Unit 3
Topic 2
Energy in ecosystems
Pupil Notes
Energy Flow in an Ecosystem
Producers and consumers
Green plants trap light energy and convert it to chemical energy in glucose. Since green
plants can produce their own food they are called producers. Animals, which have to feed on
other organisms are called consumers.
Word
Definition
Producer
An organism, such as a green plant, which can produce its own food
Consumer
An organism, like an animal, which gets energy by eating other
organisms
Herbivore
An animal that only eats plants
Carnivore
An animal that eats other animals
Omnivore
An animal that eats plants and animals
Food chain
A food chain shows the direction of energy flow between organisms. It shows the
relationship between consumers and the organisms they consume.
Energy is used for movement, growth and repair of tissue, chemical reactions, keeping
warm.
When an animal eats a plant, energy is transferred from the plant to the animal (the primary
consumer).
When the primary consumer is eaten by a second animal (the secondary consumer) energy
is again transferred to that animal and so on.
This diagram shows a food chain, the arrows indicate the direction of energy flow or transfer
between the organisms in the food chain.
Grass
Rabbit
Fox
Food webs
An ecosystem contains many interconnecting food chains.
A food web is a diagram that shows these interconnected food chains. An example of a food
web is shown below
Fox
Skylark
beetle
Grouse
Moth
Producers
Primary
consumers
Secondary
consumers
Heather
Grouse
Grouse
Moss
Beetle
Fox
Moth
Skylark
Moss
Heather
All food webs are delicately balanced.
A change in the number of one type of organism can have an effect on the number of every
other plant or animal in the food web. When the number of a particular organism in a food
web is reduced:
The numbers of its predators may decrease because their food source has decreased
The numbers of its prey may increase because they have fewer predators feeding on them
The numbers of its competitors (animals eating the same food) may increase because there
is now more food available to them
The numbers of animals having the same predators may decrease because the predators eat
more of them.
Transfer of energy
The transfer of energy from one level of a food chain to the next is not efficient, only about
10% is transferred to the next level, 90% is lost, e.g.
Grass
Rabbit
Fox
When the rabbit eats the grass some of the energy in the grass will be lost as undigested
materials in its faeces.
The rabbit uses the energy in the grass for RESPIRATION which allows it to heat its body
and move.
Once this energy is used up it is no longer available to the fox in the next stage
At each level in a food chain 90% of the energy is lost as
•
Heat
•
Movement
•
Undigested materials
Grouse
Pyramids of Numbers / Biomass / Energy
These pyramids are diagrams displaying the change in numbers, biomass or energy at
different levels along a food chain.
1. Pyramids of Numbers
Indicates the numbers of organisms at each level
There are three types of pyramid:-
Secondary consumers
Primary consumers
Decreasing
numbers
Producers
This ‘pyramid shape’ is very common when counting numbers as it is usual for there to
be a larger number of smaller living things ( e.g. Rabbits ) providing the food for smaller
numbers of larger living things ( e.g. Foxes )
This pyramid shape is not the only shape possible
b) Inverted (upside down ) pyramid, e.g. when the food chain involves parasites
parasites
aphids
Producer, e.g. rose bush
c) Irregular shape e.g. Food chain based on large tree
Secondary consumers, e.g. birds
Primary consumers, e.g.
insects
Producer, e.g. tree
Biomass
The biomass of a population is the total mass of the living things at that stage. This is
much harder to measure than counting numbers but gives a much better measure of the
loss of energy in a food chain
2. Pyramids of Biomass
A pyramid of biomass shows that the biomass of organisms decreases at each level of the
food chain:
Biomass of secondary consumers
Biomass of primary consumers
Biomass of producers
Pyramids of biomass are usually pyramid shaped because each level gets all of its biomass
from the level below, so the lower level cannot have a smaller biomass.
3. Pyramids of Energy
The total available energy at each level of a food chain is always pyramid shaped.
Since there is loss of energy at each stage, higher levels always have less available energy
than lower levels.
Decreasing
energy at each
level
Available energy from secondary consumers
Available energy from primary consumers
Available energy from producers
Competition in Ecosystems
In ecosystems, competition occurs when two or more members of a community need the
same resource.
The size of any population is limited by one or more of the following factors
Plants compete for:•
Light
•
Water
•
Soil nutrients
•
Space etc.
Animals compete for:•
Food
•
Water
•
Shelter
•
Territory
•
Mate
There are 2 types of competition:Interspecific – between different species
Intraspecific – between members of the same species
The more the niches of two living things overlap the greater the competition for resources.
Different species may reduce competition by:
Selecting different prey

Feeding at different times

Nesting in different habitats
Two species with exactly the same niche will result in the migration or extinction of one
species. This is called the competitive exclusion principle.
An example of two species with almost exactly the same niche are the grey and red
squirrels.
The grey squirrel is larger, uses wider variety of foods and is more aggressive.
This means that in areas with grey squirrels the red squirrel population is pushed out or
dies away.
Another example of two species with very large overlaps in niche are the rainbow
and brown trout.
Rainbow trout ousts brown trout. Rainbow trout are more aggressive and greedy for food
Members of the same species have ways of reducing competition and ensuring that the
best adapted survive to pass on their genes to the next generation. For example in birds
such as robins and red grouse, males set up territories from which other males are
excluded so that only the male owning the territory is allowed to hunt for food in it. The
territory is usually defended by behavioural displays rather than fighting so energy is
conserved and injury avoided.
Nitrogen Cycle
Nitrogen is needed by plants to make protein.
Plants usually get their nitrogen by absorbing nitrates through their roots.
Animals get protein by eating plants or other animals that have eaten plants.
Nitrogen compounds (like proteins) in the dead bodies of plants and animals and in animal
wastes are broken down by decomposers such as bacteria and fungi to produce ammonium
compounds
Nitrifying bacteria change the ammonium compounds into nitrites then into nitrates.
Nitrogen fixing bacteria convert nitrogen gas in the air into nitrate. Some of these bacteria
are found in the soil, other types live inside swellings called root nodules in the roots of a
type of plant called legumes. Examples of legumes are beans and clover.
Denitrifying bacteria in waterlogged soil change soil nitrate into nitrogen gas
Stage in nitrogen cycle
Nitrogen fixation
Denitrification
Decomposition
Nitrification
Organisms involved
Nitrogen fixing bacteria
Denitrifying bacteria
Decomposers, e.g. bacteria
and fungi
Nitrifying bacteria
Reactions
Nitrogen
nitrate
Nitrate
nitrogen
Protein
ammoniu
Ammonium
Nitrite
Fertilisers
Fertilisers are added to the soil to supply nitrates and increase crop yields
Nitrite
Nitrate