Exercises

1. Fill in the blanks.

• (a) Plants are called as producers because they fix carbon dioxide.
• (b) In an ecosystem dominated by trees, the pyramid (of numbers) is spindle type.
• (c) In aquatic ecosystems, the limiting factor for the productivity is light/nutrients.
• (d) Common detritivores in our ecosystem are earthworms.
• (e) The major reservoir of carbon on earth is oceans.

2. Which one of the following has the largest population in a food chain?

• (a) Producers
• Explanation: In an upright pyramid of numbers (most common), producers are always the most numerous.

3. The second trophic level in a lake is

• (b) Zooplankton
• Explanation: Phytoplankton (Producers) are the 1st trophic level; Zooplankton (Primary consumers) are the 2nd.

4. Secondary producers are

• (d) None of the above
• Explanation: Herbivores are secondary consumers. Only autotrophs are producers.

5. What is the percentage of photosynthetically active radiation (PAR) in the incident solar radiation?

• (b) 50%
• Note: Less than 50% is PAR, and plants capture only 2-10% of PAR.

6. Distinguish between:

• (a) Grazing food chain (GFC) and detritus food chain (DFC): GFC starts with producers (living plants); DFC starts with dead organic matter.
• (b) Production and decomposition: Production is the synthesis of organic matter (energy input); decomposition is the breakdown of organic matter (releasing nutrients).
• (c) Upright and inverted pyramid: Upright narrows towards the top (e.g., energy); inverted is wider at the top (e.g., biomass in the sea).
• (d) Food chain and Food web: A food chain is a single linear pathway of energy flow; a food web is a network of interconnected food chains.
• (e) Litter and detritus: Litter is fresh organic residue on the soil surface; detritus is partially decomposed organic matter.
• (f) Primary and secondary productivity: Primary is the rate of biomass production by producers; secondary is by consumers.

7. Describe the components of an ecosystem.

• Abiotic Components: Inorganic substances (air, water, soil) and physical factors (temperature, light).
• Biotic Components: Producers (autotrophs), Consumers (heterotrophs), and Decomposers (saprotrophs).

8. Define ecological pyramids and describe with examples, pyramids of number and biomass.

• Ecological Pyramid: A graphical representation of the relationship between organisms at different trophic levels.
• Pyramid of Number: e.g., Grassland (upright). Millions of grass plants support a few top carnivores.
• Pyramid of Biomass: e.g., Forest (upright). The total weight of trees > herbivores > carnivores. e.g., Sea (inverted). Biomass of fish > phytoplankton.

9. What is primary productivity? Give brief description of factors that affect primary productivity.

• The rate at which biomass or organic matter is produced per unit area by plants during photosynthesis.
• Factors: Plant species inhabiting the area, availability of nutrients, solar radiation, temperature, and soil moisture.

10. Define decomposition and describe the processes and products of decomposition.

• The breakdown of complex organic matter into inorganic substances.
• Processes: Fragmentation (by detritivores), Leaching (water-soluble nutrients move down), and Catabolism (enzymatic breakdown).
• Products: Humus (dark organic matter) and inorganic nutrients (CO2, H2O, minerals).

11. Give an account of energy flow in an ecosystem.

• Energy flow is unidirectional (Sun → Producer → Consumer).
• It follows the 10 Per Cent Law, where only 10% of energy is passed to the next level.
• Energy is dissipated as heat at each trophic level (Second Law of Thermodynamics).
• The number of trophic levels is limited because energy eventually becomes insufficient to support higher levels.