All organisms live in an environment and that environment affects how the
organism survives. The environment and the organisms that live within it form
a natural unit called an ecosystem.
Ecosystems
Ecosystems are complex but they can be simplified by dividing them up into a
number of different components (parts). The diagram below shows a woodland
ecosystem:
leaf surface
insect eating bird
tree surface
greenfly
ladybird
fungi
woodland soil
This ecosystem can be divided up into the follow components:
Habitat - the place where an organism lives e.g. woodland soil, tree surface, leaf
surface
Population - the total number of living organisms of ONE type (species) living in
a habitat e.g. fungi
Community - ALL of the living organisms living in all types of habitat e.g. fungi,
ladybirds, insect-eating birds, greenfly and trees
When all of these components are added together they form an ecosystem:
An ecosystem is a natural unit made up of one or more habitats and the
community (or communities) of organisms living there. An ecosystem is made up
of living (biotic) and non living (abiotic) components. e.g. fungi (living) and
woodland soil (non living).
Food chains and food webs
All organisms need energy to survive. Food chains and webs show where
organisms get energy from and how energy can be passed from one organism to
another. The table below shows a number of terms that apply to food chains
and webs:
Term
Meaning
Producer
Organism that can make its own food. This is done by converting
the suns light energy into chemical energy using photosynthesis
Primary consumer
Organism that eats producers
Secondary consumer
Organism that eats a primary consumer
Herbivore
Organism that eats plants
Carnivore
Organism that eats other animals
Predator
Organism that hunts and kills other animals for food
Prey
Organism that is hunted and killed
Decomposers
Bacteria and fungi that break down dead plants and animals
Food chains
 A food chain is a feeding relationship which shows the direction that energy
flows through a series of organisms.
 Food chains always start with a producer (usually green plants) that converts
the suns light energy into chemical energy by the process of photosynthesis.
 This chemical energy is passed onto animals that eat the plants. Energy is
also passed from these animals to other animals that eat them. Thus energy
is passed from one organism to the next.
e.g. food chain for an oak tree community
Oak tree
Greenfly
 The arrows(producer)
in a food chain show(primary
two things:
Ladybird
Small bird
(secondary
(tertiary
consumer)
consumer)
consumer)
1. they point from a plant or animal which is eaten to the animal which
eats it
2. they show the direction of energy flow in the food chain
Food webs
A food web is a diagram which shows how all the food chains in a community are
linked together e.g. woodland community
fox
weasel
owl
hedgehog
frog
vole
rabbit
snail
oak
primrose
What happens if all the foxes are removed?
1. There would be an increase in the numbers of frogs, hedgehogs and
weasels as they would not be being eaten by foxes.
2. This would lead to a decrease in the number of snails and voles.
3. This would give an increase in the number of primrose.
4. The rabbit population would remain the same because it would
 increase due to lack of foxes
 decrease due to increase in number of weasels.
Decomposers
Bacteria and fungi are known as decomposers because they feed on dead
plant and animal material, breaking it down. This releases nutrient chemicals,
such as nitrogen compounds, back into the soil and ensures that nutrients
aren’t lost from ecosystems.
Energy flow in food chains
Food chains can show the paths which energy flows along but they don’t show
how much energy there is at each stage of the food chain.
Pyramids of energy
A pyramid of energy is a diagram showing how much energy there is at each
stage of a food chain e.g.
grass
rabbit
fox
fox
rabbit
grass
length of bar represents the quantity of energy
The above diagram shows that as you move along a food chain there is less
energy at each stage - the grass has more energy than the rabbits and the
rabbits have more energy than the fox.
This is because energy is lost from the food chain when an organism:
Uses it for MOVEMENT
Uses it for HEAT
Converts it into WASTE MATERIAL (FAECES)
Only energy that an organism uses for growth can be passed on to the next
organism in the food chain.
Pyramid of numbers
The quantity of energy available affects the number of organisms and this is
shown using a pyramid of numbers diagram.
A pyramid of numbers is a diagram showing how much energy there is at
each stage of a food chain e.g.
fox
rabbit
grass
length of bar represents the number of organisms
The diagram shows that as you move along a food chain there are less organisms
at each stage – there are more grass plants than rabbits and there are more
rabbits then foxes.
This is because as the quantity of energy decreases the number of organisms
that can be sustained also decreases.
Pyramid of biomass
Biomass takes into account both the size of an organism and the number of
them.
A pyramid of biomass is a diagram showing the total mass of organisms at
each stage of a food chain e.g.
fox
rabbit
grass
length of bar represents the total mass of organisms
The diagram shows that as you move along a food chain the total mass of
organisms at each stage decreases – the grass plants have a greater mass than
rabbits and the rabbits have a greater mass than the foxes.
This is because there as you go up the food chain there is less energy available
for growth and therefore the total mass of organisms is greatest at the bottom
of the food chain.
Nutrient Cycles
Bacteria and fungi are known as decomposers because they feed on dead
plant and animal material breaking it down releasing chemicals such as
nitrogen compounds back into the soil. This ensures that nutrients aren’t lost
from ecosystems.
All living organisms use nitrogen to make protein. Plants get this
nitrogen from the soil when they take in nitrates. Animals take in
nitrogen when they eat plants and other animals. Nitrogen would be used up
very quickly if there were not some way in which it could be returned to the
soil when a plant or animal dies.
1 - Nitrogen in plants in the form of protein is eaten by animals.
2 - Plants or animals die or leave waste products on the ground.
3 - Bacteria begin breaking down the waste or dead organisms into a compound
called ammonia. This is called decomposition.
4. Nitrifying bacteria convert the ammonia into a compound called Nitrite.
5. A different group of Nitrifying bacteria convert the Nitrite into more useful
Nitrate
6. The Nitrate is useable by plants so they absorb it from the soil by their
roots. This is called absorption
7. There are some denitrifying bacteria who convert Nitrates into Nitrogen gas
which enters the atmosphere this process is called Denitrification.
8. Certain plants called Legumes (e.g. pea plants) have special bacteria in their
root nodules which can take Nitrogen from the air and turn it back into nitrates
this is called Nitrogen Fixation.
9. Nitrogen fixation (Nitrogen gas into Nitrates) also occurs as a result of
lightning.
Competition occurs when two organisms require the same resource.
Competition often occurs when a resource is in short supply.
Plants compete mainly for light, water and soil nutrients
Animals compete mainly for food, water and shelter
Competition between members of the same species (intraspecific competition)
can control population size, but competition between two species (interspecific
competition) can lead to one of them dying out.
Competition between cress seedlings
Aim
The aim of this experiment is to show the effects of competition on
germination of cress seedlings.
Method
1. Set up two pots as shown below:
Pot A
5 cress
seeds
50 cress
seeds
Enough cotton wool to cover
the bottom of the pot and
20 mls of water
2. Place both pots in an oven at 30oC for 2-3 days
Pot B
3. Remove pots and count how many of the seeds have germinated
Results
Pot
Number of germinated
cress seeds
% of seeds
germinated
A
B
It was necessary to calculate the percentage of seeds germinated
because__________
_______________________________________________________
Conclusion
The percentage of seeds that germinated was higher in pot ____ than pot
_____. This occurred because there was greater competition between the
seeds in pot _____ for resources like _________.