Untangling food webs: How human activity shakes up delicate ecosystems

Lesson overview

This 70-minute lesson plan for grade seven students focuses on food webs, with the aim of furthering understanding of these complex systems. The lesson encompasses wild, natural ecosystems and how changes to the population of individual species can impact the entire system. The lesson also asks students to consider their own personal position in the food web, and to consider how human actions impact the global environment. Following the lessons, students will be expected to ask themselves two big questions:

  • Are our industries and daily conveniences negatively impacting wild/natural food webs?
  • Are our food webs sustainable?

Ontario curriculum requirements

Grade 7: Understanding Life Systems: Interactions in the Environment

  • 1.1 assess the impact of selected technologies on the environment
  • 2.4 use appropriate science and technology vocabulary, including sustainability, biotic, ecosystem, community, population, and producer, in oral and written communication
  • 3.3 describe the roles and interactions of producers, consumers, and decomposers within an ecosystem
  • 3.4 describe the transfer of energy in a food chain and explain the effects of the elimination of any part of the chain

Detailed lesson plan

Warm up: what do we eat? (5-10 minutes)

Start the lesson with a discussion about food. Ask the students to share what they ate for breakfast or last night’s dinner. Then, ask students to think about where their food came from. If their meal was animal-based, what would that animal have eaten? If their meal was plant-based, where was it grown and where did those plants get their energy to grow? Students’ input will be written onto a whiteboard/chalkboard/display.

When the activity is complete, the teacher will label the foods on the whiteboard/chalkboard/display as “consumers” (animals) and “producers” (plants). Then, the concept of “decomposers” (worms and fungi) will be introduced, to link the top consumers back to the producers, and complete this simple model of a food chain. Other terms like “ecosystem”, “pollinators”, “biotic” and “abiotic” can also be introduced in the warm-up.

This warm-up exercise encourages students to consider their individual place within food webs.

Lead-in activity: simple food chains (15-20 minutes)

Divide the class (up to 30 Students) into small groups (3-4 students per group, 7-10 groups).

Give each group a package with four organism profiles and ask them to spend two minutes reading the profiles and arranging them into a top-down food chain (A eats B, B eats C, etc.).

Students will then be asked to label the ecosystem, identify organisms as consumers, producers or decomposers and suggest one abiotic and one human factor that may impact that ecosystem.

After a few minutes, the groups will then share their work with the class, discussing their food chains, and why they arranged them in that particular order.

Organism groups may include:

  • Corn (producer), owl (carnivore), grasshopper (herbivore), rat (omnivore)
  • Pond snail, bullfrog, algae, heron
  • Mouse, baboon, leopard, snake
  • Fish (herring, sardines, mackerel), tuna, sharks, zooplankton
  • Hawk, caterpillar, sparrow, trees (leaves)
  • Worms, robin, berries, cat
  • Mushrooms, grizzly bear, blueberries, deer

Outcome

Students will immediately discover that food web groupings are far more complex than linear food chains. In the last grouping, for example, both the deer and black bear will eat blueberries, deer will eat mushrooms, bear will eat deer, and mushrooms will decompose dead bear and deer and blueberry bushes.

Main activity (25 minutes)

The main activity of the lesson has the teacher handing out species cards to the all students, with each card representing a species endemic to a specific ecosystem. Each card will feature each species’ name and image. The students will then be tasked to identify which other species are their food and which are their predators.

Students will then form a circle or will disperse randomly within the classroom, as a ball of yarn is passed or thrown from species-to-species to demonstrate the energy transfer within the ecosystem. The activity will begin with the sun and then move on to the producers that feed on the sun’s radiant energy, before they are eaten by consumers, and so on. Once the ball of yarn reaches an apex predator, the yarn is cut and the ball is given back to the sun to begin a new food chain. This activity serves to visualize the interconnectedness of food webs within ecosystems.

After each species has been preyed upon by at least two other species, the teacher can ask “Who has the most yarn and why?” The answer is, of course, the sun. The sun is an abiotic (non-living) factor to life because all usable energy on earth is thanks to its radiant energy, which is transformed into chemical energy by plants.

The teacher can now alter the activity in several ways:

They may remove one species, brainstorming explanations for why it has gone extinct (i.e. impact from humans, lack of vegetation, etc.). If the species has gone extinct, how does the food web change and respond to that? How do we ensure the ecosystem is sustainable?
They may add an invasive species that is more competitive in the environment and ask the same questions.

The teacher can also pose several discussion questions:

  • What other biotic or abiotic factors might affect how species survive in an ecosystem?
  • Why should we be concerned about each species?
  • Why is biodiversity important?

Closure (10 minutes)

Teachers will wrap up the lesson by asking, “What is the human ecosystem?” The expected answer is that humans have a tremendous impact on the the global ecosystem. As you can see, there is more to this lesson plan than merely educating students on what food webs are and the interactions of their constituents; it is about making the lesson personal to the students, ultimately with the aim of inspiring them to make a positive environmental difference.

That being said, during the closing comments, teachers can also reinforce the concepts introduced in the lesson with a brief brainstorming session, exploring specific human impacts on ecosystems, such as:

  • Overfishing
  • Global warming
  • Ocean plastic pollution
  • Deforestation
  • Industrial food complex

A five-minute video will reinforce the complex and unexpected impacts species can have when added to a food web. In the video, wolves are reintroduced to the Yellowstone National Park and they dramatically change the ecosystem.

The wolves do this by driving an overpopulation of deer away from open spaces along the rivers, thereby allowing grassy areas to grow back into forested areas, and attracting a plethora of birds and land animals.

Assessment (one-page assignment, due next class)

Students will be required to consider how their individual actions have a direct impact on organisms and habitats within their ecosystems. Students can choose one of the following assignments:

  1. Create a six-step food web based on what you ate for dinner. Where did each ingredient come from? Students must make at least one connection between producers (plants), consumers (animals) and decomposers (bacteria and fungi).
  2. Choose an organism from either the lead-in or main activity. Identify the organism's habitat (forest, desert, water, grassland or tundra). Write a one paragraph summary of how human activities are affecting this organism, and its habitat at large.
  3. Keep a list of your waste over a week, (i.e. garbage, recyclables, food packaging, food waste etc.). Keep a tally of single use plastics and identify ways that you can (and will) reduce your waste. Do you use disposable cups or carry a thermos? Do you compost? How is the path of composted food waste different from food waste put in the garbage/landfill?

Resources

  • Main Activity
    • Cards/slips of paper with all of the species of the chosen environment to give to the students
    • Ball of yarn
    • Scissors

Additional links and videos

References

Food webs and food chains explanations

USDA – Agriculture in the classroom

Adapted from Project LEAP: Learning about Ecology, Animals, and Plants, College of Agriculture and Life Sciences, Cornell University, Ithaca, NY 14853.

Appendix

Classroom food web example: Predator-prey table

Spreadsheet featuring food web relationships

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