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ge Igcse Biology Coursebook Third Edition Sample Chapter Web


Cambridge IGCSE Biology,

Third edition Coursebook with CD-ROM Mary Jones and Geoff Jones

The Coursebook content has been revised and rearranged,

ensuring that it is up to date and comprehensive in its coverage,

with supplementary material clearly marked

A Workbook and Teacher’s Resource are also available

The Coursebook contains: • total coverage of the syllabus • language accessible to students of a wide range of abilities • a clear indication of the chapter content at the beginning of each chapter • activities to help students develop practical and investigative skills • study tips throughout to aid with understanding • highlighted definition boxes covering every definition required by the syllabus • in-chapter questions and end-of-chapter questions,

including exam-style questions to test students’ knowledge • summaries at the end of each chapter • a glossary of key biological terms

Completely Cambridge – Cambridge resources for Cambridge qualifications Cambridge University Press works closely with Cambridge International Examinations as parts of the University of Cambridge

We enable thousands of students to pass their Cambridge exams by providing comprehensive,

Mary Jones and Geoff Jones

Cambridge IGCSE®


Coursebook Third edition

Jones and Jones

Other components of Cambridge IGCSE Biology,

Third edition: Workbook ISBN 978-1-107-61793-2 Teacher’s Resource ISBN 978-1-107-61496-3


The accompanying CD-ROM contains: • advice on how to revise for and approach exams • self-assessment check lists for making drawings,

constructing and completing results tables,

drawing graphs and designing experiments • revision check lists for each chapter • answers to the end-of-chapter questions,

including the exam-style questions • model exam papers and mark schemes • multiple-choice self tests and answers • expanded notes on the activities from the Coursebook with full teacher/technician notes for the activities • a series of animations to aid understanding

Cambridge IGCSE Biology Third edition

Cambridge IGCSE Biology,

Third edition matches the requirements of the latest Cambridge IGCSE Biology syllabus (0610)

It is endorsed by Cambridge International Examinations for use with their exams

To find out more about Cambridge International Examinations visit www

org/cie for more information on our full range of Cambridge International A Level titles including e-book versions and mobile apps


Humans and the environment

agriculture and food production habitat destruction pollution conservation

Saving a species on the brink

In this chapter,

Black-footed ferrets used to be common on the prairies of North America (Figure 22

as humans took over more and more land to graze cattle and grow crops,

their habitat and food supply dwindled

Black-footed ferrets are predators that feed almost entirely on burrowing rodents called prairie dogs,

and farmers killed large numbers of prairie dogs because they thought they competed

Figure 22

In 1974,

the black-footed ferret was declared extinct in the wild

But in 1981,

a dog brought a dead black-footed ferret to his owners

They notified wildlife officials,

Searches for the ferrets eventually discovered a small population of them living nearby

The wild ferrets were given protection

as their numbers remained low,

it was eventually decided that they needed more help

Some of the ferrets were taken to zoos,

where they were given the right conditions to help them to breed

Many of the offspring were reintroduced into suitable habitats in several states,

where their populations are gradually growing

the black-footed ferret is out of danger

Just in time,

we have managed to reverse a population decline that would have resulted in extinction

Black-footed ferrets are adapted to hunt their prey in their underground burrows

Cambridge IGCSE Biology

© Cambridge University Press 2014

Most of the world’s supply of food is produced by growing crops or by keeping animals

During the last century,

the quantity of food produced has greatly increased

Figure 22

such as wheat and rice – between 1950 and 2006

Figure 22

farmers have tractors and other machinery to work the land

This farmer is preparing the soil for sowing seeds

Farmers are also using agricultural chemicals to help to improve the growth of their crops,

so that they can get more yield from the same area of ground

Chemical fertilisers add more mineral ions – such as nitrate ions – 0

Insecticides are sprayed onto crops to kill insects pests that might 0 1950 1960 1970 1980 1990 2000 2010 reduce yields or make the crop look less appealing to Year buyers

Herbicides are sprayed to kill weeds,

would compete with the crop plants and reduce their growth

Although all of these chemicals are expensive,

Increasing food production the cost is outweighed by the increased quantity and It is important that we increase the quantity of food that quality of the crops

because the world’s increasing population Selective breeding has also played an important part needs to be fed

There are several ways in which this has in increasing world food production

We have seen been achieved

how breeders can choose parents,

generation after Agricultural machinery has made a very big generation,

to produce new and improved varieties difference

With machinery such as tractors and of animals and plants

For example,

modern wheat combine harvesters,

one farmer can cultivate a much varieties produce far more grain per plant than older greater area of land in a much shorter time than using ones ever did

Selective breeding has also produced crop manual labour (Figures 22

3 and 22

plants that can grow in poor soils,

or that are resistant to diseases (Figure 22

Figure 22

His work is labour intensive,

as people in this region cannot afford much machinery

Negative impacts of monocultures As we increase the quantity of food that we produce,

we reduce the habitat that is available for species other than ourselves

Crop plants are usually grown as large areas of a single variety (Figure 22

This is called a monoculture

In a natural ecosystem,

there are usually many different species of plants growing,

which in turn support many different species of animals

We say that there is a high biodiversity (Figure 22

In a monoculture,

Only a few species can live where the crop is growing

© Cambridge University Press 2014

Chapter 22: Humans and the environment

Figure 22

It contains an enormous number of different species of plants and animals

Another problem with monocultures is that they can lead to an increase in the populations of organisms that are pests of the crop

For example,

leafhoppers feed on the sap of rice plants,

They also transfer plant viruses to the rice,

which cause serious diseases in the plants

If a large area of land is covered with just rice plants,

then leafhoppers have so much food available to them that their population can become enormous

Farmers can try to reduce the quantities of these pests by spraying insecticides onto the crop


this kills not only the leafhoppers,

but also other harmless insects – including predatory insects and spiders that would help to reduce the leafhopper population

It is also very expensive

leafhoppers have become resistant to the insecticides and are no longer killed by them

Many farmers try to use other methods of controlling insect pests

One approach is to use mixed cropping,

where only fairly small areas of ground are covered with the same crop at the same time of year

This makes it more difficult for insect pests to spread from one rice field to another

Figure 22

Figure 22

Huge areas of land are covered by the same kind of tree,

which only supports a small number of different species of animals

Cambridge IGCSE Biology

© Cambridge University Press 2014

Negative impacts of intensive livestock production In many parts of the world,

cattle and other livestock are kept outside

They graze on grass,

or are fed by people bringing freshly-cut vegetation for them to eat (Figure 22

be welfare issues for the livestock,

which may suffer in the crowded conditions in which they are kept

Disease can spread easily among them

In some countries,

this is dealt with by giving the animals regular doses of antibiotics – which,

increases the risk that bacteria will develop resistance to the antibiotics

The waste from the intensive farming unit can pollute land and waterways nearby

World food supplies It has been calculated that more than enough food is produced on Earth to provide every single person with more than enough for their needs

Yet many people do not get enough food

Each year,

many people – both children and adults – die because they have an inadequate diet

The fundamental problem is that,

while some parts of the world produce more than enough food for the people that live there,

in other parts of the world nowhere near enough food is produced

Food is distributed unequally on our planet

Although large amounts of food are transported from one area to another,

this is still not sufficient to supply enough food to everybody

then even if there is plenty of food around,

many people may not be able to afford to buy it

Famines can occur for many different reasons

If an area suffers drought for several years in succession,

then it becomes impossible for the people to grow crops

Their animals die,


the problem is exactly the opposite – so much rain falls that it causes flooding,

again preventing crops from growing (Figure 22


even though the weather remains normal,

the human population may grow so large that the land on which they live can no longer provide enough food for them


wars raging in an area prevent people from working the land and harvesting their crops

When the world becomes aware that an area is suffering from famine,

other countries are usually very willing to donate food supplies to the people


this will only need to happen for a relatively short time,

until things improve and people can plant their crops and become self-sufficient again

Most people would much prefer this,

rather than having to rely on handouts of food

Figure 22

In developed countries,

livestock are often farmed intensively

This means that large numbers of livestock are kept in an area that would not normally be able to support more than a very small number (Figure 22

The farmer uses high inputs to increase the production of milk,

For example,

high-energy foods are bought to feed them

Regular medication may be given to stop the development of disease

The animals may be kept in temperaturecontrolled buildings to maximise their growth rates

Intensive farming can help to provide more food,

but there are some big disadvantages

For example,

Figure 22

© Cambridge University Press 2014

Chapter 22: Humans and the environment

Figure 22

living near the village of Muzaffarpur in northern India,

had no food for four days after flooding swept away their houses and drowned their farmland

These floods happened in 2007 and were the worst in living memory

Within the past 10 000 years or so,

another organism has had an enormous impact on the environment

Ever since humans learnt to hunt with weapons,

to domesticate animals and to farm crops,

we have been changing the environment around us in a very significant way

One of the greatest effects we have had,

and the one that is most threatening to the existence of many different species of organisms,

is that we have destroyed their habitats

We have seen that each species has adaptations that help it survive in its particular habitat

If we destroy that habitat,

then it is difficult for many species to survive in a new kind of environment

Habitats are destroyed when we use land for other purposes

Here are some examples

We cut down native vegetation to make land available for growing crops,

for building houses and factories,

We damage habitats when we mine for natural resources,

such as metal ores or fossil fuels (Figure 22

All living things affect the living and non-living things around them

For example,

earthworms make burrows and wormcasts,

and therefore the plants growing in it

Rabbit fleas carry the virus which causes myxomatosis,

so they can affect the size of a rabbit population,

and perhaps the size of the fox population if the foxes depend on rabbits for food

Perhaps the biggest ever effect of living organisms on the environment happened about 1500 million years ago

At this time,

the first living cells that could photosynthesise evolved

Until then,

there had been no oxygen in the atmosphere

These organisms began to produce oxygen,

which gradually accumulated in the atmosphere

The oxygen in the air we now breathe has been produced by photosynthesis

The appearance of oxygen in the air meant that many anaerobic organisms could now only live in particular parts of the Earth which were oxygen-free,

It meant that many other kinds of organism,

which used the oxygen for respiration,

All this oxygen excreted by photosynthetic organisms could be considered to be the biggest case of pollution ever

Cambridge IGCSE Biology

Figure 22

We add pollutants to land and water,

which can kill the plants that normally live there,

Habitats can also be damaged if we remove key species from them

For example,

collecting live corals from coral reefs damages the whole coral reef habitat,

endangering the hundreds of species of other animals that depend on corals (Figure 22

Corals contain tiny photosynthetic protoctists inside their bodies,

which are the start of every food chain on the reef

© Cambridge University Press 2014


Figure 22

and to support the complex food webs

Figure 22

It has an enormous species diversity

A rainforest is a very special place,

full of many different species of plants and animals

More different species live in a small area of rainforest than in an equivalent area of any other habitat in the world

We say that rainforest has a high species diversity

When an area of rainforest is cut down,

the soil under the trees is exposed to the rain

The soil of a rainforest is very thin

It is quickly washed away once it loses its cover of plants

This soil erosion may make it very difficult for the rainforest to grow back again,

even if the land is left alone

The soil can also be washed into rivers,

silting them and causing flooding (Figure 22

Humans have always cut down trees

Wood is an excellent fuel and building material

The land on which trees grow can be used for growing crops for food,

One thousand years ago,

most of Europe was covered by forests

most of them have been cut down

The cutting down of large numbers of trees is called deforestation (Figure 22

Rainforests occur in temperate and tropical regions of the world (Figure 22


most concern about deforestation has been about the loss of tropical rainforests

In the tropics,

the relatively high and constant temperatures,

provide perfect conditions for the growth of plants (Figure 22

Figure 22

large amounts of carbon dioxide are released and soil nutrients are lost

Figure 22


© Cambridge University Press 2014

Chapter 22: Humans and the environment

Trees are cut down

Wheat crops are grown every year

The land is overgrazed

Rivers carry the topsoil away

The soil structure is impoverished and is blown or washed away

Figure 22

The removal of grass cover allows soil to be blown or washed away

clear land on which they can grow food

It is difficult to expect someone who is desperately trying to produce food,

unless you can offer some alternative

International conservation groups such as the World Wide Fund for Nature,

and governments of the richer,

developed countries such as the USA,

can help by providing funds to the people or governments of developing countries to try to help them to provide alternative sources of income for people

Many of the most successful projects involve helping local people to make use of the rainforest in a sustainable way

The greatest pressure on the rainforest may come from the country’s government in the big cities,

rather than the people living in or near the rainforest

The government may be able to obtain large amounts of money by allowing logging companies to cut down forests and extract the timber

A way of getting round this could be to allow countries to sell ‘carbon credits’ to other,

In 2009,

Indonesia did this

The idea is that other countries give money to Indonesia to use in conserving their forests,

and that these countries are then allowed to produce more carbon dioxide from their industrial activities

The loss of part of a rainforest means a loss of a habitat for many different species of animals

Even if small ‘islands’ of forest are left as reserves,

these may not be large enough to support a breeding population of the animals

Deforestation threatens many species of animals and plants with extinction

The loss of so many trees can also affect the water cycle (Figure 20

While trees are present and rain falls,

a lot of it is taken up by the trees,

and transported into their leaves

It then evaporates,

and goes back into the atmosphere in the process of transpiration

If the trees have gone,

then the rain simply runs off the soil and into rivers

Much less goes back into the air as water vapour

The air becomes drier,

This can make it much more difficult for people to grow crops and keep livestock

When people in industrialised countries get concerned about the rate at which some countries are cutting down their forests,

it is very important they should remember that they have already cut down most of theirs

Most tropical rainforests grow in developing countries,

and in some countries many of the people are very poor

The people may cut down the forests to 298

Cambridge IGCSE Biology

The thin soil is washed away by the rain

© Cambridge University Press 2014

Questions 22

Greenhouse gases The Earth’s atmosphere contains several different gases that act like a blanket,

They are sometimes called greenhouse gases

The most important of these gases is carbon dioxide

Methane is also a significant greenhouse gas

Carbon dioxide is transparent to shortwave radiation from the Sun

The sunlight passes freely through the atmosphere (Figure 22

The ground is warmed by the radiation,

Carbon dioxide does not let all of this infrared radiation pass through

Much of it is kept in the atmosphere,

This is called the greenhouse effect,

because it S is similar to the effect which keeps an unheated greenhouse warmer than the air outside

List four reasons why the quantity of food produced by agriculture has increased enormously in the last century

Explain what is meant by a monoculture

Describe two problems caused by monocultures

Describe two problems caused by intensive livestock farming

Outline the main ways in which humans destroy habitats

Explain how extensive deforestation can affect the amount of carbon dioxide in the air

Explain how deforestation can cause soil erosion and flooding

Short wavelength radiation from the Sun passes through the Earth’s atmosphere

Only some long wavelength radiation escapes from the Earth

Most long wavelength radiation is reflected back by greenhouse gases

Earth’s atmosphere

The ground absorbs short wavelength radiation,

and re-emits it as long wavelength radiation

Reflected radiation heats the Earth some more

Figure 22

Short wavelength radiation from the Sun passes through the atmosphere and reaches the ground

Some of it is absorbed by the ground,

and is re-emitted as long wavelength radiation

Much of this cannot pass through the blanket of greenhouse gases in the atmosphere

It is reflected back towards the Earth,

© Cambridge University Press 2014

Chapter 22: Humans and the environment

The glass around the greenhouse behaves like the carbon dioxide in the atmosphere

It lets shortwave radiation in,

but does not let out the longwave radiation

The longwave radiation is trapped inside the greenhouse,

making the air inside it warmer

We need the greenhouse effect

If it did not happen,

then the Earth would be frozen and lifeless

The average temperature on Earth would be about 33 °C lower than it is now


the amount of carbon dioxide and other greenhouse gases in the atmosphere is increasing (Figure 22

This may trap more infrared radiation,

and make the atmosphere warmer

This is called the enhanced greenhouse effect,

and its effect on the Earth’s temperature is called global warming

Over recent years,

the amount of fossil fuels which have been burnt by industry,

and in engines of vehicles such as cars,

This releases carbon dioxide into the atmosphere

Other gases that contribute to the greenhouse effect have also been released by human activities

These include methane,

Table 22

The concentrations of all of these gases in the atmosphere are steadily increasing

As the concentration of these gases increases,

the temperature on Earth will also increase

At the moment,

we are not able to predict just how large this effect will be

There are all sorts of other processes,

which can cause quite large changes in the average temperature of the Earth,

and these are not fully understood

For example,

every now and then the Earth has been plunged into an Ice Age

Perhaps we are due for another Ice Age soon

Perhaps the enhanced greenhouse effect might help to delay this

But most people think that we should be very worried about the enhanced greenhouse effect and global warming

If the Earth’s temperature does rise significantly,

there will be big changes in the world as we know it

For example,

This would release a lot more water into the oceans,

Many low-lying areas of land might be flooded

This could include large parts of countries like Bangladesh,

almost the whole of the Maldive islands,

and major cities such as London

A rise in temperature would also affect the climate in many parts of the world

No-one is sure just what would happen where – there are too many variables for scientists to be able to predict the consequences

It would probably mean that some countries which already have low rainfall might become very dry deserts

Others might have more violent storms than they do now

This would mean that animals and plants living in some areas of the world might become extinct

People in some places might not be able to grow crops

in waste tips and paddy fields

waste gases from digestive processes in cattle and insects

refrigerators and air conditioning systems

1900 Year

Figure 22

Cambridge IGCSE Biology

Main sources

Table 22

© Cambridge University Press 2014

It can be used as fuel (Figure 22

Although burning it for fuel does release carbon dioxide,

this carbon dioxide does not trap so much infrared radiation as the methane would have done

There might be some beneficial effects,

For example,

extra carbon dioxide in the atmosphere and higher temperatures might increase the rate of photosynthesis in some parts of the world

This could mean that higher yields could be gained from crops

One obvious way to cut down the emission of greenhouse gases is to reduce the amount of fossil fuels that are burnt

This would reduce the amount of carbon dioxide we pour into the air

Agreements have been made between countries to try to do this,

but they are proving very difficult to implement

Deforestation has also been blamed for increasing the amount of carbon dioxide in the air

It has been argued that cutting down rainforests leaves fewer trees to photosynthesise and remove carbon dioxide from the air


if the tree is burnt or left to rot when it is chopped down,

then carbon dioxide will be released from it

Methane is produced by farming activities (Figure 22

It is released by bacteria which live on organic matter,

such as in paddy fields (flooded fields which are used for growing rice),

by animals which chew the cud,

There is probably not much that we can do about this

Methane is also produced by decaying rubbish in landfill sites

We can reduce this problem by decreasing the amount of rubbish we throw away,

Figure 22

which can be piped off and used as a fuel

Questions Explain the difference between the greenhouse effect,

the enhanced greenhouse effect and global warming

Each of the following has been suggested as a way of reducing global warming

For each suggestion,

a reducing the top speed limit for cars and trucks b improving traffic flow in urban areas c'insulating houses in countries with cold climates d'increasing the number of nuclear power stations e encouraging people to recycle more of their rubbish

Figure 22

© Cambridge University Press 2014

Chapter 22: Humans and the environment

Acid rain

Fossil fuels,

were formed from living organisms

They all contain sulfur

When they are burnt,

the sulfur combines with oxygen in the air and forms sulfur dioxide

When fuels are burnt in vehicle engines,

the high temperatures cause nitrogen in the air to combine with oygen,

Sulfur dioxide is a very unpleasant gas

If people breathe it in,

it can irritate the linings of the breathing system

If you are prone to asthma or bronchitis,

sulfur dioxide can make it worse

Sulfur dioxide is also poisonous to many kinds of plants,

sometimes damaging their leaves so badly that the whole plant dies

Rainwater is usually slightly acidic,

This is because carbon dioxide dissolves in it to form carbonic acid

Sulfur dioxide and nitrogen oxides also dissolve in rainwater

They form a more acidic solution,

The pH of acid rain can be as low as 4

Acid rain damages plants

Although the rainwater usually does not hurt the leaves directly when it falls onto them,

it does affect the way in which plants grow

This is because it affects the soil in which the plants are growing

The acid rainwater seeps into the soil,

and washes out ions such as calcium,

The soil becomes short of these ions,

so the S plant becomes short of nutrients

It also makes it more difficult for the plant to absorb other nutrients from the soil

So acid rain can kill trees and other plants

The ions which are washed out of the soil by the acid rain often end up in rivers and lakes

Aluminium ions,

because they affect their gills

Young fish are often killed if the amount of aluminium in the water is too great

Other freshwater organisms are often killed,

At the same time,

the water itself becomes more acidic,

which means that many kinds of plants and animals cannot live in it (Figure 22

One of the biggest problems in trying to do anything about the problems of acid rain is that it does not usually fall anywhere near the place which is causing it

A coal-burning power station might release a lot of sulfur dioxide,

which is then carried high in the air for hundreds of miles before falling as acid rain

Sulfur dioxide produced in England might fall as acid rain in Norway

But acid rain is,

a much easier problem to solve than the enhanced greenhouse effect

The answer is simple – we must cut down emissions of sulfur dioxide and nitrogen oxides

Coal-burning power stations have been the worst culprits

The number of coal-burning power stations in some European countries has been going down and

Coal and oil burnt in power stations,

industries and homes release sulfur dioxide and nitrogen oxides into the air

Burning fossil fuels releases nitrogen oxides

Sulfur dioxide damages trees

Acid water in rivers adds toxic chemicals and causes food shortages,

both of which reduce fish life

Cambridge IGCSE Biology

Acid soils are made more acidic by acid rain

Acid water run-off brings toxic chemicals,

Figure 22

SO2 and NO2 dissolve to form acid rain

© Cambridge University Press 2014


What causes acid rain

? How does acid rain damage trees

? How does acid rain damage fish

? Summarise what is being done to try to reduce the production of acid rain

Nuclear fall-out

There are two main sources of pollution which can reduce oxygen levels in fresh water

They are fertilisers and untreated sewage

Farmers and horticulturists use fertilisers to increase the yield of their crops

The fertilisers usually contain nitrates and phosphates

Nitrates are very soluble in water

If nitrate fertiliser is put onto soil,

it may be washed out in solution when it rains

This is called leaching

The leached nitrates may run into streams and rivers

Algae and green plants in the river grow faster when they are supplied with these extra nitrates

They may grow so much that they completely cover the water

They block out the light for plants growing beneath them,

Even the plants on the top of the water eventually die

When they do,

their remains are a good source of food for bacteria

The bacteria breed rapidly

The large population of bacteria respires aerobically,

using up oxygen from the water

there is very little oxygen left for other living things

Those which need a lot of oxygen,

This whole process is called eutrophication (Figures 22

22 and 22

It can happen whenever food for plants or bacteria is added to water

As well as fertilisers,

such as slurry from buildings where cattle or pigs are kept,

or from pits where grass is rotted down to make silage,

Untreated sewage can also cause eutrophication (Figure 22

Sewage does not usually increase the

more of them are burning oil which produces less sulfur dioxide

New ways of producing energy,

which do not produce sulfur dioxide,

These include using wind power,

Where fossil fuel is burnt in power stations or other industries,

the waste gases can be ‘scrubbed’ to remove sulfur dioxide

This often involves passing the gases through a fine spray of lime

We have seen that car engines also produce nitrogen oxides

These can be removed by catalytic converters fitted to the exhaust system

In most countries,

all new petrol-burning cars now have to have catalytic converters

Accidents at nuclear power stations may release radioactive substances into the atmosphere

Exposure to large amounts of radiation from these substances can cause radiation sickness and burns

This type of radiation can also increase mutation rates in DNA in our cells,

This happens because ionising radiation – such as alpha,

beta and gamma radiation – damages the DNA molecules in living cells

Alpha is the most ionising and so causes the most damage,

but only if it gets inside the body

This is because it is not able to penetrate the skin

Gamma is the least ionising but the most penetrating

Eutrophication Many organisms live in water

They are called aquatic organisms

Aquatic habitats include fresh water,

and also marine environments – the sea and oceans

Most organisms that live in water respire aerobically,

They obtain their oxygen from oxygen gas which has dissolved in the water

Anything which reduces the amount of oxygen available in the water can make it impossible for fish or other aquatic organisms to live there

Figure 22

These have used up most of the oxygen in the water,

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Chapter 22: Humans and the environment

Water with few nutrients is rich in oxygen,

and supports a variety of animal life

Sunlight can penetrate deep into the water,

Water with high concentrations of nutrients is low in oxygen,

but it does provide a good food source for many kinds of bacteria

Once again,

Could we stop using nitrate fertilisers

? It is not really sensible at the moment to suggest that we could

People expect to have plentiful supplies of relatively cheap food

Although fertilisers are expensive,

by using them farmers get so much higher yields that they make more profit

If they did not use fertilisers at all,

their yields would be much lower and they would have to sell their crops for a higher price,

in order to make any profit at all

Some farmers use organic fertilisers,

Organic fertilisers are better than inorganic ones in that they do not contain many nitrates which can easily be leached out of the soil


they release their nutrients gradually,

giving crops time to absorb them efficiently


manures can still cause pollution,

if a lot is put onto a field at once,

at a time of year when there is a lot of rain or when crops are not growing and cannot absorb the nutrients from them

The yields obtained when using organic fertilisers are not usually as great as when using inorganic ones,

so the crops are usually sold for a higher price

Many people are now prepared to pay this extra money for food from crops grown in this way,

If nitrate fertilisers are used,

there is much which can be done to limit the harm they do

Care must be taken not to use too much,

but only to apply an amount which the plants can take up straight away

Fertilisers should not be applied to empty fields,

but only when plants are growing

They should not be applied just before rain is forecast

They should not be sprayed near to streams and rivers

animal waste and silage containing nitrates and other nutrients

No light gets through the water,

No fish can live in this water

Large populations of algae and bacteria grow

Figure 22

Nutrients flowing into the water increase algal and bacterial growth

This reduces oxygen concentration,

Study tip Effluent from properly treated sewage (page 309) does not cause eutrophication

It is raw (untreated) sewage that causes problems

Distance downstream point at which untreated sewage is discharged Figure 22

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If nothing is done about this,

then crop yields can be very badly reduced

It has been estimated that,

at least one-third of potential crops are destroyed by pests

If farmers did not use pesticides,

By definition,

a pesticide is a harmful substance

If they are not used with care,

some pesticides can do a lot of damage to the environment

For example,

DDT is a pesticide that kills insects

It is a persistent insecticide,

which means that it does not break down,

but remains in the bodies of the insects or in the soil

When a bird or other organism eats the insects,

The DDT stays in their bodies

more DDT accumulates in their tissues

If a bird of prey eats the insect-eating bird,

it too begins to accumulate DDT

Birds and other animals near the ends of food chains can build up very large concentrations of DDT in their bodies (Figure 22


DDT is also nonspecific

This means that it not only harms the insects it is meant to kill,

but is also harmful to other living things

In high concentrations it is very harmful to birds,

In Britain,

it affected the breeding success of peregrine falcons,

by making their egg shells very weak,

so that they very rarely hatched

The peregrine falcon population dropped very rapidly

Once it was realised that DDT was doing so much harm,

its use in Britain was stopped

Now DDT is banned in some parts of the world


it is still used in many developing countries,

insects would be such a problem that more people would starve or die of diseases like malaria

Other insecticides need to be developed which are as cheap and effective as DDT,

but that do not harm other living organisms

A pesticide is a substance that kills organisms which damage crops

Insects that eat crops can be killed with insecticides

Fungi that grow on crops are controlled with fungicides

Weeds that compete with crop plants for water,

light and minerals can be controlled with herbicides

Pesticides may also be used to control organisms which transmit disease,

We have seen that insects or fungi which can feed on a monoculture have an almost inexhaustible food supply

The usual limits on their population growth do not apply

The populations of the insects or fungi may grow very rapidly,

until they are so big that they cause extensive damage to the crop (Figure 22

Figure 22

can cause huge amounts of damage to a maize crop

An investigation in the 1970s showed that,

although the DDT levels in the water of an estuary were only 0

the amounts in the animals feeding in the estuary were much greater

The numbers show the amount of DDT in each kind of animal

Figure 22

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Chapter 22: Humans and the environment

Study tip Do not confuse fertilisers with pesticides

Fertilisers provide extra nutrients for crops

Pesticides kill pests

Chemical waste

Female contraceptive hormones In Chapter 17,

we saw that hormones containing oestrogen can be taken by women to stop them producing eggs,

Some of these hormones are excreted in the woman’s urine,

and eventually find their way into waterways,

Fish and other animals,

that live in water can be affected by these hormones

They can prevent the male hormone,

working effectively in the animals’ bodies

Male fish,

have been shown to produce fewer sperm if the water in which they live is polluted with female contraceptive hormones

In some cases,

they can even make a male fish change sex and become female

This might also be one of the reasons why,

the sperm count of many men is much lower now than it was 20 years ago

‘Sperm count’ is a measure of the number of sperm produced,

A study involving 26 000 men found that,

the average sperm count fell by one third

One possible explanation for this is exposure to female contraceptive hormones

Another is exposure to synthetic chemicals used for various purposes – such as making plastics – that behave in similar ways to these hormones

At the moment,

no-one knows the full explanation for this fall in sperm count

A very different kind of water pollution may result from the discharge of chemical waste into waterways

Chemical waste may contain heavy metals,

These substances are very poisonous (toxic) to living organisms,

because heavy metals stop enzymes from working

If they get into streams,

they may kill almost every living thing in that area of water

Litter is very unsightly

It can also be dangerous to S other organisms

For example,

plastic waste thrown away at sea is often mistaken for jellyfish by turtles

They eat it

As they eat more and more,

it gradually collects up in their stomachs,

because it cannot be broken down by their enzymes

Now they cannot eat their real food,

Animals can also get trapped inside plastic containers,

or may get plastic cords or bags wrapped around their bodies,

Non-biodegradable plastics

Plastics are man-made materials that we use for many different purposes

Most of them are made from fossil fuels

If you look around you now,

you will almost certainly be able to see several items made from plastics

Plastics are cheap,

lightweight and can be made into any shape and colour that we want

One big problem with plastics is that most of them are non-biodegradable

This means that decomposers cannot break them down

When a plastic item is thrown away,

Discarded plastic objects just accumulate (Figure 22

Figure 22

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Table 22

The rest of this chapter explains what we can do to limit further damage


Conservation is the process of looking after the natural environment

Conservation attempts to maintain or increase the range of different species living in an area,

We have seen that one of the greatest threats to biodiversity is the loss of habitats

Each species of living organism is adapted to live in a particular habitat

If this habitat is destroyed,

then the species may have nowhere else to live,

Tropical rainforests have a very high biodiversity compared with almost anywhere else in the world

This is one of the main reasons why people think that conserving them is so important

When tropical rainforests are cut down or burnt,

the habitats of thousands of different species are destroyed


Main causes

caused by release of carbon dioxide,

CFCs and nitrogen oxides

Possible solutions

produce less organic waste and/or collect and use methane produced from landfill sites

sulfur dioxide and nitrogen oxides from the burning of fossil fuels

use catalytic converters on cars

for wood and for land for farming,

provide alternative sources of income for people living near rainforests

draining wetlands for housing and land for farming

untreated effluent from industry

run-off from mining operations

impose tighter controls on industry and mining

sewage and fertilisers running into streams

treat all sewage before discharge into streams

leakages from undersea oil wells

impose tighter controls on shipping and the oil industry

see deforestation and wetlands above

careless use of insecticides and herbicides

development of more specific and less persistent pesticides

more use of alternative control methods,

greatly reducing populations impose controls on methods and amount of species caught for food

accidental of fishing damage to other animals such as dolphins

Name this gas

Explain what is meant by this term

Table 22

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Chapter 22: Humans and the environment

Sustainable resources

were formed millions of years ago,

from dead plants and bacteria that lived in the Carboniferous period

Their partially decayed bodies were compressed and formed coal,

This took a very long time to happen

These fossil fuels cannot be replaced

Once we have used them,

We therefore need to limit our use of fossil fuels,

to ensure that there will still be some available for future generations

One way that we can help to conserve nonsustainable resources is to recycle things that are made from them

Recycling glass Glass is made from sand (silicon oxide) and a few other chemicals,

such as lime (calcium oxide) and soda (sodium oxide)

These are non-sustainable resources

The best glass is made from especially pure sand,

which is mined from deposits made long ago

Making new glass involves heating these chemicals to very high temperatures

It releases a lot of carbon dioxide

Used glass can be crushed,

and then used to make new glass objects (Figure 22

This releases much less carbon dioxide than making new glass

It also reduces our use of the raw materials,

We have already seen how and why deforestation occurs

Another kind of habitat that is under threat is wetland,

People drain wetland so that it can be more easily farmed

We build roads and houses,

destroying whatever used to grow on that land

We farm animals in large numbers on land that cannot really produce enough vegetation to support them,

so that the land becomes a semi-desert

Many governments and also world-wide organisations such as the World Wide Fund for Nature are aware of these problems and are attempting to make sure that especially important habitats are not damaged

Most countries have special areas where people’s activities are carefully controlled,

ensuring that wildlife can continue to live there

the loss of money from agriculture in these areas can be regained by allowing tourists to visit them

The most successful projects actively involve local people,

who are usually delighted to see their environment being cared for,

so long as they can still make a living from it

People will always need to use resources that we take from our environment

These include food,

fuels (such as fossil fuels) and minerals (such as ores of copper or aluminium)

If we are careful,

then we can take and use these resources without doing too much harm to other organisms

We say that a resource is sustainable if we can keep on using it,

For example,

fish in the sea could be a sustainable resource,

as long as we do not take so many that their populations fall to dangerously low levels

Wood for fuel could be a sustainable resource,

so long as we replant trees to replace the ones we cut down,

or make sure that plenty of young trees are allowed to naturally grow


many of the resources that we take from the Earth are not sustainable

Fossil fuels are a good example of a non-sustainable resource

These fuels

Recycling plastics We have seen that plastics are made from fossil fuels

If we can reuse plastic objects,

Key definition sustainable resource – one which can be removed from the environment without it running out

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Figure 22

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that will reduce the amount of fossil fuels we have to use to make them

If we cannot reuse things,

then we can still recycle them

This means using the plastic from a used object to make a different object

Used plastic,

can be used to make fleece clothing,

packaging and many other items

Recycling paper Paper is made of cellulose fibres from plants,

Waste paper can be mixed with water and chemicals that break it down to form a pulp

This is passed through filters that remove any glue that may have been stuck to it (for example,

to hold together the pages of magazines) and then treated to remove the printing ink from it

This leaves clean cellulose fibres,

which can be made into new paper

Recycling paper does reduce the number of trees that have to be cut down to make new paper,

but in fact most paper mills use trees that are specially grown for the purpose,

and each time some are cut down new ones are planted to replace them

The main advantage of recycling paper is that it causes less water and land pollution than making new paper,


it is important to take into account the energy – usually from fossil fuels – used in collecting and transporting the used paper,

which can be quite significant

has come from bathrooms and kitchens in people’s houses and offices

Some of it has come from factories

Sewage is mostly water,

but also contains many other substances

These include urine and faeces,

Sewage should not be allowed to run into rivers or the sea before it has been treated

This is because it can harm people and the environment

Untreated sewage is called raw sewage

Raw sewage contains many bacteria and other microorganisms,

some of which may be pathogenic

People who come into contact with raw sewage,

especially if it gets into their mouths,

Raw sewage also contains many substances which provide nutrients for plants and microorganisms

We have seen how this can cause eutrophication if it gets into waterways (page 303)

It is therefore very important that sewage is treated to remove any pathogenic organisms,

before it is released as effluent

Microorganisms play an important part in all the most commonly used methods of sewage treatment

When sewage has been treated,

the water in it can be used again,

so sewage treatment enables water to be recycled

It may not be a nice thought to know that the water you drink was once inside someone else’s body,

but if we did not recycle water in this way then significant water shortages would occur in many parts of the world

Sewage is treated to make it safe

the raw sewage is passed through screens

These trap large objects such as grit which may have been washed off roads

The screened liquid is then left for a while in settlement tanks,

where any other insoluble particles drift to the bottom and form a sediment (Figure 22

There are two different ways in which the resulting liquid can now be treated

Recycling metals We get metals from ores that are found underground

Mining metal ores uses a lot of energy – usually from fossil fuels – and damages habitats

Metal ores are also a non-renewable resource

Most metals can be recycled


steel (iron) and zinc are all recycled in many parts of the world

Recycling aluminium saves 95% of the energy that would be used in mining aluminium ore and extracting aluminium metal from it

Sewage treatment

Water is a scarce resource in many parts of the world

Water that we have used can be recycled

Sewage is waste liquid which has come from houses,

industry and other parts of villages,

Some of it has just run off streets into drains when it rains

Some of it

raw sewage from house resulting liquid Figure 22

How sewage is treated

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Chapter 22: Humans and the environment

liquid after screening and settlement filter bed

Primary settlement tank Solid wastes sink and are sent to the anaerobic digester

Aeration tank Aerobic microorganisms grow and feed on the organic matter

Secondary settlement tank Microorganisms sink

The sludge at the bottom is called activated sludge and is returned to the aeration tank

Grit and stone removal Grit and stones are stopped by a screen

Activated sludge In this method (Figure 22

the liquid from the settlement tanks runs into a tank called an aeration tank

Like the trickling filter bed,

this contains aerobic microorganisms,

mostly bacteria and protoctists

Oxygen is provided by bubbling air through the tank

As in the trickling filter bed,

these aerobic microorganisms make the sewage harmless

Why is this method called ‘activated sludge’

? ‘Activated’ means that microorganisms are present

Some of the liquid from the tank,

containing these microorganisms,

is kept to add to the next lot of sewage coming in

‘Sludge’ means just what it sounds like

! It is a word which describes the semi-solid waste materials in sewage

Figure 22

The liquid is trickled onto the surface of the stones through holes in a rotating pipe

This makes sure that air gets mixed in with the liquid

The liquid trickles quite slowly