Detailed Notes – 2 – CHAPTER 12- ECOSYSTEM

12.3 DECOMPOSITION

1. Decomposition Process:

   • Decomposition is the breakdown of complex organic matter into simpler inorganic substances like carbon dioxide, water, and nutrients.
   • Detritus, which includes dead plant and animal remains, serves as the raw material for decomposition.

2. Steps in Decomposition:

   • Fragmentation: Detritivores (e.g., earthworms) break down detritus into smaller particles.
   • Leaching: Water-soluble inorganic nutrients move down into the soil and precipitate as unavailable salts.
   • Catabolism: Bacterial and fungal enzymes degrade detritus into simpler inorganic substances.
   • Humification: Leads to the accumulation of humus, a dark-colored substance resistant to microbial action, serving as a nutrient reservoir.
   • Mineralisation: Some microbes further degrade humus, releasing inorganic nutrients through mineralisation.

3. Factors Affecting Decomposition:

   • Oxygen Requirement: Decomposition is largely an oxygen-requiring process.
   • Chemical Composition: Detritus rich in lignin and chitin decomposes slower, while detritus rich in nitrogen and water-soluble substances decomposes quicker.
   • Climatic Factors: Temperature and soil moisture are key factors regulating decomposition rates.
     • Warm and moist conditions favor decomposition.
     • Low temperatures and anaerobic conditions inhibit decomposition, leading to organic material buildup.

12.4 ENERGY FLOW  

1. Energy Source in Ecosystems:

   • Solar energy is the primary source of energy for ecosystems, except for deep-sea hydrothermal ecosystems.
   • Only about 2-10% of incident solar radiation (photosynthetically active radiation) is captured by plants, sustaining the entire living world.

2. Unidirectional Energy Flow:

   • Energy flows unidirectionally from the sun to producers (plants) and then to consumers (animals).
   • This flow of energy follows the First Law of Thermodynamics, which states that energy cannot be created or destroyed, only transformed.

3. Producers and Consumers:

   • Green plants are producers in ecosystems, utilizing solar energy for photosynthesis.
   • Consumers depend on producers directly (herbivores) or indirectly (carnivores feeding on other animals).
   • Primary consumers are herbivores, while secondary and tertiary consumers are carnivores.

4. Food Chains and Webs:

   • Food chains and webs depict the interdependency of organisms in an ecosystem, with energy transfer from producers to consumers.
   • Detritus food chains (DFCs) start with dead organic matter and involve decomposers (fungi, bacteria) breaking down detritus into simpler inorganic materials.

5. Trophic Levels and Energy Transfer:

   • Trophic levels represent the position of organisms in the food chain based on their feeding relationships.
   • Energy decreases at successive trophic levels, with only about 10% of energy transferred to each level (10% energy transfer efficiency).

6. Standing Crop and Biomass:

   • Each trophic level has a standing crop, representing the mass of living organisms at a specific time.
   • Biomass is often measured in terms of dry weight for accuracy, as it eliminates water content variations.

7. Limitations in Food Chains:

   • Grazing food chains have a limited number of trophic levels due to the 10% energy transfer rule.
   • Detritus food chains do not have such limitations, as energy transfer is more efficient in detritus decomposition compared to grazing.

12.5 ECOLOGICAL PYRAMIDS

1. Shape and Representation:

   • Ecological pyramids depict food or energy relationships between organisms at different trophic levels.
   • They have a pyramid shape, with a broad base representing producers (first trophic level) and an apex representing top-level consumers (tertiary consumers).

2. Types of Ecological Pyramids:

   • Pyramid of Number: Represents the number of organisms at each trophic level.
   • Pyramid of Biomass: Represents the total biomass of organisms at each trophic level.
   • Pyramid of Energy: Represents the energy flow or transfer between trophic levels.

3. Functional Trophic Levels:

   • Trophic levels represent functional levels rather than specific species.
   • A species can occupy more than one trophic level simultaneously based on its feeding habits. For example, a sparrow can be a primary consumer when eating seeds and a secondary consumer when eating insects.

4. Upright Pyramids:

   • In most ecosystems, pyramids of number, biomass, and energy are upright, with producers > herbivores > carnivores in terms of numbers, biomass, and energy.
   • Energy decreases at higher trophic levels due to energy loss as heat in each step of energy transfer.

5. Exceptions and Inverted Pyramids:

   • Exceptions to upright pyramids include cases like counting insects feeding on a large tree, where the pyramid of numbers would be inverted due to a high number of insects compared to the tree.
   • In marine ecosystems, the pyramid of biomass may be inverted because the biomass of fishes can exceed that of phytoplankton. This is due to the rapid turnover rate of phytoplankton compared to slower-growing fish populations.

6. Pyramid of Energy:

   • The pyramid of energy is always upright because energy decreases as it flows through trophic levels due to energy loss as heat.
   • Each bar in the energy pyramid represents the amount of energy present at each trophic level over time or annually per unit area.

7. Limitations of Ecological Pyramids:

   • They do not account for species occupying multiple trophic levels.
   • They assume a simple food chain rather than a complex food web.
   • Saprophytes, which play a crucial role in decomposition, are not explicitly included in ecological pyramids.