The feeding relationships of the different organisms in an ecosystem can be shown most simply in a food chain. However, in most communities, animals will eat more than one type of organism and as such, a food web gives a more complete picture of exactly what eats what. Below are just a few ideas of how to teach both.
Wildlife documentaries contain lots of examples of real life (and often quite bloody) food chains and food webs, and asking the students to watch a few snippets before identifying the feeding relationships in each can prove a useful starting point to the lesson.
One common mistake when drawing food chains and food webs is for students to put the arrows the wrong way round and it is crucial that you emphasise that they point in the direction of energy flow up the chain. Asking students to construct food chain and food web hanging mobiles can help. You can download templates here or ask the students to make their own.
Energy transfer in food chains is inefficient; the amount of energy that is passed on is reduced at every step (trophic level). However, great care must be taken to ensure that you do not refer to the energy being lost, since energy can be neither created nor destroyed but rather is converted into some other form or store.
In food chains, much of the energy is transferred to the environment as heat during respiration but obviously some of it is also used by the organism (before it is eaten) in life processes such as movement and growth. Furthermore, not all of a food item may be ingested during feeding and, even if it is, not all parts will be digestible (e.g. lignin and cellulose).
A nice way to demonstrate energy transfer in a food chain is to pour coloured water between paper, plastic or styrofoam cups (each representing a trophic level) and reducing the volume of water transferred each time.
Begin by pouring just 10% of the total volume of water in the jug (the sun) into the first cup (producer). This demonstrates that only about 10% of the sunlight that falls on a plant is used in photosynthesis since most of it is transmitted or reflected, and some of it is simply not the correct wavelength to be absorbed by the photosynthetic pigments in the leaf (only red or blue light is absorbed).
Next, pour roughly 10% of the water in the first cup into the second (primary consumer) and the remainder into another container labelled ‘Respiration (movement, warmth, growth) and excretion’ (do not pour the water down the sink as this will only reinforce the misconception that energy is lost). Repeat this process along the food chain.
By the time you reach the final consumer, only a couple of drops of water will remain. This is a good point in the learning to ask the students why food chains are usually restricted to just three or four trophic levels and why the number of organisms generally decreases along the chain.
Dinner at the Reef
This is a fun game from Arkive in which students learn about food chains in a marine environment, predator-prey relationships and the fine balance of an ecosystem. Although primarily aimed at 7 – 11 year olds it can also be used at Key Stage 3.
Interactive learning websites
There are now a large number of interactive learning websites offering simulations and simple ‘drag and drop’ style games for teaching younger students about food chains and food webs. Simply click on any of the images below to link.