Samacheer Kalvi 9th Science Environmental Science Exercise Solutions

An ecosystem is a functional unit of nature where living (biotic) organisms interact with each other and with their physical (abiotic) environment through energy flow and nutrient cycling. Examples: pond ecosystem, forest ecosystem, grassland ecosystem.

• Biotic components: All living organisms — producers (plants), consumers (animals), and decomposers (bacteria, fungi).
• Abiotic components: Non-living factors — sunlight, temperature, water, soil, air, and minerals.

A food chain is a linear sequence of organisms through which energy and nutrients flow, starting from producers and ending with decomposers. Example: Grass → Grasshopper → Frog → Snake → Eagle.

A food web is an interconnected network of multiple food chains in an ecosystem, showing all possible feeding relationships among organisms. It provides alternative pathways for energy flow, making the ecosystem more stable.

Biodiversity is the variety of life forms at all levels of organisation — genetic diversity (within a species), species diversity (between species), and ecosystem diversity (different habitats). It is essential for ecosystem stability and human well-being.

Sustainable development is development that meets the needs of the present generation without compromising the ability of future generations to meet their own needs. It balances economic growth, environmental protection, and social equity.

Biomagnification is the progressive increase in the concentration of non-biodegradable toxic substances (e.g., DDT, mercury) at successive trophic levels in a food chain. Organisms at higher trophic levels accumulate higher concentrations.

Lindeman's 10% law states that only about 10% of the energy available at one trophic level is transferred to the next trophic level. The remaining 90% is used by organisms for metabolic processes (respiration, movement, digestion) or lost as heat to the environment.

Example: If producers capture 10,000 kJ of solar energy:

• Primary consumers receive ~1,000 kJ
• Secondary consumers receive ~100 kJ
• Tertiary consumers receive ~10 kJ

This explains why food chains rarely have more than 4–5 trophic levels.

1. Habitat destruction — Deforestation, urbanisation, and agricultural expansion destroy natural habitats. 2. Pollution — Air, water, and soil pollution harm organisms directly or indirectly. 3. Overexploitation — Overfishing, overhunting, and overharvesting deplete species. 4. Invasive species — Non-native species outcompete native species for resources. 5. Climate change — Global warming alters habitats, forcing species to migrate or face extinction. 6. Poaching — Illegal hunting of endangered species for trade (e.g., tigers, elephants).

Decomposers (bacteria and fungi) break down dead organic matter — dead plants, animals, and waste products — into simple inorganic substances. This process: 1. Releases nutrients (carbon, nitrogen, phosphorus) back into the soil. 2. Completes the nutrient cycle, making nutrients available to producers again. 3. Prevents accumulation of dead matter in the ecosystem. 4. Maintains soil fertility.

Without decomposers, nutrients would remain locked in dead organisms, and the ecosystem would collapse.

Ozone depletion is the thinning of the ozone layer in the stratosphere, primarily caused by chlorofluorocarbons (CFCs). When CFCs reach the stratosphere, UV radiation breaks them down, releasing chlorine atoms that catalyse the destruction of ozone molecules (O₃).

Effects: 1. Increased UV-B radiation reaches Earth, causing skin cancer and cataracts in humans. 2. Weakens the immune system. 3. Damages phytoplankton, disrupting aquatic food chains. 4. Affects plant growth and crop yields. 5. Accelerates ageing of materials (plastics, paints).

Causes: 1. Burning of fossil fuels (coal, petroleum) releasing CO₂. 2. Deforestation reducing CO₂ absorption. 3. Methane from rice paddies, cattle, and landfills. 4. Industrial processes releasing N₂O and other greenhouse gases.

Effects: 1. Rise in average global temperature. 2. Melting of polar ice caps and glaciers — rising sea levels. 3. Changes in rainfall patterns — floods and droughts. 4. Threat to biodiversity — species extinction. 5. Increased frequency of extreme weather events (cyclones, heat waves). 6. Spread of tropical diseases to new regions.

Structure of an Ecosystem:

An ecosystem has two main structural components:

1. Biotic (Living) Components:

• Producers (Autotrophs): Green plants, algae, and some bacteria that synthesise food using sunlight via photosynthesis.
• Consumers (Heterotrophs):
• Primary consumers (herbivores): deer, rabbit, grasshopper
• Secondary consumers (carnivores): frog, snake
• Tertiary consumers (top carnivores): eagle, lion
• Decomposers: Bacteria and fungi that break down dead organic matter.

2. Abiotic (Non-living) Components:

• Physical factors: sunlight, temperature, rainfall, wind
• Chemical factors: oxygen, carbon dioxide, nitrogen, minerals, pH of soil/water

Function of an Ecosystem:

1. Energy flow: Solar energy → Producers → Herbivores → Carnivores → Decomposers (unidirectional). 2. Nutrient cycling: Carbon cycle, nitrogen cycle, water cycle — nutrients circulate between biotic and abiotic components. 3. Productivity: Rate of biomass production at each trophic level.

Example — Pond Ecosystem:

• Abiotic: Water, dissolved oxygen, sunlight, temperature, pH
• Producers: Phytoplankton, algae, aquatic plants (Hydrilla, Vallisneria)
• Primary consumers: Zooplankton, small fish, tadpoles
• Secondary consumers: Frogs, medium fish
• Tertiary consumers: Large fish, water birds
• Decomposers: Bacteria and fungi at the bottom

Energy Flow in an Ecosystem:

1. Sun as ultimate source: The sun is the primary source of energy for almost all ecosystems. Producers capture 1–3% of solar energy through photosynthesis. 2. Unidirectional flow: Energy flows from the sun → producers → consumers → decomposers. It never flows in the reverse direction. 3. Trophic level transfer: At each trophic level, energy is used for:

• Metabolic activities (respiration, digestion, movement)
• Growth and reproduction
• Lost as heat to the environment

Lindeman's 10% Law (1942):

Only about 10% of the energy at one trophic level is transferred to the next trophic level.

Ecological Significance:

1. Limits food chain length: Since energy decreases sharply at each level, food chains rarely exceed 4–5 trophic levels. 2. Explains pyramid of energy: Energy pyramids are always upright — more energy at the base, less at the top. 3. Supports more herbivores than carnivores: Lower trophic levels have more energy, so populations are larger at the base. 4. Humans and energy efficiency: Eating grains (T2) is more energy-efficient than eating meat (T3), as more energy is available at lower trophic levels. 5. Conservation implication: Protecting producers and primary consumers is critical since all higher-level organisms depend on their energy.

Causes: Burning of fossil fuels (coal, petroleum), industrial emissions, vehicle exhaust, forest fires, agricultural burning.

Effects: Respiratory diseases (asthma, bronchitis), acid rain (SO₂ + NOₓ), global warming (CO₂), smog formation, damage to crops and buildings.

Control measures: Use clean fuels (CNG, LPG), install scrubbers and electrostatic precipitators in factories, plant more trees, enforce emission standards, use renewable energy (solar, wind).

Causes: Industrial effluents, domestic sewage, agricultural runoff (fertilisers, pesticides), oil spills, thermal pollution from power plants.

Effects: Eutrophication (algal blooms deplete oxygen), waterborne diseases (cholera, typhoid), death of aquatic organisms, biomagnification of toxins through food chains, contamination of drinking water.

Control measures: Treat sewage before discharge, regulate industrial effluents, reduce use of chemical fertilisers and pesticides, clean oil spills promptly, adopt rainwater harvesting.

Causes: Excessive use of chemical fertilisers and pesticides, dumping of industrial waste, plastic waste, mining activities, improper waste disposal.

Effects: Reduced soil fertility, contamination of groundwater, toxic chemicals entering the food chain, loss of soil microorganisms, health problems in humans and animals.

Control measures: Use organic farming methods, practise crop rotation, proper waste disposal and recycling, ban single-use plastics, bioremediation of contaminated soil.