Pyramid of biomass

Level 5 and 6
Curriculum area - Science

Outcomes

Students

• Interpret data to demonstrate biomass and number pyramids.
• Use information to draw conclusions.
• Examine an example of biological magnification.

Materials and preparation

The Australian food web displayed with the animal cut-outs on a whiteboard

Outline Part 1 Sketch the pyramid. As a class, or in small groups, the students refer to the Australian food web displayed and list all of the names of the first, second and third order consumers in the pyramid.

Discuss the decrease in numbers of different animals through the food web. Outline the following example of a count of numbers in the levels of one food chain and ask the students to account for this reduction in numbers. (At each step of the food chain, energy used by each animal to maintain body functioning is not passed through the food chain but is lost as heat.

So a reduced proportion of the sun's energy originally captured by the plant actually reaches the top order consumer in a food chain - 10% of the energy is lost at each step. Because of this energy loss the number of animals supported at each step in a food chain becomes fewer and fewer which is often represented as a pyramid of numbers or biomass.)

1 eagle 25 quolls 500 bandicoots 300 000 grasshoppers 3 000 000 plants

Part 2

Biological magnification

Students use the following information and the Wedge-tailed eagle pyramid numbers above to illustrate the effect of biological magnification.

Pretend that you are scientist that works for The Department of Farming. A chemical company has produced a chemical for the purpose of killing weeds in pastures. As the officer in charge of approving the use of the farm chemicals, you have to investigate if the chemical will endanger wildlife. The chemical is taken into the body of all insects and permanently remains in their fat deposits.

From your studies, an insect will not be affected by the recommended dosage rate of 0.0001g per plant. But in quantities of 2g the chemical does kill animals even a large animal like an eagle. Use the pyramid of numbers for the Wedge-tailed eagle food chain (see above) to calculate the amount of the chemical that could be passed up the chain. What is your recommendation about the pesticide? Explain.

Extension

Find out about DDT.