Student Notes
Williamwood High School
National 5 Biology
Unit 3 – Life On Earth
Section 3.1 Biodiversity and the Distribution of Life
Section 3.2 Energy in the Ecosystem
Section 3.3 Sampling Techniques
Section 3.4 Adaptation, Natural Selection and the Evolution
Section 3.5 Human Impact
Student Notes___________________
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Student Notes
Learning Outcomes:

At each level in a food chain 90% of energy is lost as heat, movement or undigested materials.

Definitions and comparison of pyramids of biomass, energy and numbers.

Nitrogen in ecosystems:
- Animal and plant proteins are produced from nitrates.
- The roles of nitrifying, denitrifying, root nodule and free-fixing soil bacteria.
- Decomposers convert proteins and nitrogenous wastes to ammonium and nitrate.

Competition in ecosystems:
- Interspecific competition is when individuals of different species compete for the same
resource in an ecosystem.
- Intraspecific competition is when individuals of the same species compete for exactly the
same resources.
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Student Notes
Food Chains
A food chain shows what eats what in a habitat.
Food chains show the flow of energy from a producer (plant that makes it’s own food from the sun’s
energy) through the primary consumer (a herbivore) to the secondary consumer (a carnivore).
The example shows grass seeds eaten by a vole, which is eaten by a barn owl.
The grass seed is the producer, the vole is the primary consumer and the owl is the secondary
consumer.
Food Chain Arrows
The arrows between each item in the chain always point in the direction of
energy flow - in other words, from the food to the feeder.
The energy in a food chain originates with the sun. The producer converts the light energy into
chemical energy through the process of photosynthesis, allowing it to be transferred between the
individuals in the food chain.
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Student Notes
Arrows show the flow of energy!
Loss of Energy
Energy is transferred along food chains from one stage to the next. But not all of the energy available
to organisms at one stage can be absorbed by organisms at the next one.
The amount of available energy decreases from one stage to the next, roughly 10% passed on, 90% is
lost!
Reasons for Loss of Energy
Only 10% of energy is actually passed on at each stage in a food chain, so what happens to the rest of
the energy? Energy can be lost as heat, movement or undigested material. Only 10% of the energy
available is used for growth. In plants some of the light hitting the leaf will be transmitted or reflected
and some energy is lost as heat from both photosynthesis and respiration reactions.
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Student Notes
heat
movement
Energy loss from
a food chain…
Indigestible material
Energy Transfer
In the example food chain 10,000 kj of energy from the sun is available but only 1000 kj of energy is
absorbed by the plant. The plant uses energy to grow but also to carry out cell reactions and to repair
tissues. Some of the plant material is unable to be digested by the vole so only 100 kj of energy per
1000 kj of eaten material is used by the vole to add to its mass. The vole runs around and gives out
heat so only 10 kj of energy from the vole can be used by the owl.
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Student Notes
1000kj
100kj Passed on
10kj Passed on
900kj Lost
90kj Lost
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Student Notes
Food Webs
Food chains are largely unrealistic. Producers may be eaten by many consumers, and consumers eat
many different types of food. Some secondary consumers are not carnivores but are omnivores,
eating both plant and animal material. People are omnivores.
Most food chains can be linked together in a complex way to form a food web.
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Student Notes
Removal of species
When organisms are removed from a food web the delicate balance is often disturbed and
populations of other species may rise or fall as a consequence.
Food Webs – Removing an Organism
What effect would removing the rabbit population have on the fox and slug population?
Fox: The fox numbers decrease as the fox no longer has the
rabbit as a food source and must now depend of the frog and
vole as its food source.
Slug: The slug population will increase as more
grass is available without competition from the
rabbits.
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Student Notes
Food Chains/Webs – Key Terms
Word
Meaning
Producers
Green plants - they make food by photosynthesis.
Primary
consumers
Usually eat plant material - they are herbivores. For example rabbits,
caterpillars, cows and sheep.
Secondary
consumers
Usually eat animal material - they are carnivores. For example cats,
dogs and lions.
Predators
Kill for food. They are either secondary or tertiary consumers
Prey
The animals that predators feed on.
Scavengers
Feed on dead animals. For example, crows, vultures and hyenas are
scavengers.
Decomposers
Feed on dead and decaying organisms, and on the undigested parts of
plant and animal matter in faeces.
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Student Notes
Pyramids
A food web shows relationships between organisms in a food chain or web but not
how many organisms are involved.
Sometimes a food chain can be represented using a pyramid diagram.
There are 3 types of these: Pyramid of Numbers, Biomass and Energy.
Pyramid of Numbers
The producers are always at the base of the pyramid. The top carnivore is the tip of the pyramid. The
pyramid shape is due to the decrease in the numbers at each link in the food chain. The bigger the
bar, the more individuals are represented.
This pyramid shows the number of organisms at each stage in a food chain.
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Student Notes
Pyramid of Biomass
Pyramids of numbers are often unrepresentative of real life,
imagine one large oak tree or when the top predator is a
parasite. A pyramid of the biomass of organisms at each
stage in a food chain becomes more accurate and can over
come the anomalies of parasites or single trees.
Biomass can be thought of as the weight of living material.
The most reliable measurement would of course be of dry
mass and as organisms would have to be killed to calculate
this it is not often used.
Pyramid of Energy
The most reliable pyramid is a
pyramid showing the energy
contained within organisms at each
stage in a food chain. Pyramids of
energy are very accurate but very
difficult to calculate as a sample
organism must be burned in a
calorimeter and the energy
generated multiplied by the
number of individuals in the
population.
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Student Notes
Irregularity in Pyramids
Sometimes a pyramid of numbers does not look like a
pyramid at all as it does not take into account the size of the
organisms.
A more accurate idea of the quantity of animal and plant
material in a food chain is obtained by constructing a
pyramid of biomass.
This represents the mass of all organisms at each level and
gives a much better representation of the actual quantity
of animal and plant materials at each level.
Pyramids of energy are even more accurate although
difficult to obtain.
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Student Notes
Investigating a Scottish loch the biomass of all the organisms was measured. It was
found that there was 1.5 g/m² of brown trout. Brown trout eat dragonfly nymphs.
Trout biomass was 10 times less than the biomass of the dragonfly nymphs.
Dragonfly nymphs eat mayfly nymphs. The mayfly nymphs biomass was 22 times
less than the biomass of the algae, which had a biomass of 814 g/m² and is the
producer in this food chain.
The Nitrogen Cycle
Proteins are essential in cell structure and function in both plants and animals.
Proteins are used by organisms to build cell membranes, make enzymes, hormones, antibodies and
hair fibres.
Nitrogen is essential for the formation of amino acids that are then linked to form proteins. Essential
elements such as nitrogen are finite on earth and would run out if they could not be recycled.
The nitrogen cycle is a model that explains how nitrogen is recycled.
There's lot of nitrogen in air – about 78% of the air is nitrogen.
Because nitrogen is so unreactive, it cannot be used directly by plants to make protein.
Only nitrates are useful
to plants, so we
are dependent on other
processes to
convert nitrogen to
nitrates in the
soil. This process is called
the nitrogen
cycle .
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Student Notes
Nitrogen cycle – stage 1
Nitrogen gas is converted into nitrates by nitrogen-fixing bacteria in soil or root nodules.
Lightning also converts nitrogen gas to nitrates.
Nitrogen Gas is converted into ammonia which is then converted to nitrates
by nitrifying bacteria in the soil.
Nitrogen cycle – stage 2
Plants absorb nitrates from the soil and use these to build up proteins. The plant
may be eaten by an animal and used to produce animal protein.
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Student Notes
Nitrogen cycle – stage 3
Nitrogenous waste (faeces) is broken down by decomposers. This results in nitrogen being returned
to the soil as ammonia. Nitrifying bacteria then converts ammonia into nitrites and then into
nitrates.
Nitrogen cycle – stage 4
Decomposers also break down the bodies of dead organisms resulting in nitrogen
being returned to the soil as ammonia. Nitrifying bacteria then converts ammonia
into nitrites and then into nitrates.
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Student Notes
Nitrogen cycle – stage 5
Denitrifying bacteria in the soil break down nitrates and return nitrogen to the
air.
Nitrogen Cycle – Role of Bacteria
The nitrogen cycle is made possible by the action of decomposers such as earthworms, woodlice,
fungi and bacteria. Decomposers breakdown organic material into tiny particles, thus allowing the
action of bacteria to break down compounds into even smaller molecules.
Nitrifying bacteria — converts ammonium compounds into nitrites and then into nitrates.
Root nodule bacteria / Nitrogen fixing bacteria — fix nitrogen in the air into
nitrates.
Some plants house bacteria in root nodules. The bacteria change nitrogen from
the air into nitrates, which the plant then uses.
Denitrifying bacteria — converts nitrates in soil into nitrogen gas in the air.
Decomposers — Coverts protein and waste into nitrates and ammonia.
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Student Notes
Nitrogen Cycle - Summary
Nitrogen gas is converted into nitrates by nitrogen-fixing
bacteria in soil or root nodules.
Plants use nitrates to make proteins which may be eaten by
animals.
S
p
a
arec
e
Animal bodies or waste
broken down by decomposers to
produce ammonia. Nitrifying Bacteria convert the ammonia into
nitrites then into nitrates. a
n
d
s
h
Denitrifying bacteria breake down
l
which is returned to the air.
t
e
r
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nitrates into nitrogen gas
Student Notes
Competition
Habitats have limited amounts of the resources needed
by living organisms.
Organisms must compete with others in order to get
enough of these resources to survive. If they are
unsuccessful and cannot move to another habitat, they
will die.
Competition - Animals
Animals may have to compete for…
Food
Water
Mates
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Student Notes
Competition - Plants
Plants may have to compete for…
Light
Minerals and nutrients
Space
Water
Plants do NOT compete for food as they make this through photosynthesis.
Competition - Graph
This graph shows as the number of days increased the population of both species increased however
due to competition for food, species Y has been unsuccessful so the population decreased. Species X
on the other hand has been successful in competition and gained the most food so its population
size has
increased.
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Student Notes
Interspecific Competition
Interspecific competition is when individuals of different species compete for the same resources in
an ecosystem.
Example: squirrels
The more aggressive grey squirrel has pushed out the red squirrel from its habitat.
Intraspecific Competition
Intraspecific competition is when individuals of the same species compete for the same resources in
an ecosystem.
Example: foxes
Several foxes competing for rabbits in an ecosystem.
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Student Notes
Success Criteria
I can use the correct terminology to describe the role of organisms in
food chains and food webs.
I can state the ways energy can be lost.
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I can describe what is represented by pyramids of numbers, biomass and
energy and explain any irregularity in shape.
I can explain why nitrogen is needed by organisms.
I can describe the process involved in the nitrogen cycle.
I can explain the role of microorganisms in the nitrogen cycle.
I can state what both animals and plants may be in competition for.
I can state that competition occurs when resources are in short supply.
I can state the difference between interspecific and intraspecific
competition.
I can state that competition may result in death or force an animal to
leave an ecosystem.
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