Our Environment

The One Thing

An ecosystem depends on interactions among living organisms and between organisms and their non-living surroundings. Energy flows in one direction through food chains, whereas nutrients and other materials are recycled; human activities can disrupt this balance through pollution, waste, deforestation and ozone depletion.

Reactions, Processes and Experiments

Key Terms

• Ecosystem: A system formed by the interaction of living components with non-living components in a particular area.
• Biotic components: The living parts of an ecosystem, including producers, consumers and decomposers.
• Abiotic components: The non-living parts of an ecosystem, such as air, water, soil, sunlight, temperature and minerals.
• Producers: Organisms, mainly green plants and algae, that prepare food by photosynthesis.
• Consumers: Organisms that obtain food by eating plants or other organisms.
• Decomposers: Bacteria and fungi that break down dead organisms and wastes into simpler substances, returning nutrients to the environment.
• Food chain: A sequence showing how food and energy pass from one organism to another.
• Food web: A network of interconnected food chains in an ecosystem.
• Trophic level: Each feeding position in a food chain, such as producer, primary consumer or secondary consumer.
• Ten percent law: Approximately 10 percent of the energy available at one trophic level is transferred to the next level.
• Biological magnification: The increase in concentration of non-biodegradable toxic substances at successive trophic levels.
• Biodegradable substance: A substance that can be broken down naturally by microorganisms into simpler, usually harmless substances.
• Non-biodegradable substance: A substance that cannot be decomposed easily by natural organisms and may remain in the environment for a long time.
• Ozone layer: A region of the stratosphere containing ozone gas that absorbs much of the Sun’s harmful ultraviolet radiation.
• Ozone depletion: The thinning or reduction of ozone in the atmosphere, mainly caused by chlorine- and bromine-containing chemicals.
• Ultraviolet radiation: High-energy radiation from the Sun that can cause skin damage, eye problems and harm to living organisms.
• Waste management: The collection, separation, treatment, reuse, recycling and safe disposal of waste.
• Sustainable practices: Ways of using resources that meet present needs without damaging the ability of future generations to meet theirs.

Easily Confused

• Energy flow and nutrient cycling: Energy flows in one direction through an ecosystem, whereas matter and nutrients circulate through natural cycles.
• Biodegradable and non-biodegradable substances: Biodegradable substances are broken down naturally by microorganisms; non-biodegradable substances persist and may accumulate.
• Food chain and food web: A food chain is a single feeding sequence; a food web consists of interconnected food chains.
• Biotic and abiotic components: Biotic components are living organisms; abiotic components are non-living environmental factors.
• Ozone layer and ozone depletion: The ozone layer absorbs harmful ultraviolet radiation; ozone depletion is its thinning or reduction.
• Decomposers and consumers: Consumers obtain food by eating plants or other organisms, whereas decomposers break down dead organisms and wastes and return nutrients to the environment.
• Reduce, reuse and recycle: Reduce means preventing or minimising waste, reuse means using an item again, and recycle means processing materials into useful products.

What Gets Asked

• Define or identify ecosystem components: Questions may require classification into biotic components—producers, consumers and decomposers—and abiotic components such as air, water, soil, sunlight, temperature and minerals. A common error is treating non-living factors as biotic components.
• Complete or explain a food chain: The specific example may be written as Grass -> Grasshopper -> Frog -> Snake -> Eagle. Marks are lost by placing organisms in the wrong feeding order or confusing a food chain with a food web.
• Apply the ten percent law: Given 10,000 units of energy in producers, calculate approximately 1,000 units for primary consumers, 100 units for secondary consumers and 10 units for tertiary consumers. The key error is treating energy as recycled rather than progressively reduced.
• Explain biological magnification: Pesticides and heavy metals are non-biodegradable pollutants whose concentrations increase at successive trophic levels. The specific distinction from ordinary pollution is the increase in concentration higher in the food chain.
• Compare biodegradable and non-biodegradable waste: Vegetable peels, paper, cotton and animal dung are biodegradable, while plastics, synthetic chemicals and some metals are non-biodegradable. A response should not imply that biodegradable waste is automatically harmless if it is poorly managed.
• Explain ozone depletion and its control: Answers should include O3, the stratospheric location of the ozone layer approximately 15 to 35 kilometres above Earth’s surface, chlorine- and bromine-containing chemicals, harmful ultraviolet radiation and the Montreal Protocol adopted in 1987.