Science (086) — CBSE Class 10 board question

Part 1 (1 mark):
Ponds and lakes are natural ecosystems that are self-sustaining. They contain producers, consumers, and decomposers which maintain a natural balance, breaking down dead matter and recycling nutrients on their own.

Part 2 (2 marks):
An aquarium is a human-made (artificial) ecosystem with limited space and fewer organisms. It lacks sufficient decomposers to break down dead remains and waste products. As a result, waste accumulates, water becomes polluted, and oxygen levels drop. Hence, it needs to be cleaned regularly to maintain conditions suitable for the organisms living in it.

In contrast, in a pond or lake, the large number of decomposers, adequate space, and natural balance ensure that waste is broken down automatically without human intervention.

Source: Chapter 13, Section 13.1 – Ecosystem and Activity 13.1

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• The examiner looks for the key contrast: natural ecosystem (self-sustaining) vs artificial ecosystem (not self-sustaining).
• Mention the role of decomposers — this is the core scientific reason.
• For 1+2 marks, keep Part 1 brief (1–2 lines) and Part 2 slightly more detailed (3–4 lines).
• Don't write lengthy paragraphs; crisp, point-based answers score better.