PRELIMS

Ecology and Environmental Science

Ecology is the study of the inter-relationship between living organisms and their environment (Tansley coined "Ecosystem").

I. Components of an Ecosystem

Every ecosystem consists of Biotic (living) and Abiotic (non-living) parts.

1. Biotic Components:
   • Producers: Make their own food (Green plants).
   • Consumers:
     • Primary: Feed on producers (Herbivores).
     • Secondary: Feed on primary consumers (Fox, Wolf).
     • Tertiary: Feed on secondary consumers (Tiger, Lion).
   • Decomposers: Fungi and bacteria; break down dead matter into physical elements.
2. Abiotic Components: Non-living factors like Water, Light, Temperature, Air, Humidity, and Minerals.

II. Energy Flow and Food Chain

• Food Chain: Transfer of energy from producers through a series of organisms, each eaten by the next (unidirectional flow of energy).
• Trophic Levels: Each step in a food chain — 1st (Producers), 2nd (Primary consumers), 3rd (Secondary consumers), 4th (Tertiary consumers).
• Food Web: An interconnected network of multiple, cross-linked food chains in an ecosystem; makes the ecosystem more stable than a single food chain.
• 10% Law (Lindeman's Law): Only about 10% of energy is transferred from one trophic level to the next; the rest is lost as heat/respiration — explains why food chains rarely exceed 4-5 levels.
• Biotic Community: Various populations of living organisms in a definite place.

II(a). Ecological Pyramids

Graphical representation of trophic structure; producers form the base.

• Pyramid of Numbers: Count of organisms at each trophic level; usually upright, but inverted in a tree ecosystem (few trees support many insects/birds).
• Pyramid of Biomass: Total mass of organisms at each level; usually upright, but inverted in aquatic/pond ecosystems (small phytoplankton biomass supports larger fish biomass).
• Pyramid of Energy: Always upright — energy loss at each trophic level is unavoidable (per the 10% Law), so it can never be inverted.

II(b). Biomes and Ecosystem Types

• Biome: A large geographic region with a distinctive climate, flora, and fauna (e.g., Tropical Rainforest, Desert, Grassland, Tundra, Taiga).
• Ecosystem types: Natural (forest, grassland, pond, ocean — self-sustaining) vs Artificial/Man-made (crop fields, aquarium — dependent on human input).
• Ecotone: Transition zone between two different ecosystems (e.g., mangroves between land and sea); shows edge effect — greater species diversity than either adjoining community.
• Ecological Niche: The functional role and position of a species within its habitat (its "occupation").

III. The Nitrogen Cycle

A critical process for converting atmospheric Nitrogen into usable forms for plants.

1. Nitrogen Fixation: Free converted into nitrogenous compounds by living organisms.
2. Ammonification: Formation of Ammonia from organic compounds (Proteins/Nucleic Acids) by microorganisms.
3. Nitrification: Ammonia converted into nitrites and nitrates by nitrobacteria.
4. Denitrification: Conversion of fixed nitrogen (Nitrates) back into free (e.g., Pseudomonas).

UPSC Relevance

Ecology and Biogeochemical cycles are fundamental to GS-III Environment.

• Energy Transfer: The hierarchical structure of consumers (Primary to Tertiary) and the role of decomposers (recyclers).
• Nitrogen Cycle: Identifying the bacterial roles—especially the symbiotic vs. denitrifying (Pseudomonas) bacteria—is a standard Prelims requirement.
• Ecosystem Terminologies: The distinction between Biotic and Abiotic components and Tansley's foundational definition.
• Pyramid Inversions: A favorite trap — Number pyramid (tree ecosystem) and Biomass pyramid (pond/aquatic ecosystem) can invert; the Energy pyramid never inverts.