Activity - Ecological Interactions: The Food Cobweb

Skills: Ordering, inferring, predicting, critical thinking.
Activity - Ecological Interactions: The Food Cobweb - Articles



Plants and animals live in interacting, intertwined communities. There is a characteristic set of species in different environments. For example, certain species of trees, shrubs, ground cover, arthropods, reptiles, mammals, birds etc. live in a temperate forest environment. A completely different set of creatures live in a marsh, or a grassland or an agroecosystem. However, the relationships between these groups can be defined by the ecological role they play, the flow of energy between them and the cycling of nutrients between them. This is a fancy way of saying "everything is connected"! And if you change one part of the system, something else changes. In an ecosystem management decision, you hope you know what those consequences of your actions are!) This is important in managing agroecosystems as well.

Who are the players in every Ecosystem?

  • Sun - Energy Source
  • Producers - Capture energy and make food All plants
  • Primary Consumers - Eat the producers Herbivores
  • Secondary Consumers - Eat the primary consumers Carnivores
  • Tertiary Consumers - Eat the secondary consumers Carnivores

Can you see where predator and prey fits into this scheme?

Many organisms eat just about anything, plant or animal. These are omnivores, like my friend Bob. Organisms that feed primarily on decaying organic matter are scavengers or detritivores. Each organism has a certain way of making a living it its environment, that is it has its niche.

A simple food chain, in which chemical energy and nutrients are passed along might be:

  1. GRASS --> COW -->YOU! (So thank the grass next time you eat a burger!)

How many other examples can you give?

However, most transactions are more complex than these examples. Can you make even these simple ones more complex? Once all the organisms that rely on another are mapped out, the relationship looks more like a food web than a straight-line chain.

Let's see how organisms rely on each other within a system.

Learning Objectives

  1. See in what ways living things are connected.
  2. Understand that systems are more or less complex in their connections.
  3. Learn what happens if certain living components of the system are removed.
  4. Construct a simple food web that might be found in an agroecosystem
  5. Consider the pest management implications of food webs

Materials Needed

  • Pre-made cards showing the ecosystem, name of organism, classification, and food.
  • Pre-made cards that have an arrow, stating "Energy Flow"
  • Big pieces of paper for students to draw out their own food webs

(You can also do this exercise with balls of yarn, see below.)


If using card layout

  1. Give students all the cards representing marsh ecosystem connections
  2. Lay out the species joined with "Energy Flow" arrow cards to each of their stated foods.
  3. Diagram the food web thus created.
  4. Pull out a species, thus erasing the connections it has to other species. What happens?

If doing the ball of yarn technique

  1. Give the students the marsh ecosystem playing cards with the individual's role listed
  2. Each student has a card (Make as many cards as you have students.)
  3. Each student then identifies who they need to be connected to to live and takes a piece of yard that they both hold onto. Repeat with as many different food types as they eat.
  4. Teacher reaches in and removes one species from the group.
  5. Students note what happens to them and everyone around them.

IPM Application

  1. Have the students create & explain their own food web in an agricultural or garden setting.
  2. Include species that are pests of the plants or animals; or weeds with their herbivores.
  3. Include predators upon these species
  4. Have students discuss how they would manage these pests without disrupting the environment.


Time required: Full class period for introduction to food webs, and a second period for IPM application.


  • What happened when one of the organisms is removed from the food web?
  • Is there the same amount of disruption for every species removed?
  • What situations in the real world might impact members of the food web?
  • What other factors can affect the system as a whole (not just individual species)?


  • Discuss the effect of biodiversity in a food web.
  • Compare the agricultural ecosystem with the marsh ecosystem.
  • Are some species more in danger of being knocked out of a system than others?
  • Why or why not?
  • Are some ecosystems more fragile than others?
  • What might determine this?
  • What might the effect of an invasive species (plant, animal or disease) be on an ecosystem?
  • What considerations might you take into account when choosing IPM tactics for management of pests in various ecosystems?


**Portions of this exercise borrowed from "Purple Loosestrife Project: Coordinators Handbook", Section 4, Part One, Lesson 4, Michigan State University.