APES Notes 1.11 - Food Webs & Chains

Main Ideas, Concepts, and Lessons

Unit Goal (1.11: Food Webs & Food Chains)

By the end of this unit, students should be able to:

• Describe food chains and food webs
• Identify organisms in food webs/chains by their trophic levels
• Use a visual model (food web) to describe an environmental concept

Energy and Matter Flow in Ecosystems

• Food webs show how energy and matter move from organism to organism through feeding relationships.
• Predation includes herbivory: even if an herbivore eats a plant, it’s still considered predation because one organism uses another as an energy source.
• Arrows indicate direction of flow
  • Students do not need to memorize “arrow points to eater vs. eaten.”
  • Instead: the arrow shows the direction of energy.
  • The organism the arrow points to is the one taking in energy and matter.

Example: Grasshopper and Corn

• A grasshopper eats corn
• The grasshopper gains energy/glucose used for movement and growth
• The grasshopper also takes in nitrogen and phosphorus to build body tissue
• This illustrates energy/matter flow direction: corn → grasshopper

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Food Chain vs. Food Web

Food Chain

• A single pathway of energy/matter flowing from one organism to another.
• Example pathway: grass → rabbit → fox

Food Web

• Made of at least two connected food chains sharing a common link (at least one shared organism).
• Example: adding grass → grasshopper → mouse → fox
• Because the chains connect through a shared organism (fox), the result is a food web.
• Key point: food webs don’t have to be complex—the requirement is two or more linked chains connected through one common organism.

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Trophic Levels (and How Organisms Can Occupy Multiple Levels)

• Organisms can be primary, secondary, or tertiary consumers depending on what they eat.

Example Using Owls

• Rabbit is a primary consumer (eats grass)
• If an owl eats a rabbit, the owl is a secondary consumer
• Robin is a secondary consumer (eats a grasshopper → grasshopper is a primary consumer that eats grass/producer)
• If an owl eats a robin, the owl is a tertiary consumer

Lesson: Food webs help show how an organism may change trophic level depending on its prey.

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How Changes Spread Through a Food Web (Trophic Cascades)

Core Concept

• Changes in one population can affect other organisms both above and below in the food web.

Example: Python Population Increase

If pythons increase:

• Their prey (rats and frogs) decline
• With fewer predators, grasshoppers increase
• More grasshoppers eat corn, so corn declines
• Ripple effects may continue to other organisms that depend on corn
• If rats and frogs decline, another predator species (referenced as an additional “ego population” in subtitles—likely another organism dependent on rats/frogs) may also be affected

This ripple effect is called a trophic cascade:

• A dramatic increase/decrease in a top predator causes cascading effects through lower trophic levels.

Common Example: Loss of a Top Predator (Wolves)

• Wolf loss → deer increase → plants deer eat decrease
• Less vegetation/ground cover results (e.g., trees may have fewer leaves; some trees may die)
• Ecosystem impacts spread to organisms that rely on vegetation for habitat or food
• A common described impact is defoliation (loss of vegetation) due to herbivore increase (deer) without wolf predation control

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Practice FRQs / Assigned Tasks (Checklist Method)

Q 1.11: Video Practice (2 FRQs)

FRQ 1: Describe a Direct Effect

• Task: Describe a direct effect that a decline in the frog population would have on another organism in the food web.
• Method implied: Use the food web connections/arrows to determine:
  • what organisms frogs feed on,
  • what organisms feed on frogs,
  • and how a frog decline affects those connected trophic levels.

FRQ 2: Identify an Organism by Trophic Role

• Task: Identify an organism that is both a secondary and a tertiary consumer in the food web.
• Method implied: Find an organism that fits both roles by showing it feeds at:
  • the secondary consumer level in one relationship, and
  • the tertiary consumer level in another relationship.

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Speakers / Sources Featured

• Mr. Smeeds (teacher/presenter)