How Does One Change Lead to Another? (12.7) — "How Nature Works in Harmony"

1. 14 marks

   Trace the full cascade shown in Fig. 12.13, starting from pond pollution.

   Hint. Five linked steps, ending with farmers reaching for a specific tool that itself causes more harm.

   ✦ Pollution causes many plants in a pond to start dying → with fewer plants, less oxygen is produced in the water → this leads to a drop in the fish population → reduced fish population means fewer consumers to eat insects, so insects increase in number → these insects spread to nearby farmlands, damaging crops → farmers are compelled to use pesticides to protect their crops, which may again adversely affect the environment, continuing the cascade further.

2. 24 marks

   Describe the Indian bullfrog story from Activity 12.9 as a real-world parallel to Fig. 12.13's cascade, and state how the Government of India responded.

   Hint. Same shape of cascade — a population removed by humans, insects rising, and a chemical response causing further harm — but this time the numbers are historical fact, not a diagram.

   ✦ In the 1980s, India was a major exporter of frog legs, especially from the Indian bullfrog, and this large-scale harvesting caused a real decline in frog populations. Since frogs eat insects, fewer frogs meant a rise in agricultural pests, which forced farmers to use more synthetic pesticides — harming the environment, soil and water quality, and affecting environmental and human health overall. The Government of India responded by banning the export of frog legs, to prevent further ecological damage.

3. 33 marks

   Define ecosystem balance, in the chapter's own words, and explain why it is described as 'dynamic, not fixed.'

   Hint. The chapter deliberately avoids the word 'stable' or 'unchanging' — it uses a different word for how balance behaves.

   ✦ 'An ecosystem stays in balance when interactions among organisms and their environment keep populations and resources stable.' The chapter describes this balance as dynamic rather than fixed because it is actively maintained through ongoing interactions (like the fish-dragonfly-pollinator chain, or predator-prey relationships) rather than being a permanently locked, unchanging state — which is exactly why it 'can be disrupted by natural or human-made changes' at any point.

4. 43 marks

   Both the pond-pollution cascade (Fig. 12.13) and the bullfrog story start with a change to ONE population. What do both examples show about how far the effects of removing or reducing one population can reach?

   Hint. Compare the starting point (plants/frogs) to the final consequence (pesticide use harming the wider environment) in each case.

   ✦ Both examples show that a change to a single population can cascade well beyond its immediate surroundings — in Fig. 12.13, pollution affecting pond plants eventually leads to farmland pest damage far from the pond itself, and in the bullfrog story, harvesting one species (frogs) for export eventually affected soil, water quality and human health nationwide through increased pesticide use. Neither cascade stays contained to where it started, which means an ecosystem's populations really are tightly interconnected.

5. 52 marks

   What natural or human-made changes could disrupt an ecosystem's dynamic balance, based on the examples given across this chapter?

   Hint. Pull examples from across the elephant-corridor story, the pond-pollution cascade, and the bullfrog export story.

   ✦ Natural changes could include disease (as in a later exercise about the Indian hare) or shifts in rainfall and temperature (as in the elephant-corridor opening story); human-made changes include pollution (the pond cascade), deforestation and habitat loss (elephants entering farms), and large-scale harvesting of a single species (the Indian bullfrog's frog-leg export) — since this chapter shows all of these can disrupt the stable, dynamic balance an ecosystem otherwise maintains.

Solutions written by the tuition.in editorial team and checked against NCERT Curiosity — Textbook of Science for Grade 8, Chapter 12 (hecu112.pdf), Reprint 2026-27, pages 190-209. Questions are referenced from the NCERT textbook for identification.