National 4 Unit 3
Life on Earth
Revision Notes
Key Area 1 – Animal and plants species depend on each
other
Food webs are made up of lots of food chains.
Plants  Rabbits  Foxes is an example of a food chain from the
food web above. The foxes eat the rabbits and the rabbits eat the
plants.
If an organism is wiped out of the food web (for example through
disease) then the organisms around it might increase, decrease or
stay the same in number.
For example, if the population of rabbits were to be wiped out then
the fox’s numbers might decrease because there would be less food
for them.
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Key Area 2 – Impact of population growth and natural
hazards on biodiversity
Biodiversity means the variety of life.
Habitat destruction
Habitat destruction is the process in which natural habitat is
rendered functionally unable to support the species present.
Deforestation
Deforestation describes the removal of vast areas of natural
forest for the benefit of humans. This can result in habitat
destruction, a reduction in soil fertility and poor soil structure
leading to a decrease in biodiversity.
Over-fishing
Overfishing is a form of overexploitation in which fish stocks are
depleted to unacceptable levels, regardless of water body size.
Intensive agriculture
Intensive agriculture occurs when farms produce large qualities of
(either animal or plant) food but to do this they have to spray large
volumes of pesticides and fertilisers which can pollute the local area
(especially nearby rivers).
Genetic pollution
Genetic pollution is the term used when genes from the genetically
engineered organisms mix with natural organisms.
Oil spills
An oil spill is the release of oil into the environment, especially
marine areas, due to human activity, and is a form of pollution.
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Key Area 3 – Nitrogen cycle
Nitrogen is essential for all organisms to make proteins.
Nitrogen has to be cycled around the ecosystem or it would run
out. Bacteria are involved in converting the nitrogen found in dead
plants and animals and converting it into nitrates.
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Key Area 4 – Fertiliser design and environmental impact
of fertilisers
Nitrogen can be added to the soil in the form of nitrate fertilisers,
manure or compost. When crops are harvested, nitrogen is taken out
of the cycle so needs to be replaced. Some farmers also add clover
plants as they have special roots that can covert nitrates into
nitrogen.
Nitrogen, phosphorus and potassium help plants grow and because of
this are very important parts of fertilisers.
Nitrogen in fresh water increases algal growth, blocking out the
light. This causes death of organisms, decrease of oxygen and means
less life can be supported.
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Key Area 5 – Adaptations for survival
Adaptations can be structural, physiological or behavioural and help
organisms survive and reproduce in their environment:
1. Behavioural - responses made by an organism that help it to
survive/reproduce
2. Physiological - a body process that helps an organism to
survive/reproduce
3. Structural - a feature of an organism’s body that helps it to
survive/reproduce
The Emperor Penguin lives in the Antarctic environment, which is
very cold. The ground is covered in ice and snow and food is only
available in the sea.
Behavioural adaptations
Adult male penguins huddle together during winter. This helps them
to avoid heat loss and survive until spring.
They breed during the winter so that their offspring reach
independence in summer when more food is available.
Physiological adaptations
Males can go without food for around 100 days. This allows them to
survive during winter when no food is available.
A penguin’s resting heart rate can slow down greatly during a dive so
that it can conserve oxygen underwater and spend more time finding
food.
Structural adaptations
Penguins have a short stiff tail. They can lean backwards and balance
on their heels and their tail. This reduces heat loss from their feet
to the ground.
The colour of a penguin’s body helps to camouflage it when it is
swimming. From above, its dark back blends in against the sea and
from below, its light front blends in against the sky.
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Key Area 6 – Learned behaviour in response to stimuli
linked to species survival
Communication
Bee-keepers have long puzzled over the mysterious little
performance, which bees stage for their hive-mates when they
return home from a foraging mission.
On entering the hive after gathering nectar, a bee will run around in
a tight figure of eight dance, waggling its abdomen as it does so. All
the other bees gather around, apparently scrutinising the ceremonial
manoeuvre.
The direction the bees point while performing the dance indicates
the direction of the food source in relation to the Sun; while the
intensity of the waggles indicate how far away it is.
E.g. Bees do a waggle dance to show others where food is.
Imprinting
Newly hatched birds of some species learn to recognise their
mother and form an attachment to her. They will then follow her
around and gain food and protection. This learned behaviour is called
imprinting.
E.g. Ducklings will follow the first creature they see after hatching
(they see that organism as mum).
Migration
Migration is the usually seasonal movement of animals in pursuit of
food, suitable breeding sites or to escape bad weather or other
environmental conditions. Mass migrations - such as the wildebeest
crossing the Mara River, or Pacific salmon heading upstream to mate
- create some of the world's greatest wildlife spectacles.
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E.g. Swallows fly to Africa in winter due to shorter daylight hours,
colder temperatures and less food.
Swarming
Swarming happens when animals gather or travel together in large
numbers. Its most familiar examples are in insects, such as locusts
and midges, flocking birds and shoaling fish. Some animals swarm as
a defence against predation, others, such as locusts and bees, only
form swarms in specific circumstances. Swarming can be carried out
by the smallest and simplest micro-organisms, such as bacteria, and
even by humans.
E.g. Locusts move in large numbers.
Associative learning
The monarch butterfly has a bright orange colour and an unpleasant
taste to birds. As a result of feeding experiences, birds avoid eating
orange butterflies as they have learned to associate their colour
with the unpleasant taste.
Habituation
Habituation allows animals to avoid wasting energy by responding to
repetitive harmless stimuli.
A snail will withdraw its body when its shell is tapped. This
withdrawal response is an innate behaviour that helps it to avoid
predation. If the snail’s shell is tapped repeatedly, but the snail is
not attacked by a predator, then the snail stops making the
withdrawal response.
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