Unit 3: Life On Earth
Biodiversity and Human Impact
National 5
Unit 3
Topic 5 Human impact on the environment
Pupil Notes
Increasing Population
The increase in the human population means that more food is needed.
Human attempts to produce more food have been very successful but have also brought about
a wide range of problems.
These problems include:
•
The use of Fertilisers
•
The use of Pesticides
•
Deforestation
•
Overfishing
Fertilisers
Fertilisers are added to soil to increase plant growth and crop yield. Fertiliser can be washed
(leached) from the soil into waterways like lakes and rivers. The fertiliser causes a huge
increase in the growth of algae in the water – this is called an algal bloom and the process of
increased algal growth is called eutrophication.
The algal bloom on the surface blocks light from reaching other water plants causing their
death.
When the algae die they provide food for bacteria. With an increased food source the number
of bacteria in the water greatly increase and they use up oxygen. The oxygen concentration
falls to such low levels that animals in the water can no longer survive.
The flow chart summarises this process:
Fertiliser is leached into the water
Fertiliser causes increased growth
of algae (algal bloom)
Bacteria feed on dead algae and
use up oxygen in the water
Water animals such as fish die due
to lack of oxygen
Unit 3: Life On Earth
Biodiversity and Human Impact
National 5
Pesticides
Pesticides are chemicals sprayed onto crops to stop them being damaged by a variety of
pests such as caterpillars.
Pesticides build up over time in the bodies of animals which eat them.
Since animals higher in the food chain eat lots of animals lower in the food chain that have
already stored pesticides in their bodies, the pesticides become more concentrated along food
chains until they can reach fatal levels in the final consumers.
e.g. the diagram represents the accumulation of the now banned pesticide DDT in an aquatic
food chain
Birds
25
ppm
Increase of 10
million times
in DDT
concentration
along this
food chain
DDT in
large fish
2 ppm
DDT in small
fish 0.5 ppm
DDT in plankton 0.04 ppm
DDT in water 0.000003 ppm
(ppm = parts per million)
Birds at the top of the food chain were either killed or laid eggs that were not viable.
Unit 3: Life On Earth
Biodiversity and Human Impact
National 5
Indicator Species
Indicator species are organisms whose presence or absence indicates the level of pollution in
an area – for some organisms, e.g. bloodworms in water, the fact that they are present
indicates pollution while for others, e.g. lichens, the fact that they are absent from trees
indicates air pollution.
Rivers can be polluted with organic waste, e.g. sewage. Bacteria feed on this and use up
oxygen causing death of other organisms such as fish. Some of the invertebrate indicator
species whose presence indicates polluted or clean water are shown below:
Increasing
level of
pollution
Level of pollution
Indicator species present
Very low
Stonefly or mayfly
Low
Freshwater shrimp
Moderate
Water louse
High
Bloodworms
Very high
Rat-tailed maggot
Indicators of Air Pollution
Lichens are plants that grow in exposed places such as tree bark. Air pollutants like sulphur
dioxide from burning fossil fuels dissolves in rainwater and damages lichens preventing them
from growing.
This makes lichens natural indicators of air pollution.
In places where no lichens are growing, this is often a sign that the air is heavily polluted
with sulphur dioxide.
Unit 3: Life On Earth
Biodiversity and Human Impact
National 5
Biological control
Biological control of pests may be used as an alternative to controlling them by use of
chemical pesticides. In biological control, a natural predator or parasite of the pest is
introduced to the area affected by the pest. These will then kill the pest and reduce its
numbers. The table shows examples of biological control:
Pest organism
Rabbits
Aphids
Scale insects
Organism used in biological
control
Myxmatosis virus
Ladybirds
ladybirds
There are potential disadvantages to use of biological control where particularly when the
predator is not native to the area and has been introduced to control a pest. These include:
1. Lacking natural predators in the area, the introduced predator’s numbers may increase
uncontrollably so that it becomes a pest itself.
2. As well as killing the pest, the introduced predator may kill other harmless organisms
and decrease their numbers.
Genetically Modified (GM) Crops
These are crops that have had genes from other organisms introduced to them to improve
their characteristics. Genetically modified (transgenic) plants may

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be resistant to disease or drought
be resistant to herbicides and pesticides
grow faster
produce new drugs to order
have greater nutritional value – e.g. golden rice has been developed to contain
vitamin A and is produced in areas where vitamin A deficiency is common
Effect of GM crops and biological control on intensive farming
Growing large numbers of just one crop species in an area as happens in farming, requires the
use of fertilisers and pesticides. It is hoped that



biological control will make the need for pesticides
by producing crops that produce their own anti-pest chemicals, less pesticides will
need to be used
Crops may be produced that have less need of fertilisers because they grow faster or
are able to use nitrogen in the air.