Ecosystem

Key Concepts

Ecosystem Structure and Function

An ecosystem is a functional unit of nature where living organisms interact among themselves and with their physical environment.

• Categories: Terrestrial (forest, grassland, desert) and Aquatic (pond, lake, wetland, river, estuary).
• Stratification: Vertical distribution of different species occupying different levels (e.g., trees → shrubs → herbs).
• Key Aspects: Productivity, Decomposition, Energy flow, and Nutrient cycling.

Productivity

• Primary Production: Amount of biomass or organic matter produced per unit area by plants during photosynthesis.
• Gross Primary Productivity (GPP): Total rate of production of organic matter.
• Net Primary Productivity (NPP): GPP minus respiration losses (R). NPP = GPP - R. It is the biomass available for heterotrophs.
• Secondary Productivity: Rate of formation of new organic matter by consumers.
• Global NPP: ~170 billion tons. Oceans contribute only 55 billion tons despite covering 70% of Earth's surface.

Decomposition

Breakdown of complex organic matter into inorganic substances (CO2, H2O, nutrients).

• Detritus: Raw material (dead leaves, bark, animal remains).
• Steps:
  1. Fragmentation: Detritivores (e.g., earthworm) break detritus into smaller particles.
  2. Leaching: Water-soluble nutrients move down into the soil.
  3. Catabolism: Bacterial and fungal enzymes degrade detritus into simpler inorganic substances.
  4. Humification: Formation of dark, amorphous humus (resistant to microbes, nutrient reservoir).
  5. Mineralisation: Release of inorganic nutrients from humus.
• Factors: Faster if detritus is rich in Nitrogen/Sugars; slower if rich in Lignin/Chitin. Warm and moist environments favour decomposition.

Energy Flow

• PAR: Photosynthetically Active Radiation (< 50% of incident solar radiation).
• First Law of Thermodynamics: Energy is transferred, not created or destroyed.
• Second Law of Thermodynamics: Energy is lost as heat at each transfer (increasing disorder).
• 10 Per Cent Law: Only 10% of energy is transferred to the next trophic level.
• Food Chains:
  • Grazing Food Chain (GFC): Producer → Herbivore → Carnivore.
  • Detritus Food Chain (DFC): Begins with dead organic matter. Major conduit in terrestrial ecosystems.
• Food Web: Natural interconnection of food chains.

Ecological Pyramids

Express relationship between organisms at different trophic levels in terms of number, biomass, or energy.

• Pyramid of Number: Usually upright (except for a big tree supporting many insects).
• Pyramid of Biomass: Usually upright; Inverted in sea (phytoplankton biomass < fish biomass).
• Pyramid of Energy: Always upright; energy is lost at each step.
• Limitations: Does not account for same species in multiple trophic levels, assumes simple food chains, and ignores saprophytes.