Student Experience 3: In what ways are organisms being affected in Sunrise Farm pond?

Purposes

To illustrate the trophic relationships within the pond.
To introduce students to a representation of the pond that they can use to simulate effects.
To illustrate the extended effects of a disturbance to a food web.
To help students visualize the effects of oxygen depletion in the farm pond ecosystem.

Description

Assign students roles for different organism populations in the pond ecosystem. For example, in a class of 24 students, students may be assigned as follows to reflect natural population sizes:

2 perch
4 minnows
6 mayfly larvae
6 underwater plants
6 algae

Show students a ball of string or yarn, and explain how the string will represent the connections among organism populations. After student groups have arranged themselves in a circle, one of the producers in the circle receives a ball of yarn from the teacher. Ask students to consider why the yarn would start at the producers.

Then, the student representing a particular producer describes the relationship between herself and another student. For example, “algae” connecting to “mayfly larvae” may say, “I am connected to mayfly larvae because mayfly larvae get energy by consuming algae,” while passing the ball of yarn to the mayfly larva. This articulation is very important because the string web representation (unlike the food web with arrows) does not represent the direction of energy flow. For example, to continue, a “mayfly larva” might say, “I’m connected to underwater plants because I also consume hornwort and pondweed,” or “I’m connected to minnows, because minnows consume mayfly larvae.” Students should use the class-created food web as the basis for these connections.

Students should continue until they have connected all organism populations appropriately. Based on the number of students and configuration, webs will vary. The figure below shows a birds-eye view of a sample string web:

After the web is formed and relationships articulated, use the class-generated list of events from Student Experience 2 to provide multiple scenarios, which students simulate and discuss. If a scenario involves or affects an organism, the student(s) representing that organism should gently tug on the string. Facilitate a discussion about what the tug represents and the ways that others in the web feeling the tug are affected. For example, a decreased minnow population will result in a “tug” on perch as well as the mayfly larvae, and it is important for students to consider the differences in the impacts that these two tugs represent. In the case of perch, they are losing a food source, whereas the mayfly larvae will now have increased opportunity to survive given that fewer minnows are consuming them. As students simulate the different scenarios, continue to ask students to pause and contribute observations to describe the effects. As students make observations, document the corresponding effect, perhaps in a flowchart format to facilitate students making connections between the cause and effect, and to help students visualize the sequence of events in the pond.

In scenarios that include a decomposer, you (the teacher) should take on the role of the decomposer to avoid the misconception that decomposition is something that happens in the absence of a decomposer. Because decomposers consume at all levels, the presence of the decomposer is best represented in this case by the teacher on the periphery of the string web. Direct the students’ attention to what is happening during the process of decomposition (i.e., decomposers consuming dead organisms) and the impacts (i.e., depleted oxygen levels).

Much of this student experience is driven by student thinking and what students are experiencing in the web; therefore, discussions may vary. See sample excerpt for an example of classroom discussion related to the food web.

Questions to Ask Students

What would happen if all of the minnows disappeared?
What would happen if there were no algae in the pond?
What happened after the fertilizer entered the water?
- What caused the algae to grow?
- What happened to the algae when they died?
- Where would decomposers play a role in this web?
In what ways are organisms impacted by the increased decomposition that is occurring in the pond?
- Which organism would likely be impacted most immediately by the increased decomposition? Why?
- How would the death of the mayfly larvae impact the other organisms? Why?

Crosscutting Concepts

Cause and Effect Using the string web builds on students’ previous experience of listing the connections and impacts at Sunrise Farm Pond. As noted above, it is particularly important to have students articulate the connection when a “tug” is felt between two organisms, identifying the cause and directionality, and indicating whether the effect would be potentially beneficial or harmful to the affected organism.

Science Practices

Developing and Using Models In this experience, students are introduced to another model of the ecosystem at Sunrise Farm Pond. Ask students to consider how this model differs from the model they have been developing. For example, students may suggest that the string web model is more interactive, which should be highlighted as an affordance of the model, as it allows students to test ideas. A possible limitation of the model that students may mention is that there is not a written record to refer back to; however, recording these cause-and-effect relationships in a flow chart captures information gathered while using the string web.

Student Thinking

Students may find it challenging to understand “underlying causal patterns,” meaning that students often think populations are affected only by those directly linked in a food web and do not readily trace the effects.
Similarly, students tend to overlook indirect effects of abiotic factors on organisms and their feeding relationships, such as the extended impacts of pollutants on an ecosystem, which means that students may find it challenging to make a connection between the nutrients entering the water and the oxygen depletion.
Students’ understandings may also appear somewhat limited when examining the directionality of effects. For example, students are more likely to trace effects up through a food web rather than down. Therefore, students may be less likely to consider how algae and underwater plants would be impacted by fewer mayfly larvae.
Students view ecosystems as universally fragile, believing that a food web disturbance would affect all populations similarly. In this scenario, with the limited number of organisms that students are focusing on, students may further believe that all populations are affected similarly. For this reason, it may be helpful to remind students of other organisms who live above the water (and therefore do not rely on the dissolved oxygen).

Implementation Tips

It may be helpful to have students wear some kind of sign indicating their assigned organism population, perhaps by creating cards with the organism listed on the front and its food sources listed on the back. Alternatively, you may want to have students grouped together by organism so that the role of each student is clear. However, it is important to acknowledge this limitation of the model, because in the pond the organisms would not necessarily be grouped in this way.
Draw students’ attention to the fact that as a group, they represent a population of organisms. The suggested number of students per organism population outlined above is intended to represent the relative size of each population, though it is also important to note the limitation of this model.
Due to the complexity of decomposers’ role in the web, we do not recommend having students take on the role of a decomposer population. However, it is important to acknowledge decomposers and their role at all trophic levels.
We suggest a flowchart, but if students are familiar with a particular graphic organizer to represent cause-and-effect scenarios, it may be helpful to use that to help students understand the relationship between the events that have occurred and the impacts on organisms.