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Biology Core

Ecology

Biology Unit 10
25 min read
IAT Core Framework
High Yield

Core Concept

Ecology is the study of the complex interactions between organisms and their physical environment. It seamlessly scales from the individual to the population, the community, the ecosystem, and finally the entire biosphere. The central themes are the Flow of Energy, the Cycling of Nutrients, and the Dynamics of Growth and Conservation.

Ecosystem Components

  • Abiotic: Non-living factors (Light, temperature, water, soil).
  • Biotic: Living factors (Producers, consumers, decomposers).

1. Organisms and Populations

A. Adaptations

  • Morphological: Physical features (e.g., thick fur in polar bears, sunken stomata in xerophytes).
  • Physiological: Internal processes (e.g., kangaroo rats conserving water through concentrated urine; Altitude Sickness compensation).
  • Behavioral: Actions (e.g., migration in birds; desert lizards basking in sun to regulate body temperature).

B. Population Attributes

Age Pyramids:

  • Expanding: Triangular shape (Broad base of pre-reproductive individuals).
  • Stable: Bell-shaped (Nearly equal pre-reproductive and reproductive individuals).
  • Declining: Urn-shaped (Fewer pre-reproductive individuals).

Population Growth Models

  • Exponential (Geometric) Growth: Occurs when resources are unlimited.
    Equation: dN/dt = rN   (where r = intrinsic rate of natural increase). Graph: J-shaped.
  • Logistic (Sigmoid) Growth: The realistic model; resources are strictly limited.
    Equation: dN/dt = rN &left[
    K - NK
    &right]
    (where K = Carrying Capacity: Maximum population size an environment can support).
    Graph: S-shaped (Sigmoid). Also called Verhulst-Pearl Logistic Growth.

Life History Variation

  • r-selected: Small size, short lifespan, high reproduction (e.g., insects, oysters). Adapt to unstable environments.
  • K-selected: Large size, long lifespan, low reproduction (e.g., humans, elephants). Adapt to stable environments hitting carrying capacity.

C. Population Interactions

Keystone Species: Species that have a disproportionately large effect on their environment relative to their abundance (e.g., Sea star Pisaster, Sea otters).

Interaction Species A Species B Example
Mutualism + + Lichen (Algae/Fungi), Mycorrhiza
Competition − − Abingdon tortoise and Goats (Galapagos)
Predation + − Tiger and Deer; Sea star Pisaster
Parasitism + − Liver fluke, Cuscuta (Dodder)
Commensalism + 0 Orchid on Mango branch; Barnacles on Whale
Amensalism − 0 Penicillium inhibiting bacteria

Gause’s Competitive Exclusion Principle: Two species directly competing for the exact same resource cannot securely coexist; the inferior one is eventually eliminated.

2. Ecosystem

A. Productivity & Decomposition

Primary Productivity:

  • GPP (Gross): Total rate of organic matter production during photosynthesis.
  • NPP (Net): GPP minus respiration losses (R). NPP = GPP - R. (This is the biomass actually available to consumers).

Decomposition (Detritus → Inorganic):

  • Steps: Fragmentation (by detritivores) → Leaching (water-soluble nutrients percolate down) → Catabolism (enzymatic breakdown) → Humification (forming dark amorphous humus) → Mineralization (release of minerals).
  • Factors: Decomposition is noticeably slower if detritus is rich in Lignin/Chitin. It is significantly faster in warm and moist environments.

B. Energy Flow & Pyramids

  • 10% Law (Lindeman): Only a mere 10% of energy is efficiently transferred to the next trophic level; 90% is irreversibly lost as heat.
  • Ecological Pyramids:
    • Pyramid of Energy: ALWAYS strictly upright (due to unidirectional energy flow).
    • Pyramid of Number: Usually upright, but noticeably inverted for a single large tree supporting many birds and parasites.
    • Pyramid of Biomass: Upright on land; but inverted in Aquatic ecosystems (Biomass of fishes > Biomass of phytoplankton).

C. Ecological Succession

  • Definition: Gradual and fairly predictable change in the species composition of a given area.
  • Primary Succession: Starts from bare rock/sand (extremely slow).
  • Secondary Succession: Starts in areas where natural biotic communities have been destroyed (abandoned farmland, burned forest) - much faster.
  • Pioneer Species: The first species to colonize barren land (e.g., Lichens on rocks, Phytoplankton in water).

3. Biodiversity and Conservation

A. Patterns of Biodiversity

  • Latitudinal Gradients: Species diversity significantly decreases as we move from the equator to the poles. (Tropics are environmentally stable, evolutionarily older, and receive far more solar energy).
  • Species-Area Relationship (Alexander von Humboldt): Within a region, species richness increases with area, but strictly only up to a limit.
log S = log C + Z log A
Slope Z is usually 0.1 to 0.2. However, for massive large continents, Z is much steeper (0.6 to 1.2).

B. Biodiversity Loss & Conservation

The "Evil Quartet":

  1. Habitat loss & fragmentation (Most important cause).
  2. Over-exploitation.
  3. Alien species invasion (e.g., Lantana, Nile Perch).
  4. Co-extinctions.

Conservation Strategies:

  • In-situ (On-site): Protecting the whole ecosystem natively.
    • Biodiversity Hotspots: Regions with exceptionally high species richness and high endemism, which are under constant threat (34 globally).
    • Includes National Parks, Biosphere Reserves, and Sacred Groves.
  • Ex-situ (Off-site): Protecting threatened species outside their natural habitats. Includes Zoos, Botanical Gardens, Cryopreservation (stored at -196°C), and Seed Banks.

4. Common Mistakes

  • Logistic Growth: Forgetting that when N = K (population hits carrying capacity), the growth rate (dN/dt) logically becomes Zero.
  • Decomposition Factors: Thinking that anaerobic conditions somehow speed up decomposition. No, lack of oxygen strongly inhibits it.
  • Biomass Pyramid: Mistakenly assuming the aquatic biomass pyramid is always upright. Remember: Phytoplankton turn over very fast, so their standing crop/biomass is surprisingly low compared to the fish they support.
  • Energy Pyramid: Never pick an "Inverted" option for the Pyramid of Energy. It violates thermodynamic laws and is physically impossible.

5. IAT Exam Focus Points

  • Interaction Nuances: Deeply focus on Competition (specifically Resource partitioning demonstrated by MacArthur) and recognizing trick Commensalism examples.
  • Calculation-based Z values: Knowing exactly what happens to the slope of the Species-Area curve when the area becomes massive (like entire continents → steeper slope).
  • Decomposition Steps: IAT frequently asks the correct chronological order of the 5 decomposition steps.
  • Conservation Categories: You absolutely must be able to distinguish between In-situ (National Parks/Biosphere Reserves) and Ex-situ (Zoos/Cryopreservation).
  • Productivity units: Memorize g/m2/yr or kcal/m2/yr. Note that the NPP of the whole biosphere is ~170 billion tons, but oceans contribute a mere 55 billion (despite covering 70% of Earth).

6. Practice Mock Test

Ready to test your knowledge?

Take a quick 15-question assessment specifically designed for Ecology. Challenge yourself with IAT-level questions.

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End of Chapter

Ecology

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