Energy transfer between trophic levels (Edexcel A-level Biology B)

This lesson describes how energy is transferred between trophic levels using the terms net and gross primary productivity and calculates the efficiency of this transfer. The PowerPoint and accompanying resources have been designed to cover points 10.2 (i) and (ii) of the Edexcel A-level Biology B specification and the content of the lesson also accounts for the loss of energy between different levels and describes the farming practices that act to reduce these losses.

Due to the fact that the productivity of plants is dependent on photosynthesis, a series of exam-style questions have been written into the lesson which challenge the students to explain how the structure of the leaf as well as the light-dependent and light-independent reactions are linked to GPP. All of the exam questions have mark schemes that are included in the PowerPoint to allow students to immediately assess their understanding. A number of quick quiz competitions as well as guided discussion points are used to introduce the formulae that calculate NPP and N and to recognise the meaning of the components. Once again, this is immediately followed with an opportunity to apply their understanding to selected questions and the students will have to calculate the efficiency of energy transfer.

The remainder of the lesson focuses on the ways that energy is lost at each trophic level and the biology behind the following farming practices are discussed:

• raising herbivores to reduce the number of trophic levels in a food chain
• intensely rearing animals to reduce respiratory losses in human food chains
• the use of fungicides, insecticides and herbicides
• the addition of artificial fertilisers

The ethical issues raised by these practices are also considered and alternative methods discussed such as the addition of natural predators and the use of organic fertilisers like manure