Organisms and Populations NCERT Line by Line Notes | NEET 2026 Ecology

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Organisms and Populations

NCERT Line-by-Line Breakdown for NEET 2026

Unit 5: Ecology and Environment

Ecology is the study of interactions among organisms and between the organism and its physical (abiotic) environment. Ramdeo Misra is revered as the Father of Ecology in India. The four levels of biological organization are: Organisms, Populations, Communities, and Biomes.

1. Major Abiotic Factors

The key elements that lead to variation in physical and chemical conditions are:

1. Temperature (Most Ecologically Relevant)
Affects enzyme kinetics (Basal Metabolism).
• Eurythermal: Tolerate wide range (Mammals).
• Stenothermal: Restricted to narrow range (Polar bears).
2. Water
Life is unsustainable without it. For aquatic organisms, pH and Salinity are important.
• Euryhaline: Wide salinity tolerance (Salmon).
• Stenohaline: Narrow salinity tolerance (Sharks).

Responses to Abiotic Factors

How do organisms cope with stressful conditions?

  • Regulate: Maintain constant internal body temp/osmotic conc. (Homeostasis). Birds & Mammals.
  • Conform: Body temp changes with ambient temp. (99% of animals, nearly all plants). Small animals (shrews/hummingbirds) are rarely found in polar regions due to heat loss (Surface area: Volume ratio).
  • Migrate: Move temporarily to hospitable area (e.g., Keoladeo National Park hosts Siberian cranes).
  • Suspend:
    • Hibernation: Winter sleep (Bears).
    • Aestivation: Summer sleep (Snails/Fish).
    • Diapause: Suspended development (Zooplankton).

2. Adaptations

Morphological, physiological, or behavioral attributes that enable an organism to survive and reproduce.

A. Desert Adaptations

  • Kangaroo Rat: Meets water requirement through internal fat oxidation (water is byproduct). Concentrates urine.
  • Desert Plants (Opuntia): Thick cuticle, sunken stomata (CAM pathway – stomata closed during day), leaves reduced to spines.

B. Cold Adaptations

🐻 Allen’s Rule: Mammals from colder climates generally have shorter ears and limbs to minimize heat loss.

Blubber: Thick layer of fat in seals acting as insulator.

C. Altitude Sickness (Physiological)

At high altitude (>3,500m like Rohtang Pass), low atmospheric pressure causes nausea, fatigue, heart palpitations.
Adaptation: Body increases RBC production, decreases binding affinity of hemoglobin, and increases breathing rate.

3. Population Attributes

A population has attributes that an individual organism does not.

  • Birth Rate & Death Rate: Expressed as change per capita numbers.
  • Sex Ratio: Ratio of males to females.
  • Age Pyramids: Plotting age distribution.
    • Expanding: Triangular (High pre-reproductive).
    • Stable: Bell-shaped.
    • Declining: Urn-shaped (Low pre-reproductive).

Population Growth Models

Population density (N) changes due to: Natality (B), Mortality (D), Immigration (I), Emigration (E).

Nt+1 = Nt + [(B + I) – (D + E)]

Exponential Growth (J-shaped)Logistic Growth (S-shaped)
Unlimited resources.Limited resources (Realistic).
Formula: dN/dt = rNFormula: dN/dt = rN (K-N/K)
r = Intrinsic rate of natural increase.K = Carrying Capacity (Max supportable population).
[Image of population growth curves J shaped and S shaped]

4. Population Interactions

InteractionSpecies ASpecies BExamples
Mutualism++Lichens (Algae+Fungi), Mycorrhiza, Fig & Wasp (Co-evolution).
Competition––Flamingoes & Fish competing for Zooplankton.
Gause’s Exclusion Principle: Two closely related species competing for same resource cannot co-exist.
Predation+–Tiger & Deer. Prickly Pear Cactus & Moth (Biological control).
Parasitism+–Cuscuta (Ectoparasite), Liver fluke (Endoparasite). Brood parasitism (Cuckoo & Crow).
Commensalism+0Orchid on Mango branch, Cattle egret & Cattle, Sea anemone & Clown fish.
Amensalism–0Penicillium secreting penicillin killing bacteria.

📝 Rapid Fire MCQs

Q1. Which of the following is not an adaptation for parasites?

  • A) Loss of unnecessary sense organs
  • B) Presence of adhesive organs
  • C) Developed digestive system
  • D) High reproductive capacity
Click to check Answer

Answer: C) Developed digestive system (Parasites often lose digestive systems).

Q2. The logistic population growth is expressed by the equation:

  • A) dN/dt = rN
  • B) dN/dt = rN (K-N/K)
  • C) dN/dt = rN (N-K/N)
  • D) dN/dt = rN (K-N/N)
Click to check Answer

Answer: B) dN/dt = rN (K-N/K).

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