) Carrying capacity of the environment
  8. c) Mutualism
  9. c) Two species compete for the same limiting resources
  10. d) Commensalism

Revise these notes thoroughly. Understanding the definitions, examples, and the underlying concepts of interactions and adaptations is key. Good luck with your preparation!

Read more