Chapter 12: How Nature Works in Harmony (Biology)

It is the other way round: a population is part of a community, and a community is part of an ecosystem. An ecosystem also includes the non-living surroundings.

(iii) is wrong.

• Dead matter piles up: fallen leaves, dead plants, dead animals and animal waste would not rot. They would collect everywhere.
• Nutrients are locked up: the nutrients in dead matter would never return to the soil. The soil would slowly become poor.
• Plants suffer: without nutrients, plants would grow less. Then the herbivores, and then the carnivores, would have less food, and the whole food web would decline.

Decomposers (bacteria and fungi) break dead matter down into simple substances. They recycle nutrients back into the soil for plants to use again, and they keep the ecosystem clean.

Dead matter would pile up and nutrients would not be recycled, so the soil would become poor and plants, and then animals, would decline. Decomposers recycle nutrients and clean up the ecosystem.

Mangroves grow thickly along the coast, with dense trunks, branches and tangled roots, including stilt and breathing roots.

• This "green wall" acts as a natural barrier. The waves lose much of their speed and force as they push through it, so by the time they reach the village they are weaker.
• The roots hold the soil, so the coast is not washed away as easily.

That is why mangroves are called the bio-shields of the coast, and why protecting and planting them matters.

The dense trunks and roots of mangroves slow down and weaken the waves and hold the soil, acting as a natural shield for the coast.

• Grasshoppers increase: frogs ate them, and now nothing controls their numbers. With many more grasshoppers, the grass may be eaten up quickly, which can later cause the grasshoppers to decline too.
• Snakes decrease: they lose their food, unless they can find other prey such as mice or lizards.

Removing one link affects every other link in the chain.

Grasshoppers would increase (no predator) and eat more grass; snakes would decrease (loss of food).

Possible reasons:

• fewer flowering plants for nectar;
• loss of the host plants on which butterflies lay eggs and caterpillars feed;
• use of pesticides;
• cutting of bushes and wild patches;
• pollution, or unusual heat or rain.

Steps students can take:

• plant native flowering plants (such as ixora, hibiscus and marigold) and host plants (such as curry leaf, lemon, Calotropis and Aristolochia);
• stop using pesticides;
• leave a small corner of the garden wild;
• keep a shallow dish of wet sand or mud, where butterflies drink and take in minerals;
• record the butterflies seen each week.

Likely causes are fewer nectar and host plants, pesticides and habitat loss. Remedies: grow native flowering and host plants, avoid pesticides, keep wild patches and wet sand, and monitor the butterflies.

• Without decomposers: dead plant matter would not be broken down. The nutrients in the soil would soon be used up and never returned, so even the producers could not keep growing.
• Without consumers: there would be no pollination or seed dispersal by animals, and nothing to keep plant growth in balance.

An ecosystem works only because its parts depend on one another, with energy and nutrients passing between them.

Producers need nutrients that decomposers recycle (and services such as pollination by consumers); without them, nutrients would run out and the cycle would stop.

This depends on what you observe. A sample record:

The park has many more kinds of living things, with richer soil, more water and shade. Its food webs are more complex. The roadside has fewer species, mostly hardy ones that can cope with pollution, heat and disturbance.

A park has many more living things, richer soil and more water; a roadside has fewer, hardier species because of pollution, heat and paving.

Fields are necessary: they produce the food for a huge and growing population. But harmful practices damage the land and water that farming itself depends on:

• too much chemical fertiliser and pesticide;
• growing a single crop year after year;
• over-pumping groundwater;
• burning crop stubble;
• clearing all hedges and trees.

These harm the soil, pollute water and air, and kill pollinators, birds and helpful insects.

Farming is sustainable when it keeps the land productive for the future:

• crop rotation and mixed cropping;
• compost and organic manure;
• drip irrigation and rainwater harvesting;
• natural pest control;
• keeping trees and hedges for birds and pollinators.

Fields are needed for food, but overuse of chemicals and water and the loss of biodiversity harm them; methods such as crop rotation, organic manure, water saving and natural pest control keep them productive for the future.

• Fox decreases: the hare is its food. Foxes may also turn to other prey.
• Grass and plants increase: fewer hares eat them.
• Deer may increase: there is more grass for them, and this can then help the animals that eat deer.

A change in one population spreads through the whole food web.

Foxes would decline (less food), grass and plants would grow more, and deer might increase with more food available.