A garden food web
Correct answer: Energy flow through the ecosystem
All animals will be affected to some extent. We looked for accurate descriptions of impacts that suggested:
Conditional systems thinking (recognises that what happens next may not be straightforward to predict)
Simple systems thinking (begins with the most obvious change – usually to spiders – and relates other changes to that)
Caterpillars: if there's a decrease in wasp numbers then there will be an increase in caterpillar numbers, in turn eating more food, leaving less for others.
Three substantially correct but not linked changes
NOTE: Some students described one or two correct changes (22%)
Beetles because they only eat beans (name and reason)
Some students named beetles but didn't explain (5%).
Developing a nature of science idea via the key competency "using language, symbols and texts"
As we have found in another recently developed item Feeding relationships in the Southern Ocean most students did not know that the arrows on food webs represent energy flow through an ecosystem. While this is a small point when considered in isolation, learning to ask questions such as "What do these arrows represent"? and "Who decided they should be like that?" is grounded in the nature of science idea that scientists put meaning into symbols in specific ways for particular purposes in specific contexts. Understanding this is one part of developing science-specific competencies in "Using language, symbols and texts".
There is also a practical reason to emphasise this point. When students think arrows represent "what eats what" they can get confused about how to read the systems interactions portrayed in a food web. For example, if part of the food web reads: wasp larvae → spiders → birds, then misinterpretations might look like the following two examples:
(incorrect idea that wasps eat spiders)
If there is more spiders there will be less birds because there is lots more spiders to eat the birds.
(incorrect idea that spiders eat birds)
Six percent of students knew how to "read" the food web but did not complete the Venn diagram correctly. It seems that this type of visual text for representing sets and subsets is not familiar to some students. Features of fish (Level 4) and Comparing animals (Level 3) also discuss this problem.
Practice in question answering
Some students need more practice in writing answers that are sufficiently specific to clearly explain the effect:
Spiders will die of starvation.
Five percent of students described at least one example of an effect on a plant, although the question asked them to consider three animals.
A common misconception
One student told us the question was inappropriate, showing how persistent this well known misconception can be:
There is only one type of animal – birds. The rest are insect foods and spiders.