S4.3.8 Chapter 8

Biodiversity and people

Why does it matter if a forest is cleared or a river is polluted, and how can people protect living things?

Your guess

Pick one before you read on. It is fine to be wrong.

Your guess

A It matters little, because other species just move in and fill the places of those that are lost.
You picked A. Read on. At the end, the chapter comes back to your guess.
B It matters only for the few species that people eat.
You picked B. Read on. At the end, the chapter comes back to your guess.
C The variety of living things falls, and species that depend on each other can be lost too. People can protect places, species and seeds.
You picked C. Read on. At the end, the chapter comes back to your guess.

By the end you can

Explain how habitat loss, pollution and invasive species reduce biodiversity
Evaluate ways to conserve species, such as reserves and seed banks

Biodiversity is the variety of living things in a place, or in the whole world. It counts how many different species there are. It also counts the variety of alleles within each species.

Why does it matter? Living things depend on each other, as you saw in the last chapter. A varied community is usually better able to cope when something changes. We also depend on biodiversity for food, clean water, pollination of crops, medicines and fertile soil. Many people also value nature for its own sake.

People reduce biodiversity in several ways. The biggest is habitat loss. A forest is cleared for farmland or buildings, a wetland is drained, or a river is dammed. The species that lived there lose their food and shelter. Some move away, and some die out.

Example numbers. The farmland replaced part of the forest.

Worked example

A forest site has 60 species. The farmland that replaced it has 15 species. What percentage fewer species does the farmland have?

The farmland has 60 − 15 = 45 fewer species.
45 out of 60 is 45 ÷ 60 = 0.75.
As a percentage, 0.75 × 100 = 75%.

Answer: 75% fewer

Example numbers. Each year 10 km² of the forest is cleared.

Worked example

A forest of 400 km² loses 10 km² each year. What percentage of the forest is left after 6 years?

The area cleared in 6 years is 10 × 6 = 60 km².
The area left is 400 − 60 = 340 km².
340 out of 400 is 340 ÷ 400 = 0.85, which is 85%.

Answer: 85%

Habitat loss also breaks a big habitat into small patches. A small patch holds fewer animals of each kind. The animals cannot reach each other to find mates, so the alleles are not mixed. Each small group has less variety of alleles, and close relatives breed together. So the group copes less well with disease or change.

The second cause is pollution. Oil, chemicals and plastic harm living things in water and on land. Fertiliser is another. When rain washes fertiliser from fields into a river, the water becomes rich in nitrates, as you saw in the nitrogen cycle. This causes eutrophication.

Draw it first

How fertiliser can kill fish

1 Fertiliser washes into a river. Algae use its nitrates and grow fast, so they cover the water.
2 The algae shut out the light, and the plants below them die. Then the algae die too. Decomposers feed on the dead algae and use up the oxygen in the water.
3 Fish and other animals that need oxygen die.

The third cause is an invasive species. People carry living things to new places, on ships or as garden plants. In the new place the newcomer may have no predators or diseases to keep it in check. It may eat the local species, or take their food and space. Rats and cats on islands, and a floating water plant that covers a lake, are examples.

The fourth cause is hunting and fishing. If animals are taken faster than they can breed, their number falls. The fifth is climate change caused by people, from the carbon dioxide put in the air by burning fuels. Warmer seas bleach corals, and habitats shift.

Example numbers, in thousands of fish, for a fish stock in a sea. The dashed line marks year 7, when a limit on fishing begins.

The graph shows that overfishing can be reversed. The stock fell until a limit on fishing began, and then it rose. Not every case ends this way. Some species are slow to breed, and some are taken for too long.

Conservation means protecting living things and their habitats. There are several methods. Each has gains and limits.

A nature reserve is an area where building, farming and hunting are controlled. It protects a whole habitat and all the species in it. But it costs money to guard. A small reserve can hold only a few animals of each kind. People who live near it may lose land they used.

A habitat corridor is a strip of habitat that joins two reserves or patches. Animals can move along it to find food and mates, and alleles are mixed. But it takes land, and a road can cut across it.

A seed bank stores seeds of many plants in cold, dry conditions. If a plant dies out in the wild, it can be grown again from the seeds. But the bank saves the seeds, not the animals and the habitat that go with the plant. Some seeds do not keep for long, and some kinds cannot be stored this way.

A breeding programme breeds endangered animals in a protected place, and may release some into the wild. Records are kept so that the animals are not bred with close relatives. But it is costly, and the young can only be released if a safe habitat is left for them.

Laws can also help. A law may forbid hunting a rare species, or limit how much of a fish stock may be taken. But a law works only if it is kept. It is hard to watch wild places, and some people lose the living they made.

What each method keeps. No method is enough alone, so they are used together.

Try it

Make a small seed bank

You need: seeds of 3 kinds, such as a dry bean, a pea and a pumpkin seed, paper envelopes or folded paper, a jar with a lid, a pencil, a cup of soil or wet cloth to test the seeds.

Be safe: Do not eat the seeds you store or test, and keep them away from small children. Use only seeds from food plants. Wash your hands after you touch soil.

Dry the seeds in the shade for two days. Fold each kind into its own paper, and write its name and the date on it.
Put the papers in the jar and close the lid. Keep it in a cool, dry place for a few weeks.
Take 10 seeds of one kind. Sow them in damp soil or on a damp cloth in a cup. Keep it damp and warm.
After a week, count how many have sprouted. Work out the percentage.

What you will see: Most of the seeds sprout, but not all. A seed bank must test its seeds from time to time, because some seeds die in storage.

If you cannot try it, this is what happens.

A wrong idea many people have

It does not matter if a species dies out, because other species will fill its place.

Why it feels right: Living things have come and gone throughout the history of the Earth, so it seems natural.

The test that breaks it: Take the fox in the food web. If the rabbits are lost, the fox has nothing to eat. A new species does not appear in a few years to take their place. New species take thousands of generations to form.

What is true: Species that are lost cannot be replaced quickly. Other living things that depend on them may be lost too, so protecting biodiversity matters.

Key words

biodiversity: the variety of living things in a place or in the world, including how many species there are.
eutrophication: too many nitrates in water, which makes algae grow. When the algae die, the oxygen is used up and animals die.
conservation: protecting living things and their habitats.
nature reserve: an area where building, farming and hunting are controlled, to protect its living things.
habitat corridor: a strip of habitat that joins two patches, so animals can move between them.
seed bank: a store of seeds, kept cold and dry, so that plants can be grown again if they die out.
breeding programme: breeding endangered animals in a protected place, and sometimes releasing them into the wild.

Back to your guess

The answer is C: The variety of living things falls, and species that depend on each other can be lost too. People can protect places, species and seeds.

When one species is lost, others that depend on it may follow. People cannot always replace what is lost, but they can protect places, animals and seeds.

Keep it

Copy this into your exercise book.

Habitat loss, pollution, invasive species, overhunting and climate change reduce biodiversity. Reserves, corridors, seed banks, breeding programmes and laws help, and each has limits.

Draw: Five boxes, one for each method of conservation, and under each one thing it keeps and one limit.