CBSE Class 10 Science 2019 · Paper 31/4/1 · Question 9(a)

(a) A natural water body is a self-sustaining ecosystem. Its decomposers (micro-organisms such as bacteria and fungi) break down dead remains and wastes and recycle their nutrients, so it cleans itself. An aquarium is an artificial ecosystem without enough decomposers (and without a complete food chain), so uneaten food, wastes and dead matter pile up and pollute the water; it therefore needs regular cleaning. (b) Decomposers are micro-organisms, such as bacteria and fungi, that break down the complex organic substances of dead remains and wastes into simple inorganic substances, which return to the soil. If they were completely eradicated, dead bodies and wastes would not decompose and would pile up, and the nutrients in them would not be returned to the soil. Justification: the soil would lose its fertility and become unsuitable for plants; producers, and therefore all food chains, would suffer, and the ecosystem would become unbalanced.