This food chain has four trophic levels, it shows the feeding
relationship in a community but they do not tell us how many
organisms are involved at each trophic level.
We can express this information in a pyramid of numbers:
Birds
Zebras 4
200
Ticks 800
Grass 1000
A pyramid of numbers is a graphical model describing the number of
organisms that exist at each trophic level in a community or an ecosystem.
Task 1: Draw pyramids of numbers of the following food
chains:
1. Arthropods feed on tiny algae, sardines feed on tiny
arthropods and dolphins feed on sardines.
2. Antelope feed on grass, lions feed on antelope and fleas
live on lions and suck their blood.
3. Greenflies feed on rose bushes, ladybirds feed on
greenflies and swallows feed on ladybirds.
4. Caterpillars feed on an oak tree, blackbirds feed on the
caterpillars and buzzards feed on the blackbird.
The problem with the pyramid of numbers is that it does not
take into account the size of the organism at each trophic
level. The solution to this problem is to express the
information in a pyramid of biomass:
A pyramid of biomass is a graphical model describing the distribution of
biomass at each trophic level in a community or an ecosystem.
Biomass is the mass of living things per square meter.
Birds 1000g/m2
Ticks 2000g/m2
Zebras 3000g/m2
Grass 4000g/m2
Task 2: Draw a pyramid of biomass of the following food chain:
grass (5,000 g/m2), rabbits (800 g/m2), foxes (100 g/m2), fleas (40 g/m2).
The drawbacks of the pyramid of the biomass is that it can
vary with the seasons, for example the biomass of a beech
tree is far greater in summer than it is in winter.
The best way to show what is happening in the feeding
relationships of a community or an ecosystem is to use a pyramid of energy.
A pyramid of energy is a graphical model describing the amount of energy
transferred from one trophic level to the next in a community or an
ecosystem.
Task 3: Draw a pyramid of energy of the following food chain:
Water weeds
tadpoles
small fish
pike
87000kJ/m2/year is passed from the water weeds to the tadpoles. The
tadpoles pass on 14000 kJ/m2/year to the small fish. The small fish pass on
1600kJ/m2/year to the pike and the pike passes 67kJ/m2/year on.
If the tadpoles got 87000 kJ from the water plants but only passed on
14000 kJ to the small fish. Where did the other 73000 kJ go to?
14000 kJ growth
50000 kJ excreted
23000 kJ respiration
87000 kJ food