Nature is not a collection of random events — it is an intricate, interconnected system where every living and non-living component plays a role. Ecosystems, food webs, nutrient cycles, and natural balance are the themes of this chapter.
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Key Concepts and Definitions
Ecosystem: A community of living organisms (plants, animals, microorganisms) interacting with their non-living environment (soil, water, air, sunlight). Types: forest, grassland, pond, ocean, desert.
Biotic components: Living parts — producers, consumers, decomposers.
Abiotic components: Non-living parts — temperature, water, light, soil, minerals.
- Producers (Autotrophs): Make food via photosynthesis. Example: green plants, algae.
- Consumers (Heterotrophs): Eat other organisms.
- Primary consumers (herbivores): Eat plants — grasshoppers, rabbits.
- Secondary consumers: Eat primary consumers — frogs, foxes.
- Tertiary consumers: Eat secondary consumers — hawks, large fish.
Decomposers: Break down dead matter; recycle nutrients to soil. Example: bacteria, fungi.
Food Chain: Linear sequence of who eats whom. Energy flows one way: Producer → Primary → Secondary → Tertiary Consumer.
Food Web: Network of interconnected food chains — more realistic. Shows what happens if one species is removed.
10% Law: ~90% of energy is lost as heat at each trophic level; only ~10% passes forward. This limits food chains to 4-5 levels.
- Nutrient Cycles:
- Water cycle: Evaporation → Condensation → Precipitation → Runoff.
- Carbon cycle: CO2 absorbed (photosynthesis) → released (respiration, decomposition, combustion).
- Nitrogen cycle: N2 fixed by Rhizobium → NH3 → absorbed by plants → returned by decomposers → denitrification back to N2.
Biodiversity: Variety of life in an area. High biodiversity = more stable ecosystem.
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Worked Examples
Food chain for a grassland — trophic levels.
Grass (Producer, L1) → Grasshopper (Primary, L2) → Frog (Secondary, L3) → Snake (Tertiary, L4) → Eagle (Quaternary, L5).
10% law — energy to snake in above chain if grass = 10,000 J.
Grasshopper: 10% of 10,000 = 1,000 J. Frog: 100 J. Snake: 10 J.
Why are decomposers essential?
Without them, nutrients stay locked in dead matter, soil fertility drops, plants cannot grow, and the ecosystem collapses. Decomposers are nature's recyclers.
All frogs removed from a pond?
Insects overpopulate, devouring plants. Frog predators (birds, snakes) decline. This is a trophic cascade.
Role of Rhizobium in the nitrogen cycle?
Most organisms cannot use N2 directly. Rhizobium in legume nodules converts N2 → NH3, which plants absorb for protein synthesis.
Why do humans accumulate more DDT than grass?
DDT is not broken down and concentrates at each trophic level (biomagnification). By the time it reaches humans (high trophic level), it is thousands of times more concentrated.
How does the carbon cycle maintain balance?
Photosynthesis absorbs CO2; respiration and decomposition release it. Normally balanced — but human combustion has raised atmospheric CO2, disrupting the balance.
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Key Formulas / Facts
- 10% Law: Energy at next level = 10% of energy at current level
- Photosynthesis: 6CO2 + 6H2O + light → C6H12O6 + 6O2
- Nitrogen fixation: N2 → NH3 (by Rhizobium)
- Biomagnification: toxin concentration increases at higher trophic levels
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Common mistakes
> Arrows in a food chain point FROM the organism eaten TO the one eating it — showing energy flow direction. Decomposers are a critical trophic level, not optional. A food web is a network, not a single chain.
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Summary
Ecosystems are balanced communities of biotic and abiotic components. Energy flows through trophic levels via food chains and webs, with 90% lost at each transfer. Decomposers recycle nutrients. Nutrient cycles (water, carbon, nitrogen) maintain the chemistry of life. Biodiversity strengthens stability. Human activities disrupt this natural harmony.