Food Web II

In the Great Lakes, many different food chains interact to form complex food webs. A food web shows what fish, animals and organisms eat — sometimes multiple species — and how energy is passed from one group to another. The complexity of food webs may help to ensure survival in nature. If one organism in a chain becomes scarce, another may be able to fill its role. Students learn, however, that if too many links in the food web are broken, the changes will affect every other link including humans.

Grade level: 4-8 grades

Performance Expectations:

• MS-LS2-1 Ecosystems: Interactions, Energy and Dynamics. Observe how lower links in a food web affect the highest links by analyzing and interpreting data to provide evidence for the effects of resource availability on organisms and populations of organisms in an ecosystem.
• MS-LS2-2 Ecosystems: Interactions, Energy and Dynamics. Compare a food web to a food chain and list similarities and differences by explaining and predicting patterns of interactions among organisms across multiple ecosystems.
• MS-LS2-3 Ecosystems: Interactions, Energy and Dynamics. Diagram a food web to describe the cycling of matter and flow of energy among living and nonliving parts of an ecosystem.

For alignment, see: NGSS Summary (PDF)

Lesson Objectives

• Diagram a food web.
• Compare a food web to a food chain and list similarities and differences.
• Discuss predator–prey and consumer–producer relationships using vocabulary words.
• Make predictions about roles each link plays in the overall food web.
• Observe how lower links in a food web affect the highest links.

Background

Freshwater marshes and wetlands provide an ideal setting to study aquatic food webs. These nutrient-rich areas produce more organic material, or biomass, than any other ecosystem. Typical marsh conditions stimulate aquatic plants to grow, which serve as an abundant source of energy and provide food and habitat for a variety of organisms.

Food webs can have many different feeding levels. Different species make up the various levels, depending on the type of food they produce or consume. Common terms for these levels include: producers (green plants); primary consumers (organisms and small animals that feed on live plants or dead plant and animal debris); secondary consumers (i.e., small fish or frogs that may be predators (catch their food alive) or prey (animals that are hunted or caught for food); and top level consumers or top predators (including humans) that prey upon other animals.

See the lesson Make the Connection: Food Webs for more background.

In a simple food chain, aquatic bugs eat the plants, and small fish eat the bugs. Big fish eat the little fish, and people catch and eat the big fish. As mentioned, however, organisms often feed on more than one species. This interaction is important, because if one organism declines or disappears, the organisms that feed on it are not necessarily lost; they can find other sources of food.

No matter what the complexity of a food web, when one organism consumes another, this process transfers energy through the various levels. However, with each transfer (consumption of one organism by another), energy is lost in digestion processes. The result is that it takes larger numbers of organisms at the base of a food web (e.g., producers) to support fewer organisms at the top (e.g., top predators).

At times, organisms at the bottom or middle of the web may die before they are consumed. In this case, the biomass supports the base of the food web by providing nutrients for new plant growth. 

Activity

• Who’s Hungry?
  Summary: Students use body movement and pantomime to simulate the feeding motions of freshwater organisms and demonstrate the interconnectedness of a food web.
  Time: 30-60 minutes