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IGCSE BIOLOGY TOPIC 21 HUMAN INFLUENCE ON ECOSYSTEM

Food supply
Improvements in farming:
With the advancement of technology, new methods of farming have been introduced in order
to maximise yield:
● Use of machinery – agricultural machinery can be used in the place of people. This is
quicker and more efficient, thus larger amounts of land can be farmed at once.
● Chemical fertilisers – fertilisers increase the amount of nutrients in the soil for plants,
meaning that they can grow larger and produce more fruit, increasing the yield.
● Insecticides and herbicides – these chemicals kill off unwanted insects and weed
species. This means that there is less damage done to plants and fruit lost to insects, as
well as reducing competition from other plant species.
● Selective breeding – animals and crops which produce a large yield are selectively bred
to produce a large number of organisms with a high yield.
Farmers also grow crops in a monoculture, which means that only one species of crop is grown
at once. This is done to maximise efficiency and simplicity. It does, however, have a negative
impact on the surrounding ecosystems. This is because there is a loss of biodiversity as only
one species is grown. This can harm food chains and reduce the population of some species.
Intensive farming:
Animals and crops can both be farmed intensively. With crops, this means farming in a
monoculture using high amounts of pesticides, herbicides and fertilisers to maximise
production. Animals are kept in high densities and energy loss is limited by restricting
movement and keeping a constant optimum temperature so that the organisms do not waste
energy to thermoregulate. High amounts of antibiotics are also used to prevent diseases.
Intensive farming is not sustainable, however, and damages the environment: livestock produce
large amounts of methane gas, which is a main contributor to global warming, whilst crops
take up huge amounts of space, meaning that forests and other habitats must be destroyed to
make space for farming.
Food shortage:
When people do not receive enough food, famine occurs. This can be caused by a variety of
factors, including natural disasters, such as drought and flooding, increasing population,
poverty, and unequal food distribution.

As the world human population increases, food production must also be increased to sustain
the population. This is a problem as more land is required to grow crops and animals, meaning
that deforestation is happening at an increasing rate, and there is also an increasing amount of
greenhouse gases emitted from animal production. Greenhouse gases cause global warming,
which is a worldwide issue that leads to increased natural disasters, such as tropical storms and
drought, as well as rising sea levels, which floods homes and decreases the amount of
habitable land.
Habitat destruction
Many habitats are destroyed by humans to make space for other economic activities, or by
pollution from these activities. Consequently, the biodiversity of many places is decreasing.
This interrupts food chains and webs and means that more species may die because their prey
is gone.
Main causes of habitat destruction:
● Clearing land for farming and housing – crops, livestock and homes all take up a large
amount of space. As there is an increasing population and demand for food, the amount
of land available for these things must be increased by clearing habitats such as forests
(deforestation).
● Natural resource extraction – natural resources such as wood and stone must be
gathered to make different products. Therefore many trees are cut down, destroying
forest habitats. In addition, some resource extraction takes up a large amount of space,
for example mining, which means that the land must be cleared first.
● Marine pollution – human activities lead to the pollution of marine habitats. In many
places, oil spills and other waste pollutes the oceans, killing sea life. In addition,
eutrophication can occur when fertilisers from intensively farmed fields enters
waterways. This causes a huge decrease in biodiversity as most species die.
Deforestation:
Deforestation involves cutting down large amounts of trees to gather as resources for
manufacturing or to clear space for other economic activities. This has a large amount of
undesirable effects:
● Extinction – habitat destruction can lead to the extinction of species that lived there.
● Soil erosion – without roots to anchor the soil, it is carried away by the wind and heavy
rains. This decreases the fertility of these areas.

● Flooding – forests prevent flash flooding by slowing the time that water takes to reach
the ground. They also allow the water to be absorbed into the soil.
● Increased carbon dioxide in the atmosphere – Trees absorb carbon dioxide from the
atmosphere during photosynthesis. If there are fewer trees, less carbon dioxide is
absorbed, thus there is more in the atmosphere. This increases global warming.
Pollution
Human activities have led to the pollution of land, water and air. This has a variety of negative
outcomes, including global warming and habitat destruction. Pollution comes from a variety of
sources, including industry and manufacturing processes, waste and discarded rubbish,
chemicals from farming practices, nuclear fall-out, and untreated sewage. Plastics have a large
negative impact on both land and water habitats due to their non-biodegradability. Animals
often try to eat plastic or become caught in it, leading to injuries and death, which can affect
whole the food chain. As plastics take a long time to break down, they accumulate in habitats
which causes an increasing problem.
Eutrophication and water pollution:
Bodies of water, such as lakes, rivers and oceans, become polluted in a variety of ways, such as
oil spills, discarded rubbish, and chemicals. One of the major pollutants to lakes and rivers is
chemicals from fertilisers. When fertilisers are washed off fields into waterways,
eutrophication occurs:

  1. Fertilisers are washed from fields into the waterways by rain. This brings an excess of
    nutrients into the habitat.
  2. The nutrients cause plants to grow rapidly, and there is an algae bloom across the
    surface.
  3. Algae covers the surface of the water, preventing sunlight from passing through. This
    means that plants cannot photosynthesise to produce energy so they begin to die.
  4. As there are less plants to photosynthesise, less oxygen is released into the water. The
    dead plants are broken down by decomposers, which use up the remaining oxygen from
    the water.
  5. The lack of oxygen causes organisms such as fish to die, reducing the biodiversity of the
    habitat.

    Another cause of water pollution is female contraceptive hormones. These hormones are
    excreted from the body in urine and then make their way into the water supply, as they are not
    filtered out by sewage treatment plants. When they reach male aquatic organisms, such as fish
    and frogs, which are very sensitive to the hormones, it causes feminisation. This is where male
    organisms begin to produce eggs and lose the ability to reproduce. Consequently, a small
    amount of offspring is produced which can harm the species survival and also disrupts the food
    chain for animals that usually feed off these organisms. In addition, these hormones can reduce
    the sperm count in men, which causes fertility problems.
    Air pollution:
    The main pollutants of air are methane and carbon dioxide. These are released into the
    atmosphere due to farming practices and manufacturing, especially during the burning of fossil
    fuels. Although these gases are released in small quantities naturally, human activity has greatly
    increased the rate of their emission.
    Air pollution leads to a number of environmental problems: carbon dioxide and methane
    contribute to the increasing rate of global warming and climate change, as well as causing acid
    rain. Global warming occurs when greenhouse gases rise into the atmosphere and form a layer
    around the Earth, preventing heat from the Sun escaping the atmosphere. This means that the
    climate of Earth becomes hotter. Acid rain is caused by carbon dioxide dissolving in rain water
    to form carbonic acid, and sulphur dioxide dissolving to make sulphuric acid. Acid rain then falls
    and accumulates in bodies of water and soils, causing a change in pH which may harm
    organisms living there. The acid also corrodes infrastructure and damages tree bark and leaves.
    To reduce acid rain, sulphur dioxide is removed from gases before they are released into the
    atmosphere. Sulphur can also be removed from fossil fuels before burning, although this
    process is expensive. Soils and water can be made less acidic by adding powdered limestone or
    slaked lime.
    Conservation
    It is important to reduce the negative impacts that humans have on the environment to
    conserve the biodiversity of ecosystems. This means increasing the sustainability of resources
    and manufacturing. Sustainable resources are those which can be taken from the environment
    without the risk of them running out, i.e. they can be produced naturally as quickly as they are
    harvested. Resources such as coal and oil are not sustainable as fossil fuels are non-renewable.
    Others, such as wood and fish, can be harvested sustainably with the help of quotas, education
    and re-stocking.

    Development should also happen sustainably. Sustainable development is defined as
    development which meets the needs of people today, as well as providing for an increasing
    population without damaging the environment. Development must be managed carefully to
    maintain sustainability. This involves cooperation between different players, from local councils
    and companies, to governments and NGOs, each of whom may have differing demands. For
    example, some companies may want to continue harvesting resources to protect jobs and
    profits, whereas other people may disagree as it could damage habitats.
    Reducing waste:
    Unsustainable practices can lead to large amounts of waste. This waste can be reduced by
    introducing recycling programmes. Paper, glass, plastic and metals can all be reused and
    recycled, which greatly reduces the amount of these materials polluting the environment or in
    landfills. It also reduces the demand on sourcing raw materials, which in turn reduces the
    amount of habitat destruction.
    Sewage treatment:
    Water treatment happens in three stages: sedimentation, filtration and chlorination. In
    sedimentation, larger solids separate out from the water to form sludge, whilst water and
    lighter particles which float on top are drained into another tank. This liquid is then passed
    through gravel and sand filters in filtration to remove any particles still left in the water.
    Finally, chlorine is added to the water to kill microorganisms such as bacteria which makes the
    water safe to consume.
    Conservation of endangered species:
    When the number of surviving organisms in a species becomes very low, the species is classed
    as endangered. This is harmful to a species as it greatly reduces the gene pool by decreasing
    the number of alleles available. This makes the species more susceptible to disease and less
    able to adapt to changes. Species can become endangered for a variety of reasons, such as
    habitat destruction, climate change, hunting, pollution, and competition from introduced
    species.
    If an endangered species is not protected, it can become extinct. There are many measures that
    can be used to protect a species from extinction:
    ● Monitoring – endangered animals can be monitored. This allows the number of
    organisms left to be tracked over time and can also highlight their preferred habitats
    and migration patterns, allowing important habitats to be protected.
    ● Education – this allows people to understand why a species is becoming extinct so that
    protective measures can be implemented.

    ● Captive breeding programmes – endangered animals can be bred in captivity where
    their chance of survival is greater. They can later be reintroduced to habitats in the wild.
    ● Seed banks – seeds from endangered plant species can be preserved so that the plants
    may be grown in the future.
    Conservation is important to prevent the extinction of species and to protect the environment.
    It is important to protect these things to improve sustainability and to make sure that there are
    resources available in the future.

MAIN NOTES

Food supply
Improvements in farming:
With the advancement of technology, new methods of farming have been introduced in order
to maximise yield:
● Use of machinery – agricultural machinery can be used in the place of people. This is
quicker and more efficient, thus larger amounts of land can be farmed at once.
● Chemical fertilisers – fertilisers increase the amount of nutrients in the soil for plants,
meaning that they can grow larger and produce more fruit, increasing the yield.
● Insecticides and herbicides – these chemicals kill off unwanted insects and weed
species. This means that there is less damage done to plants and fruit lost to insects, as
well as reducing competition from other plant species.
● Selective breeding – animals and crops which produce a large yield are selectively bred
to produce a large number of organisms with a high yield.
Farmers also grow crops in a monoculture, which means that only one species of crop is grown
at once. This is done to maximise efficiency and simplicity. It does, however, have a negative
impact on the surrounding ecosystems. This is because there is a loss of biodiversity as only
one species is grown. This can harm food chains and reduce the population of some species.
Intensive farming:
Animals and crops can both be farmed intensively. With crops, this means farming in a
monoculture using high amounts of pesticides, herbicides and fertilisers to maximise
production. Animals are kept in high densities and energy loss is limited by restricting
movement and keeping a constant optimum temperature so that the organisms do not waste
energy to thermoregulate. High amounts of antibiotics are also used to prevent diseases.
Intensive farming is not sustainable, however, and damages the environment: livestock produce
large amounts of methane gas, which is a main contributor to global warming, whilst crops
take up huge amounts of space, meaning that forests and other habitats must be destroyed to
make space for farming.
Food shortage:
When people do not receive enough food, famine occurs. This can be caused by a variety of
factors, including natural disasters, such as drought and flooding, increasing population,
poverty, and unequal food distribution.
www.pmt.education
As the world human population increases, food production must also be increased to sustain
the population. This is a problem as more land is required to grow crops and animals, meaning
that deforestation is happening at an increasing rate, and there is also an increasing amount of
greenhouse gases emitted from animal production. Greenhouse gases cause global warming,
which is a worldwide issue that leads to increased natural disasters, such as tropical storms and
drought, as well as rising sea levels, which floods homes and decreases the amount of
habitable land.
Habitat destruction
Many habitats are destroyed by humans to make space for other economic activities, or by
pollution from these activities. Consequently, the biodiversity of many places is decreasing.
This interrupts food chains and webs and means that more species may die because their prey
is gone.
Main causes of habitat destruction:
● Clearing land for farming and housing – crops, livestock and homes all take up a large
amount of space. As there is an increasing population and demand for food, the amount
of land available for these things must be increased by clearing habitats such as forests
(deforestation).
● Natural resource extraction – natural resources such as wood and stone must be
gathered to make different products. Therefore many trees are cut down, destroying
forest habitats. In addition, some resource extraction takes up a large amount of space,
for example mining, which means that the land must be cleared first.
● Marine pollution – human activities lead to the pollution of marine habitats. In many
places, oil spills and other waste pollutes the oceans, killing sea life. In addition,
eutrophication can occur when fertilisers from intensively farmed fields enters
waterways. This causes a huge decrease in biodiversity as most species die.
Deforestation:
Deforestation involves cutting down large amounts of trees to gather as resources for
manufacturing or to clear space for other economic activities. This has a large amount of
undesirable effects:
● Extinction – habitat destruction can lead to the extinction of species that lived there.
● Soil erosion – without roots to anchor the soil, it is carried away by the wind and heavy
rains. This decreases the fertility of these areas.
www.pmt.education
● Flooding – forests prevent flash flooding by slowing the time that water takes to reach
the ground. They also allow the water to be absorbed into the soil.
● Increased carbon dioxide in the atmosphere – Trees absorb carbon dioxide from the
atmosphere during photosynthesis. If there are fewer trees, less carbon dioxide is
absorbed, thus there is more in the atmosphere. This increases global warming.
Pollution
Human activities have led to the pollution of land, water and air. This has a variety of negative
outcomes, including global warming and habitat destruction. Pollution comes from a variety of
sources, including industry and manufacturing processes, waste and discarded rubbish,
chemicals from farming practices, nuclear fall-out, and untreated sewage. Plastics have a large
negative impact on both land and water habitats due to their non-biodegradability. Animals
often try to eat plastic or become caught in it, leading to injuries and death, which can affect
whole the food chain. As plastics take a long time to break down, they accumulate in habitats
which causes an increasing problem.
Eutrophication and water pollution:
Bodies of water, such as lakes, rivers and oceans, become polluted in a variety of ways, such as
oil spills, discarded rubbish, and chemicals. One of the major pollutants to lakes and rivers is
chemicals from fertilisers. When fertilisers are washed off fields into waterways,
eutrophication occurs:

  1. Fertilisers are washed from fields into the waterways by rain. This brings an excess of
    nutrients into the habitat.
  2. The nutrients cause plants to grow rapidly, and there is an algae bloom across the
    surface.
  3. Algae covers the surface of the water, preventing sunlight from passing through. This
    means that plants cannot photosynthesise to produce energy so they begin to die.
  4. As there are less plants to photosynthesise, less oxygen is released into the water. The
    dead plants are broken down by decomposers, which use up the remaining oxygen from
    the water.
  5. The lack of oxygen causes organisms such as fish to die, reducing the biodiversity of the
    habitat.
    www.pmt.education
    Another cause of water pollution is female contraceptive hormones. These hormones are
    excreted from the body in urine and then make their way into the water supply, as they are not
    filtered out by sewage treatment plants. When they reach male aquatic organisms, such as fish
    and frogs, which are very sensitive to the hormones, it causes feminisation. This is where male
    organisms begin to produce eggs and lose the ability to reproduce. Consequently, a small
    amount of offspring is produced which can harm the species survival and also disrupts the food
    chain for animals that usually feed off these organisms. In addition, these hormones can reduce
    the sperm count in men, which causes fertility problems.
    Air pollution:
    The main pollutants of air are methane and carbon dioxide. These are released into the
    atmosphere due to farming practices and manufacturing, especially during the burning of fossil
    fuels. Although these gases are released in small quantities naturally, human activity has greatly
    increased the rate of their emission.
    Air pollution leads to a number of environmental problems: carbon dioxide and methane
    contribute to the increasing rate of global warming and climate change, as well as causing acid
    rain. Global warming occurs when greenhouse gases rise into the atmosphere and form a layer
    around the Earth, preventing heat from the Sun escaping the atmosphere. This means that the
    climate of Earth becomes hotter. Acid rain is caused by carbon dioxide dissolving in rain water
    to form carbonic acid, and sulphur dioxide dissolving to make sulphuric acid. Acid rain then falls
    and accumulates in bodies of water and soils, causing a change in pH which may harm
    organisms living there. The acid also corrodes infrastructure and damages tree bark and leaves.
    To reduce acid rain, sulphur dioxide is removed from gases before they are released into the
    atmosphere. Sulphur can also be removed from fossil fuels before burning, although this
    process is expensive. Soils and water can be made less acidic by adding powdered limestone or
    slaked lime.
    Conservation
    It is important to reduce the negative impacts that humans have on the environment to
    conserve the biodiversity of ecosystems. This means increasing the sustainability of resources
    and manufacturing. Sustainable resources are those which can be taken from the environment
    without the risk of them running out, i.e. they can be produced naturally as quickly as they are
    harvested. Resources such as coal and oil are not sustainable as fossil fuels are non-renewable.
    Others, such as wood and fish, can be harvested sustainably with the help of quotas, education
    and re-stocking.
    www.pmt.education
    Development should also happen sustainably. Sustainable development is defined as
    development which meets the needs of people today, as well as providing for an increasing
    population without damaging the environment. Development must be managed carefully to
    maintain sustainability. This involves cooperation between different players, from local councils
    and companies, to governments and NGOs, each of whom may have differing demands. For
    example, some companies may want to continue harvesting resources to protect jobs and
    profits, whereas other people may disagree as it could damage habitats.
    Reducing waste:
    Unsustainable practices can lead to large amounts of waste. This waste can be reduced by
    introducing recycling programmes. Paper, glass, plastic and metals can all be reused and
    recycled, which greatly reduces the amount of these materials polluting the environment or in
    landfills. It also reduces the demand on sourcing raw materials, which in turn reduces the
    amount of habitat destruction.
    Sewage treatment:
    Water treatment happens in three stages: sedimentation, filtration and chlorination. In
    sedimentation, larger solids separate out from the water to form sludge, whilst water and
    lighter particles which float on top are drained into another tank. This liquid is then passed
    through gravel and sand filters in filtration to remove any particles still left in the water.
    Finally, chlorine is added to the water to kill microorganisms such as bacteria which makes the
    water safe to consume.
    Conservation of endangered species:
    When the number of surviving organisms in a species becomes very low, the species is classed
    as endangered. This is harmful to a species as it greatly reduces the gene pool by decreasing
    the number of alleles available. This makes the species more susceptible to disease and less
    able to adapt to changes. Species can become endangered for a variety of reasons, such as
    habitat destruction, climate change, hunting, pollution, and competition from introduced
    species.
    If an endangered species is not protected, it can become extinct. There are many measures that
    can be used to protect a species from extinction:
    ● Monitoring – endangered animals can be monitored. This allows the number of
    organisms left to be tracked over time and can also highlight their preferred habitats
    and migration patterns, allowing important habitats to be protected.
    ● Education – this allows people to understand why a species is becoming extinct so that
    protective measures can be implemented.
    www.pmt.education
    ● Captive breeding programmes – endangered animals can be bred in captivity where
    their chance of survival is greater. They can later be reintroduced to habitats in the wild.
    ● Seed banks – seeds from endangered plant species can be preserved so that the plants
    may be grown in the future.
    Conservation is important to prevent the extinction of species and to protect the environment.
    It is important to protect these things to improve sustainability and to make sure that there are
    resources available in the future.

Habitat destruction

Removal of habitats:

  • Farmland is not natural habitat but, at one times, hedgerow, hay meadows and stubble fields were important habitats for plants and animals.
  • Intensive agriculture has destroyed many of these habitats; hedges have been grubbed out to make fields larger, a monoculture of solage grasses has replaced the mixed population of a hay meadow and planting of winter wheat has denied animals access to stubble fields in autumn.
  • As a result, populations of butterflies, flowers and birds have cashed.
  • The development of towns and cities (urbanisation) makes a great demand on land, destroying natural habitats.
  • The crowding of growing populations into town leads to problems of waste disposal.
  • The sewage and domestic waste from a town of several thousand people can cause disease and pollution in the absence of effective means of disposal, damaging surrounding habitats.

Extraction of natural resources:

  • An increasing population and greater demands on modern technology means we need more raw materials for the manufacturing industry and greater energy supplies.
  • Fossil fuels such as coal can be mined, but this can permanently damage habitats, partly due to the process of extraction, but also due to dumping of the rock extracted in spoil heaps.
  • Oil spillages around oil wells are extremely toxic.
  • Once the oil seeps into the soil and water systems, habitats are destroyed.
  • Mining for raw materials such as gold, iron, aluminium and silicon leaves huge scars in the landscape and destroys large areas of natural habitat.

Marine pollution:

  • Marine habitats around the world are becoming contaminated with human debris.
  • This includes untreated sewage, agricultural fertilisers and pesticides.
  • Oil spills still cause problems but is gradually reducing.
  • Plastics are a huge problem: many are non-biodegradable so they persist in the environment.
  • Others form micro-particles as they break down and these are mistaken by marine organisms for food and are indigestible. They stay in the stomach, causing sickness, or prevent the gills from working efficiently.
  • Where fertilisers and sewage enter the marine environment, ‘dead zones’ develop where there is insufficient oxygen to sustain life.
  • Any form of habitat destruction by humans, even where a single species is wiped out, can have an impact on food chains and food webs because other organisms will use that species as a food source, or their numbers will be controlled through its predation.

Deforestation:

The removal of large numbers of trees results in habitat destruction on a massive scale.

  • Reasons why: for timber, to make way for agriculture, roads and settlements and for firewood.
  • Animals living in the forest lose their homes and sources of food; species of plant become extinct as the land is used for other purposes such as agriculture, mining, housing and roads.
  • Soil erosion is more likely to happen as there are no roots to hold the soil in place. The soil can end up in rivers and lakes, destroying habitats there.
  • Flooding becomes more frequent as there is no soil to absorb and hold rainwater. Plant roots rot and animals drown, destroying food chains and webs.
  • Carbon dioxide builds up in the atmosphere as there are fewer trees to photosynthesis, increasing global warming. Climate change affects habitats.

Pollution

Sources and effect of pollution of land and water.

Insecticides:

  • eg DDT controls spread of malaria by killing mosquitos which carry the protoctist parasites that cause the disease.
  • Remains in the environment after it has been sprayed and can be absorbed in sub-lethal doses by microscopic organisms – can enter food chains and accumulate as it moves up them.
  • Persist for a long time in the soil, rivers, lakes and bodies of animals, including animals.

Herbicides:

  • If herbicides do not break down straight away, they can leach from farmland into water systems. such as rivers and lakes, where they kill aquatic plants, removing the producers from food chains.
  • Herbivores lose their food source and die or migrate. Carnivorous animals are then affected as well.
  • May blow onto surrounding land and kill plants other than weeds putting rare species of wild flowers at risk.

Nuclear fall-out:

  • Leak from a nuclear power station or nuclear explosion
  • Radioactive particles carried by the wind or water and gradually settle in the environment.
  • If radiation has long half-life, it remains in the environment and is absorbed by living organisms. The radioactive material bioaccumulates in food chains and can cause cancer in top carnivores.

Sources and effects of pollution of water.

Chemical waste:

  • Many industrial processes produce poisonous waste products.
  • Electroplating produces waste containing copper and cyanide. If these chemicals are released into rivers they poison the animals and plants and could poison humans who drink the water.
  • Any factory getting rid of its effluent into water systems risks damaging the environment.
  • Some detergents contain a lot of phosphate. This is not removed by sewage treatment and is discharged into rivers.
  • The large amount of phosphate encourages growth of microscopic plants (algae).

Discarded rubbish:

  • The domestic waste from a town of several thousand people can cause disease and pollution in the absence of effective means of disposal.
  • Much ends up in landfill sites, taking up valuable space, polluting the ground and attracting vermin and insects, which can spread disease.
  • Air pollution can be caused by burning waste.

Sewage:

  • Diseases like typhoid and cholera are caused by certain bacteria when they get into the human intestine.
  • The faeces passed by people suffering from these diseases will contain the harmful bacteria.
  • If this bacteria get into drinking water they may spread the disease to hundreds of other people.
  • For this reason, among others, untreated sewage must not be emptied into rivers.

Fertilisers:

  • When nitrates and phosphates from farmland and sewage escape into water they cause excessive growth of microscopic green plants.
  • This may result in a serious oxygen shortage in the water, resulting in the death of aquatic animals – a process called eutrophication.

Eutrophication:

  • A major problem with the use of fertilisers occurs when they are washed off the land by rainwater into rivers and lakes.
  • This leaching causes an increase in the levels of minerals such as nitrate and phosphate in the water, a process called eutrophication.
  • Eutrophication encourages the growth of algae. These form a green bloom over the water surface, preventing sunlight reaching other water plants.
  • These plants die because they are unable to carry out photosynthesis.
  • Bacteria decompose the dead plants, respiring and using up the oxygen in the water as they do this.
  • The low oxygen levels make it difficult for aquatic insects and fish to live, and eventually the lake may be left completely lifeless.

The degree of pollution of river water is often measured by its biochemical oxygen demand (BOD).

This is the amount of oxygen used up by a sample of water in a fixed period of time.

The higher the BOD, the more polluted the water is likely to be.

It is possible to reduce eutrophication by using:

  • Detergents with less phosphates.
  • Agricultural fertilisers that do not dissolve so easily.
  • Animal wastes on the land instead of letting them reach rivers.

Plastics and the environment:

  • Plastics that are non-biodegradable are not broken down by decomposers when dumped in landfill sites or left as litter.
  • This means that they remain in the environment, taking up valuable space or causing visual pollution.
  • Discarded plastic bottles can trap small animals; nylon fishing lines and nets can trap birds and mammals such as seals and dolphins.
  • As the plastic in water gradually deteriorate, they fragment into tiny pieces, which are eaten by fish and birds, making them ill.
  • When plastic is burned, it can release toxic gases.

The greenhouse effect and global warming:

  • Carbon dioxide is produced by burning of fossil fuels.
  • Methane is produced from the decay of organic matter and as a waste gas from digestive processes in cattle.
  • Carbon dioxide and methane are greenhouse gases.
  • They are called greenhouse gases as they trap heat in the earth’s atmosphere in the same way a greenhouse traps heat.
  • As the concentration of these gases increase in the atmosphere more heat is trapped, making the atmosphere warmer. This is called enhanced greenhouse effect.
  • It is causing global warming –Earth’s average temperature is rising.

Global warming is causing the following problems:

  • Melt polar ice caps, causing flooding of low-lying land;
  • Change weather conditions in some countries by increasing flooding or reducing rainfall;
  • Cause the extinction of some species that cannot survive at higher temperatures.

Causes and effects on the environment of acid rain

The ‘greenhouse effect’ and global warming:

  • Increase in carbon dioxide and methane concentrations in the atmosphere have caused an enhanced greenhouse effect.
  • With emissions being produced daily, a large imbalance is being created which is enhancing the greenhouse effect and making it stronger.
  • As there are naturally occurring greenhouse gases in the atmosphere that help keep the Earth warm, additional amounts of these gases leads to more heat being trapped on the planet.
  • This extra heat is causing global warming as well as affecting the Earth’s weather patterns.

Pollution by contraceptive hormones:

  • When women use the contraceptive pill, the hormones in it (oestrogen or progesterone) are excreted in urine and become present in sewage.
  • The process of sewage treatment does not extract the hormones, so they end up in water systems such as rivers, lakes and the sea.
  • Their presence in this water affects aquatic organisms as they enter food chain.
  • male frogs and fish can become ‘feminised’ (they can start producing eggs in their testes instead of sperm).
  • This causes an imbalance between numbers of male and female animals.
  • Drinking water, extracted from rivers where water from treated sewage has been recycled, can also contain the hormones.
  • This has been shown to reduce the sperm count in men, causing a reduction in fertility.

Conservation

Sustainable resource: is one that is produced as rapidly as it is removed from the environment so that it does not run out.

Sustainable development: is development providing for the needs of an increasing human population without harming the environment.

Non-renewable resources such as fossil fuels need to be conserved because the stocks of them on the planet are finite.

It can be conserved in the following ways:

  • By increasing the use of renewable energy (wind energy, solar energy, hydroelectric power).
  • By improving the efficiency of energy use (better insulation, smaller car engines, more public transport).

Sustaining forest and fish stocks:

  • Some resources, such as forests and fish stocks can be maintained with careful management.
  • This may involve replant land with new seedlings as mature trees are felled and controlling the activities of fishermen operating where fish stocks are being depleted.
  • There are three main ways of sustaining the numbers of key species. These are:

1) Education

  • Local communities need to be educated about the need for conservation. One they understand its importance, the environment they live in is more likely to be cared for and the species in it protected.

2) Legal quotas

  • In Europe the Common Fisheries Policy is used to set quotas for fishing, to manage fish stocks and help protect species that were becoming endangered through overfishing.
  • Quotas were set for each species of fish taken commercially and also for the size f fish. This was to allow fish to reach breeding age and maintain or increase their populations.

3) Restocking

  • Where populations of a fish species are in decline, their numbers may be conserved by a restocking programme.
  • This involves breeding fish in captivity, then releasing them into the wild.
  • However, the reasons for the decline in numbers need to be identified first.
  • if pollution was the cause of the decline, the restocked fish will die as well, issue of pollution needs to be addressed first.

Recycling:

  • Products such as paper, glass, plastic and metal can be reused or recycled.

Sewage treatment:

  • Microorganisms, mainly bacteria and protoctista, play an essential part in the treatment of sewage to make it harmless.
  • Sewage contains bacteria from the human intestine that can be harmful.
  • These bacteria must be destroyed in order to prevent the spread of intestinal diseases.
  • Sewage also contain substances such as soap and detergent from household wastes and chemicals from factories. These too must be removed before the sewage effluent is released into the rivers.
  • Inland towns have to make their sewage harmless in a sewage treatment plant before discharging the effluent into rivers.
  • A sewage works removes solid and liquid waste from the sewage, so that the water leaving the works is safe to drink.
  • In a large town, the main method of sewage treatment is by the activated sludge process.

Sustainable development:

  • This is a complex process, requiring the management of conflicting demands. As the world’s population grows, so does the demand for the extraction of resources from the environment.
  • However, this needs to be carried out in a controlled way to prevent environmental damage and strategies need to be put in place to ensure habitats and species diversity are not threatened.
  • Planning the removal of resources need to be done at local, national and international levels.
  • This is to make sure that everyone involved with the process is aware of the potential consequences of the process on the environment, and that appropriate strategies are put in place, and adhered to, to minimise any risk.

Endangering species and causing their extinction:

  • Anything that reduces the population of a species endangers it (puts it at risk of extinction).

Climate change:

  • This is a natural, uncontrollable process, but processes like global warming are made worse by human activity.

Habitat destruction:

  • Can be caused by a number of things – pollution biggest factor, fishing activity and dredging ships.
  • Lead to destruction of habitats leaving species homeless.

Pollution:

  • Global warming caused by pollution leading to rapid changes in climate in certain.
  • As a result the conditions will change, causing the environment to change and the species being no longer suited to it and struggle to survive.
  • Eg polar bear – arctic ice melting – cannot swim very well.

Introduced species:

  • Some species of animal are not introduced deliberately into different ecosystem, but find they way in due to man’s activities and then upset food chains.

Hunting:

  • Extreme hunting species of animals can cause extinction.

Conservation programmes:

  • If the population of a species drop, the range of variation within the species drops, making it less able to adapt to environmental change.
  • The species could, therefore, be threatened with extinction.
  • When animal populations fall, there is less chance of individuals finding each other to mate.

Habitats can be conserved in a number of ways:

  • Using laws to protect the habitat.
  • Using wardens to protect the habitat.
  • Reducing or controlling public access to the habitat.
  • Controlling factors, such as water drainage and grazing, that may otherwise contribute to destruction of the habitat.
  • Monitoring and protecting species and habitats.
  • Captive breeding and reintroductions – possible to boost a species numbers by breeding in captivity and releasing the animals back into the environment.
  • Seed banks – way of protecting plant species from extinction. They include seed from food crops and rare species. They act as gene banks.

Reasons for conservation programmes (CP – not pokemongo related, word just too long cbf writing it):

Reducing extinction:

  • CP strive to prevent extinction. Once a species become extinct its genes are lost forever.
  • So we are also likely to deprive the world of genetic resources.
  • The chances are that we will deprive ourselves not only of the beauty and diversity of species but also of potential sources of valuable products such as drugs.

Protecting vulnerable environments:

  • CP are often set up to protect threatened habitats so that rare species living there are not endangered.

Maintaining ecosystem functions:

  • There is a danger of destabilising food chains of a single species in that food chain is removed.
  • Crops are grown for food, extraction of drugs and the manufacture of fuel.
  • Crop growth has major impacts in ecosystems, causing the extinction of many species and reducing the gene pool.

2 thoughts on “IGCSE BIOLOGY TOPIC 21 HUMAN INFLUENCE ON ECOSYSTEM”

    1. There are many adaptations, but first know that mesophytes live in areas with moderate conditions and are neither adapted to particularly dry nor particularly wet environments. They include:-
      1. Broad / wide / flat lamina; to provide large S.A; for absorption of CO2/sunlight;

      2. Thin leaves; to ensure short distance for CO2 to reach photosynthetic cells/palisade/mesophyll;

      3. Guard cells/ presence of stomata; for efficient diffusion of CO2 into leaf/ oxygen out of leaf/gaseous exchange/ water vapour/ transpiration.

      4. Transparent cuticle/epidermal cells; for penetration of light; into palisade cells/ photosynthetic
      cells.

      5. Palisade cells contain many chloroplasts; to trap light (for photosynthesis);

      6. Chloroplasts have chlorophyll; for trapping light energy;

      7. Leaves have veins; Xylem to conduct water to photosynthetic cells; and phloem to translocate
      the photosynthates to other parts of the plant;

      8. Air spaces in spongy mesophyll; for gases to circulate/diffuse easily/ for CO2 to diffuse into
      palisade cells;

      8. Mosaic arrangement of leaves; enables all leaves to trap sunlight;

      9. Palisade cells are next to upper epidermis for maximum light absorption (by chloroplasts)
      10. They have fibrous root systems to absorb water.

      Note that when answering this question you compare them with xerophytes and hydrophytes

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