Questions
10
Q1
What is an ecosystem? List its components and distinguish between natural and human-made ecosystems with examples.
An ecosystem is formed by the interaction of all living organisms in an area with the non-living (physical) constituents of their environment, maintaining a balance in nature. It has two types of components: (i) Biotic components — all living organisms such as plants, animals, microorganisms, and human beings. (ii) Abiotic components — non-living physical factors such as temperature, rainfall, wind, soil, and minerals. Natural ecosystems (forests, ponds, lakes) occur naturally; human-made ecosystems (gardens, crop-fields, aquariums) are created and maintained by humans.
Q2
What are producers, consumers, and decomposers? Explain the role of each in the ecosystem with examples. How are these groups interdependent?
Producers are organisms that produce organic compounds (sugar, starch) from inorganic substances using sunlight via photosynthesis — all green plants and certain bacteria. Consumers depend on producers either directly or indirectly for food — classified as herbivores, carnivores, omnivores, and parasites. Decomposers (bacteria and fungi) break down dead remains and waste products, converting complex organic substances into simple inorganic substances that return to the soil for reuse by plants. These groups are interdependent: producers provide energy to consumers; consumers regulate producer populations; decomposers recycle nutrients back for producers.
Q3
What is a food chain? Explain the concept of trophic levels. Give an example of a food chain found in a grassland ecosystem and state the trophic levels of each organism in it.
A food chain is a series of organisms feeding on one another at various biotic levels, representing the transfer of food energy from producers through a sequence of consumers. Each step or level of the food chain is called a trophic level. Trophic levels from base to top: first (producers/autotrophs), second (primary consumers/herbivores), third (secondary consumers/small carnivores), fourth (tertiary consumers/larger carnivores). A grassland food chain: . Here, grass is the producer (1st trophic), grasshopper is primary consumer (2nd), frog is secondary consumer (3rd), snake is tertiary consumer (4th), and eagle is the top carnivore.
Q4
What is a food web? How does it differ from a food chain? Why is a food web considered a better representation of feeding relationships in an ecosystem?
A food web is a complex, branching network of interconnected food chains in an ecosystem where organisms are typically eaten by more than one kind of consumer. Unlike a food chain, which is a single straight-line sequence (e.g., ), a food web shows multiple feeding links — for example, a rabbit may be eaten by a fox, an owl, or a snake; a snake may eat frogs, rats, or rabbits. A food web is a better representation because in nature, no organism depends on only one other organism for food, and most organisms can occupy more than one trophic level in the ecosystem.
Q5
Explain the flow of energy in an ecosystem. State the 10 percent law and explain why food chains rarely have more than four trophic levels.
Energy flows unidirectionally through an ecosystem: sunlight producers herbivores carnivores top carnivores. It never flows backwards — energy captured by producers does not return to the sun, and energy passed to consumers does not flow back to previous levels. At each trophic level, energy is lost as heat through respiration, metabolism, and undigested matter. According to the 10 percent law, only about 10% of the food energy at one trophic level becomes available as body mass for the next level. Since energy diminishes so sharply — after the fourth level, negligible usable energy remains — food chains rarely exceed three or four trophic levels.
Q6
What is biological magnification? Explain with an example how harmful chemicals enter the food chain and accumulate at each trophic level. Will the levels of magnification be the same at all trophic levels?
Biological magnification (biomagnification) is the phenomenon where non-biodegradable chemicals like pesticides accumulate progressively in higher concentrations at each higher trophic level in a food chain. For example, when pesticides are sprayed on crops, they wash into the soil or water bodies. In water, aquatic plants absorb small amounts of these chemicals. When small fish eat many aquatic plants, the pesticide accumulates in their bodies. Larger fish that eat several small fish concentrate even more pesticide. Since humans occupy the top level in many food chains, the maximum concentration accumulates in human bodies. The levels of magnification are not the same — they increase at each higher trophic level.
Q7
What is ozone? How is it formed, and what is its role in protecting life on Earth? Explain how CFCs cause ozone depletion and what steps have been taken to address this problem.
Ozone () is a molecule made of three oxygen atoms. At ground level it is a poisonous gas, but in the stratosphere it forms a protective layer that shields Earth from harmful ultraviolet (UV) radiation from the Sun. Ozone is formed when high-energy UV radiation splits molecular oxygen () into free oxygen atoms: . These free atoms then combine with molecular oxygen: . Ozone depletion is caused primarily by chlorofluorocarbons (CFCs), synthetic chemicals used as refrigerants and in fire extinguishers. CFCs released into the atmosphere break down ozone molecules. In 1987, UNEP forged an agreement to freeze CFC production at 1986 levels. Today, it is mandatory for manufacturers to produce CFC-free refrigerators worldwide.
Q8
Distinguish between biodegradable and non-biodegradable substances. Give examples of each. Why is the increasing proportion of non-biodegradable waste a serious environmental concern?
Biodegradable substances are those that can be broken down by biological processes — through the action of bacteria, fungi, and other decomposers — into simpler, harmless substances that return to the soil. Examples: vegetable peels, paper, cotton cloth, food waste, fallen leaves, wood, animal dung. Non-biodegradable substances are those that cannot be broken down by biological processes and persist in the environment for decades or centuries. Examples: plastics (polythene, PVC), glass, metals, pesticides (DDT), electronic waste. The increasing proportion of non-biodegradable waste is dangerous because these materials accumulate in the environment, cannot be recycled by natural processes, may harm organisms (animals ingest plastics), and can release toxic substances — or enter the food chain via biological magnification.
Q9
What are the problems caused by non-biodegradable waste? Explain different methods of waste disposal. How can individuals contribute to reducing the waste management problem?
Non-biodegradable waste causes persistent land and water pollution, harm to animals (ingestion or entanglement), soil contamination, release of hazardous materials from electronic waste, and contributes to biological magnification when toxic chemicals enter food chains. Plastics clog drains and create breeding grounds for disease vectors. Waste disposal methods include: segregation into biodegradable and non-biodegradable (at source), composting of biodegradable waste, recycling of non-biodegradable materials (plastics, metals, glass), proper landfill management, and treatment of sewage before discharge. Individuals can help by: carrying reusable cloth bags instead of plastic, segregating waste at home, reducing use of disposable items, composting kitchen waste, walking or cycling instead of using vehicles, and switching off unnecessary lights and fans.
Q10
What would happen if all organisms of one trophic level are removed from an ecosystem? Will the impact be the same at every trophic level? Explain with an example.
If all organisms of one trophic level are removed, the entire ecosystem would be disrupted. The organisms at the level above would lose their food source and die, while organisms at the level below would experience unchecked population growth. The impact is NOT the same for all trophic levels. Removing producers (first trophic level) would be catastrophic — all consumers would die because there would be no source of food energy. Removing top carnivores would cause herbivore populations to explode, leading to overgrazing and eventual destruction of vegetation. For example, if all grasshoppers (primary consumers) were removed from a grassland, frogs (secondary consumers) would lose their food and die, while grass (producers) would grow unchecked initially.