Our environment includes all the biotic (living) and abiotic (non-living) components that surround and interact with organisms. This chapter focuses on ecosystems, food chains and webs, energy flow, and environmental issues such as ozone depletion.
Ecosystem
An ecosystem is a self-sustaining unit comprising a community of living organisms (biotic) and their physical environment (abiotic) interacting as a system.
- Biotic components:
- Producers (Autotrophs): Green plants and algae that make food by photosynthesis.
- Consumers (Heterotrophs): Organisms that consume other organisms.
- Herbivores (Primary consumers): Eat plants (e.g., grasshoppers, deer).
- Carnivores (Secondary consumers): Eat herbivores (e.g., frogs, small fish).
- Top carnivores (Tertiary consumers): Eat secondary consumers (e.g., eagle, large fish).
- Decomposers: Break down dead organic matter (bacteria, fungi) and return nutrients to the environment.
Abiotic components: Sunlight, water, air, temperature, soil, minerals.
Food Chain and Food Web
Food chain: A linear sequence showing how energy and nutrients pass from one organism to the next. Arrow shows direction of energy flow: Producer → Primary consumer → Secondary consumer → Tertiary consumer.
Example: Grass → Grasshopper → Frog → Snake → Eagle.
Food web: A network of interconnected food chains, representing the complex feeding relationships in an ecosystem.
Trophic level: Each step in a food chain is a trophic level. Producers are at trophic level 1; primary consumers at level 2; and so on.
Energy Flow — 10% Law
Lindeman's 10% Law: Only about 10% of energy is transferred from one trophic level to the next. The remaining 90% is used up in the organism's own metabolic activities (respiration, movement, heat) or is lost as heat.
- So if 10,000 J of energy is available at the producer level:
- Primary consumers receive: 1,000 J
- Secondary consumers receive: 100 J
- Tertiary consumers receive: 10 J
This is why food chains rarely have more than 4–5 trophic levels, and why fewer carnivores exist in nature compared to herbivores.
Biological Magnification
Biological magnification (Biomagnification): The increase in concentration of a non-biodegradable chemical (e.g., DDT, mercury) at each successive trophic level. This happens because organisms cannot break down these substances, and each consumer accumulates the pollutant from all the organisms it has eaten. Top predators (including humans) have the highest concentration of such pollutants in their bodies.
Ecosystem Management
Biodegradable wastes: Broken down by microorganisms (e.g., food scraps, paper, dung).
Non-biodegradable wastes: Cannot be broken down naturally (e.g., plastics, pesticides, glass). These cause long-term environmental damage and biomagnification.
Ozone Layer and Its Depletion
The ozone layer (in the stratosphere, ~15–35 km altitude) absorbs most of the Sun's harmful ultraviolet (UV) radiation, protecting living organisms.
Ozone (O3) formation: O2 + UV → O + O; O + O2 → O3.
Ozone depletion: Caused primarily by chlorofluorocarbons (CFCs) — synthetic chemicals used in refrigerators, air conditioners, and aerosols. UV radiation breaks CFCs, releasing chlorine atoms that destroy ozone:
Cl + O3 → ClO + O2 (one Cl atom can destroy thousands of O3 molecules).
- Consequences of ozone depletion:
- Increased UV radiation reaching Earth.
- Higher rates of skin cancer, cataracts.
- Damage to marine ecosystems (UV kills phytoplankton).
- Reduced crop yields.
Montreal Protocol (1987): An international agreement to phase out the production of CFCs and other ozone-depleting substances.
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In the food chain Grass → Deer → Tiger, if 10,000 J of energy is available in grass, how much energy reaches the tiger? Using the 10% law: Deer receive 10,000 x 10% = 1,000 J. Tigers receive 1,000 x 10% = 100 J. Only 100 J, or 1% of the original energy, reaches the tiger.
Explain why vegetarian diets are considered more energy-efficient than meat-based diets. Eating plants (trophic level 1) gives humans more energy (10%) than eating animals (trophic level 2 or 3, where only 1% or 0.1% of original energy remains). A vegetarian diet supports more people per unit land area.
DDT was sprayed on water bodies to control mosquitoes. Trace its path through a food chain. Water → Aquatic plants (low DDT) → Small fish (higher DDT) → Large fish (very high DDT) → Osprey/humans (highest DDT). Each level accumulates DDT from all organisms it consumes.
Why are decomposers essential to an ecosystem? Decomposers (bacteria and fungi) break down dead plants and animals, releasing nutrients (nitrogen, phosphorus, carbon) back into the soil and atmosphere. Without them, nutrients would remain locked in dead matter and producers could not grow.
A food web includes: grass eaten by deer and rabbits; deer eaten by tigers; rabbits eaten by foxes and eagles; foxes eaten by tigers. If all tigers were removed, what would happen? Deer population would increase (no predator), overgrazing would destroy grass, and the entire ecosystem could collapse — showing the importance of top predators in maintaining balance.
Why is it important to reduce the use of CFCs? CFCs persist in the atmosphere and migrate to the stratosphere where UV radiation breaks them, releasing chlorine radicals that catalytically destroy ozone. Each chlorine atom can destroy up to 100,000 ozone molecules, significantly thinning the protective ozone layer.
A farmer uses pesticides on crops. The pesticide is non-biodegradable. Trace how it enters the human body. The pesticide washes into water bodies → absorbed by phytoplankton → eaten by zooplankton → eaten by small fish → eaten by larger fish → consumed by humans. At each step, the pesticide concentration increases (biomagnification).
Key Facts
- 10% Law (Lindeman): only 10% energy transferred per trophic level
- Ozone: O3; shields Earth from UV radiation; found in stratosphere
- CFCs cause ozone depletion
- Biomagnification: DDT, mercury concentrate at higher trophic levels
- Montreal Protocol, 1987: international treaty to protect ozone layer
- Decomposers: bacteria and fungi; recycle nutrients
Common mistakes
Students often confuse food chain (linear) with food web (network). Also, the arrows in a food chain represent the flow of energy, NOT "who eats whom" in the opposite direction — the arrow always points FROM the organism being eaten TO the eater (i.e., in the direction of energy flow). Don't confuse biodegradable (broken down by microbes) with non-biodegradable (persists in environment).
Summary
Ecosystems consist of producers, consumers, and decomposers interacting with their abiotic environment. Energy flows from producers upward through trophic levels, with only 10% transferred at each step. Non-biodegradable pollutants undergo biomagnification. The ozone layer protects life from harmful UV radiation, and CFCs are the primary cause of its depletion, addressed by the Montreal Protocol.