NCERT Solutions Curiosity Chapter 12 –208End-of-chapter questions — Keep the curiosity alive

The wrong statement is (iii) An ecosystem is part of a community.

It is the other way round. Fig. 12.19 shows three circles one inside another:

• A population is one kind of organism in a habitat at a given time — say all the grass plants in a field.
• A community is all the different populations sharing that habitat — the grass, the grasshoppers, the frogs, the microbes. So a community is larger than a population: statement (i) is correct.
• An ecosystem is that community together with the abiotic components it interacts with — the soil, water, sunlight and temperature. So a community is smaller than an ecosystem: statement (ii) is correct.

The forest would first fill up with dead matter, and then slowly starve.

Why they are essential: decomposers are the only organisms that carry matter backwards in the ecosystem. Every other arrow in a food web points upward — grass to grasshopper to frog. Decomposers alone return material to the start of the chain. Microorganisms like fungi and bacteria break down the complex substances in dead plants and animals into simpler ones, and this process returns important nutrients to the soil. Plants grow in soil, and many of the nutrients in soil come from decomposition.

Selvam is right — the mangroves acted as a living wall between the sea and the village.

A mangrove forest sits in the shallow water at the coast. Its trees stand on dense, arching stilt roots, and thousands of small roots also stick up out of the mud. A wave coming in has to push through all of them. Each root takes a little of the wave's energy, so by the time the water reaches the village behind the forest it is slower and lower.

This is exactly what the chapter says about the Sundarbans: they protect us by slowing down strong winds and waves during storms and floods. Villages that had cleared their mangroves for fuelwood or ponds had nothing to slow the same wave, so they were hit harder.

Grasshoppers will increase and snakes will decrease. The frog is the link that joins them, and removing it cuts the chain in the middle.

Grasshoppers increase because the frog was their predator. With nothing eating them, more grasshoppers survive and breed, and their numbers climb. They then eat far more grass, so the grass itself begins to suffer — the effect passes downward as well.

Snakes decrease because the frog was their food. The energy the snake used to get came up through the frog, and that route is now closed. Some snakes may switch to other prey such as mice or birds if the ecosystem has them; where it does not, the snake population falls.

A butterfly needs two quite different things — nectar plants for the adult and host plants on which the caterpillar can feed. Fewer butterflies usually means one of these has gone.

Possible reasons

• Flowering plants were removed, or the garden was replanted with plants that give little nectar.
• Weeds and shrubs at the edges were cleared — those were the caterpillars' host plants.
• Insecticide or pesticide was sprayed in the garden or on a neighbouring field; it kills caterpillars along with pests.
• The grass and hedges were mown just when caterpillars were feeding on them.
• No water and no bare damp soil for the butterflies to drink from.
• Heavy dust, smoke or a very dry season.

Steps students can take

Because energy could still enter such a system, but matter could never move — and an ecosystem needs both.

Follow what happens. The producers photosynthesise and grow. They take nutrients out of the soil to do it. Eventually each plant dies. Now:

• there are no decomposers, so the dead plants do not break down. The nutrients they took from the soil stay locked inside them for ever;
• the soil therefore runs out of nutrients, and the next generation of plants cannot grow;
• dead plant material piles up over everything, blocking light and space from any seedling.

Consumers matter too, though in a different way. Many plants depend on animals to pollinate their flowers and to disperse their seeds. With no animals, most flowering plants would set few seeds and could not spread to new ground.

Method: stand in each place for ten minutes at the same time of day. Write down every living thing you can see or hear, and every non-living thing that affects them. Then compare.

Sample answer — a park and a roadside near our school:

How they differ: the difference in the living things follows from the difference in the non-living ones. Soft moist soil and shade let roots, earthworms and leaf litter build up, so the park has producers, several kinds of consumer, and decomposers working in the soil. Tar and compacted soil block roots and water, and heat and dust make conditions harsh, so only a few tough species survive at the roadside. The roadside is also a place where humans keep interfering — sweeping, cutting, dumping — so the community there never gets time to build up.

The statement is correct on both counts, and the two halves are connected.

They are necessary. Humans have been practising farming for thousands of years to grow food, and as the population grew, our dependence on agriculture increased. Between 1950 and 1965 India faced a food crisis due to low crop production; tractors, machines, synthetic fertilisers and pesticides in the mid-20th century — the Green Revolution — helped raise food production and made the country food secure. No natural ecosystem could feed a population of this size.

They must be made sustainable. Those same methods are now considered unsustainable because of the overuse of synthetic chemicals, excessive groundwater extraction, and growing only one type of crop for commercial gain. The result is soil degradation: less humus, fewer friendly microorganisms, erosion, a falling water table, pests that have developed resistance, and fewer pollinators.

What "sustainable" means here: using the field in a way that leaves the soil as good next year as it is this year. Compost and manure, crop rotation and mixed cropping instead of monoculture, efficient irrigation, spraying only when needed, and organic and natural farming methods that keep interference with natural ecosystems to a minimum.

Fig. 12.20 shows this web: the Sun feeds the grass and plants, which feed the hare and the deer; the hare is eaten by the fox and the deer by the eagle. So a fall in the hares moves along both branches.