Food chains and food webs involve the
transfer of energy from one organism to another.
The ultimate source of energy for most
ecosystems is the Sun.
Plants can use the energy in sunlight
to produce their own food. They are called producers.
Animals consume (eat) plants or other
animals for their food and energy.
They are called consumers.
A food chain always starts with a producer.
For example, grass (a producer) is eaten by rabbits (first
consumers), which in turn are eaten by a fox (second consumer).
Usually living things are part of more
than one simple food chain. These interlinked food chains
are called food webs.
3.
Pyramids
of number and biomass
The number of organisms at each step in
a food chain can be shown as a pyramid
of numbers. For example, the food chain:
is shown as a pyramid of numbers in the diagram below.
A more accurate description of the food
and energy transfer in a food chain can be shown by a pyramid
of biomass. This shows the total mass (biomass) of
organisms at each stage (trophic
level) of the food chain.
Pyramids of biomass, unlike pyramids
of numbers, are almost always neat pyramids with the biomass
decreasing from one trophic level to the next. This shows
that energy (food) is wasted at each step in a food chain.
This wasted energy is due to:
– respiration using up energy for movement and warmth;
– energy (heat) lost to the surroundings;
– energy and materials lost in urine and faeces.
Food production can be made more
efficient by:
– reducing the number of stages in food chains (e.g.
eating cereals, rather than meat from animals fed on cereals);
– restricting the energy loss from animals reared
for meat (e.g. limiting their movements).
4.
Recycling
Recycling happens normally in the environment. Waste products,
dead plants and dead animals are broken down by decomposers
(e.g. microbes, fungi, crows and earthworms.
An example of this is when leaves die in the autumn. They
fall and become leaf mould, carbon dioxide, water and nitrogen
because of the action of microbes. These products enrich the
soil and the environment for other
plants and microorganisms.
5.
Energy
chains and important cycles
The source of energy for every food chain
is the Sun.
Energy from sunlight allows plants (producers)
to photosynthesise. This converts light energy into chemical
energy in carbohydrates, fats and proteins in plants.
The chemical energy in these substances
is then passed along the food chain to animals (consumers).
Eventually, energy is lost from the chain
as the animals and plants die and decay releasing carbon
dioxide, water and nitrogen.
Superimposed on this energy chain there
is a continual cycling of elements. These include the carbon
cycle and the nitrogen cycle.
