A soil food web

This task is about understanding relationships in a food web.

There are a lot of living things in the soil. Nematodes, for example, are tiny worm-like animals that live in the soil. Even though we don’t notice them, there are millions of them there, and they are of different types. Some are herbivores and some are carnivores.

The diagram shows some feeding relationships that begin under the ground. This sort of diagram is called a food web.

Use the food web to answer the following questions.

a)	What do the arrows on this food web represent?
b)	What two things shown in the food web are the main energy source for everything else?
	1. 2.
c)	Name two types of carnivores in this food web. 1. ______________________________ 2. ______________________________
d)	Name two decomposers in this food web. 1. ______________________________ 2. ______________________________

e)	i)	Name one thing that could become more common if predatory nematodes all die for some reason.
	ii)	Why do you think that?
f)	i)	Name one thing that could become less common if predatory nematodes all die for some reason.
	ii)	Why do you think that?
g)	i)	Do you think it would be good or bad for plants if root-living nematodes were to die out? (Circle one.) Good Bad It depends
	ii)	Why do you think that?

Answers/responses:

			Y10 (06/2008)
a)		Flow of energy Lower level response: correctly describes feeding relationship, e.g., is eaten by [Do not accept: any answer that does not clearly identify the direction of the relationship, e.g., what eats what.]	very difficult difficult
b)	i) ii)	Any 2 of: Plant roots Material shed from plants	2 correct – moderate 1 correct – easy
c)	i) ii)	Any 2 of: Arthropods/Beetles/Springtails Birds/Blackbirds/Thrushes Predatory nematodes Single celled animals/Amoeba	2 correct – easy 1 correct – very easy
d)	i) ii)	Fungi Bacteria	2 correct – moderate 1 correct – easy
e)	i) ii)	Any 1 of: Fungi Bacteria Single celled animals/Amoeba Because they are eaten by predatory nematodes NB: This answer considers only direct relationships. Some students may take a more complex or long term view, as described in Systems thinking. No trial students gave this sort of response.	Identifies organism and provides reason – difficult Identifies organism only – moderate
f)	i) ii)	Any 1 of: Arthropods/Beetles/Springtails or Birds/Blackbirds/Thrushes Because predatory nematodes are food for them or Because there may be fewer arthropods (as they cannot eat predatory nematodes) which in turn provide food for the birds (or other well reasoned indirect relationship).	Identifies organism and provides reason – difficult Identifies organism only – difficult
g)	i) ii)	Accept any answer Describes correct multiple impacts on system, e.g., Because some roots strangulate others for more nutrients so it will depend on which area and what habitat the roots were in (it depends) Describes correct direct relationship, e.g., Because that would stop plants dying because of their roots being eaten (good)	very difficultt difficult

Teaching and learning:

Communicating in science
The resource probes students’ ability to use their knowledge to interpret a food web – a familiar structure that scientists the world over use to summarise energy inter-relationships between living things in an ecosystem. Thinking about both direct and indirect relationships is an important part of systems thinking and food webs provide a rich context for exploring students’ ability to shape and express ideas about systems, based on the provided information.

Links between existing knowledge and interpretation skills
Question c) could be seen as a straightforward knowledge question. However it was possible for students to work out correct answers if they knew the definition of a carnivore, or where flesh eaters would be positioned on a food web. They could get the right answers even if they did not initially know the specific animals named and their responses showed that the question was easy for them.

Question d) was an identically shaped knowledge question, but this time the given information provided less obvious clues. To work out the answer, if they did not already know about the ecological role of fungi and bacteria, students would need to know that material shed from plants is already considered "dead" and that decomposers feed on such material. As the results show, this was somewhat more difficult for students, but we cannot know for sure if this was because they don’t know the names of decomposers per se, or if they don’t have sufficient theoretical background to "read" the answers off the provided food web.

Diagnostic and formative information:

Communicating in science
When framed from a Using Language, Symbols and Texts (ULST) perspective, knowing what the arrows in food chains and food webs symbolise is an important Nature of Science idea. However, many students still cannot identify "energy flow" as the big idea of this very common systems model. The student responses suggest that some may actually know but cannot express the idea succinctly, i.e., they described the feeding relationship rather than the energy flow. Fewer than 50 of the 184 trial students gave these sorts of responses, so even at this level the question was difficult. Next steps below suggest one way this important element of ULST could be explored to develop students’ competency in this aspect of science communication.

Systems thinking
Correct explanations for Question f) were rated difficult or very difficult and they almost always related to the short term, immediate effects typical of direct relationships. Again there is an opportunity here to develop students’ ability to consider indirect relationships and so foster systems thinking skills (see Next steps).

Question g) provides a further opportunity to reinforce emergent systems thinking. It did not matter whether students saw the change as good, bad or contingent (it depends), provided that they justified their choice. We were particularly interested in the "it depends" responses, because a previous food web item, A garden food web, raised questions about whether, if a prompt that gave them "permission" to answer this way was provided, more students might recognise that the consequences of a systems change can’t necessarily be predicted.

Thirty six students selected "good", 78 "bad", and 42 "it depends". Over half the students (53 percent) either did not try to or could not give a plausible justification for their choice of good, bad or it depends. This type of critical thinking needs practice.

	Students with incorrect or missing reasons	Students identifying correct direct relationship(s)	Students giving dynamic systems reasons
Good	11	24	0
Bad	61	13	4
It depends	24	10	8

Choosing "it depends" as their first response was not necessarily an indicator that students recognised a systems dynamic at work. By systems dynamic we mean that the first impacts are not necessarily the only changes that could take place. Students who did realise this could either sketch at least one more step as an indirect relationship to the initial change, or they could see that two possible immediate changes might inter-relate in unpredictable ways. As the table shows, just 12 students gave a dynamic system response, and it is interesting that two thirds of this small group had first chosen "it depends". (It also seems interesting that no students who first chose "good" gave a dynamic system response, but the numbers are very small.) These patterns beg further investigation but give promising hints that "it depends" thinking might be a way to encourage students to be more adventurous or playful in their thinking about possible consequences of systems change.

Examples that illustrate "it depends" type responses show that we were not expecting sophistication.

Because if the root-living nematodes died out then some of the good plants (e.g., flowers) may live longer but then the bad plants (e.g., fungi, weeds) would also possibly live longer.
Both have reasons for or against. You need to work out whether it would be worth losing plants or having better soil.

Students’ answers were very much "of the moment" whereas systems constantly adjust and evolve – an initial change that seems favourable might trigger a series of events that are ultimately not so. For example, the student who wrote the first response above would not need a great deal of prompting to think through what might happen for the flowering plants if weeds did flourish.

Science understandings
Some students did not differentiate between root and predatory nematodes, which led them to draw incorrect conclusions. Attention to detail would seem to be important here.

Next steps:

Shaping explanations

Students may benefit from practice at shaping explanations as short logically sequenced pieces of written text.
Systems thinking
Students could practise "could be" (as opposed to "is") thinking by practising describing and writing chains of rolling consequences, rather than stopping after an immediate change has been identified. UK "learning to learn" expert Guy Claxton suggests that one simple way to foster a lifelong learning orientation is to try replacing some "is" statements with "could be" explorations. The context of this food web question would lend itself well to that strategy.
Interpreting arrows on a food web
If students struggle to articulate the meaning of the arrows on a food web you could try this simple teaching sequence which begins with a concrete idea rather than going straight to the abstract explanation of the symbolisation:

Have each student or small group take one relationship in its symbolic form (two living things connected by one arrow) and write an explanation of this relationship in their own words.
List and compare all the words and phrases that the students use to explain their selected relationships.
Discuss with them which version is closest to what a scientist might say and explain why.
Try applying the scientists’ way of saying it to another relationship on the food web.

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A soil food web

A soil food web