Food Webs & Energy Flow

How energy enters ecosystems and moves through producers, consumers, and decomposers

            📚 Science
                            🎓 Grade 8
                                        ⏱️ 30 minutes
                    

Learning Objectives

Explain how energy enters most ecosystems and how producers convert light energy into chemical energy.
Interpret arrows in a food web as the direction of energy transfer and identify producers, consumers, and decomposers.
Describe why less energy is available at higher trophic levels and connect this to smaller populations of top predators.

            Progress
            5 sections
        

1
Engage: trace energy from food to the Sun
                            ~3 minutes                        

Essential question

How does energy enter an ecosystem, and how does it move through a food web?

Big idea (accuracy check)

Energy is not created by ecosystems. Energy mostly enters as sunlight and is transformed into chemical energy stored in food. As organisms use energy, much of it leaves the ecosystem as heat.

Vocabulary

• Producer (autotroph): makes its own food (usually using sunlight).
• Consumer (heterotroph): gets energy by eating other organisms.
• Decomposer: breaks down dead organisms and waste.
• Trophic level: a feeding level in a food web (producer, primary consumer, etc.).
• Biomass: the total mass of living material in an area.

Quick start question (1 minute)

Trace the energy in one bite of food back to its original source. (Hint: most paths start with the Sun.)

2
Explain: producers, photosynthesis, and one-way energy flow
                            ~9 minutes                        

                📖
                Photosynthesis (how energy gets into most food webs)            

Most ecosystems start with producers that capture sunlight and store that energy in sugars. This converts light energy into chemical energy.

                                    $$\ce{6CO2 + 6H2O + light -> C6H12O6 + 6O2}$$
                            
Photosynthesis: carbon dioxide plus water plus light produces sugar (glucose) and oxygen.

Energy flow in an ecosystem

  Energy enters and moves through an ecosystem
  Flow diagram: Sunlight to producers (plants and algae) by photosynthesis, then to consumers by eating, then to decomposers from dead matter. At every step, some energy leaves the system as heat from respiration.

  Energy flow in an ecosystem
  Energy enters mainly as sunlight, is transformed, and is lost as heat at each step.

    Sunlight
  
    Producers
    plants, algae
  
    Consumers
    herbivores
  
    Consumers
    carnivores
  
    Decomposers
    fungi, bacteria
    detritivores
  
  light energy
  eating
  eating
  waste + death

    heat (respiration)
    heat
    heat
    heat
  
  Energy flows one-way: in as sunlight, out as heat. Matter (atoms) can be recycled.
            
Flow diagram: Sunlight to producers by photosynthesis, then to consumers by eating, then to decomposers from dead matter. At every step, some energy leaves as heat from respiration....

                💡
                Energy flow vs matter cycling            

Energy flows one-way: in as sunlight, out as heat. Matter (atoms like carbon, nitrogen, and water) can be recycled through decomposers and the environment.

Question 1

In most ecosystems, what is the original source of the energy stored in food?

Heat from Earth's core
Sunlight
Oxygen in the air
Soil minerals

Question 2

Photosynthesis converts ______ energy into ______ energy stored in food.

3
Apply: read and build a food web
                            ~9 minutes                        

Food chains and food webs

• A food chain is one path of energy flow.
• A food web shows many connected paths.

How to read arrows

In food webs, arrows show the direction that energy moves: from the organism being eaten to the organism that eats it.

Example: If a rabbit eats grass, the arrow goes grass -> rabbit.

Food Webs: Crash Course Kids #21.2

Example food web

  Example food web and energy arrows
  Food web example: algae and aquatic plants feed zooplankton and snails. Zooplankton and snails feed small fish. Small fish feed larger fish and herons. Decomposers receive dead matter from all organisms. Arrows point from food to eater, showing energy transfer.

  Food web: arrows show energy moving to the eater
  If A is eaten by B, the arrow goes A to B.

    Algae

    Aquatic plants
  
    Zooplankton

    Snails
  
    Small fish

    Large fish
  
    Heron
  
    Decomposers (fungi, bacteria) break down dead matter
  
  Dashed arrows show dead matter and waste flowing to decomposers (energy still moves, matter is recycled).
            
Food web example: algae and aquatic plants feed zooplankton and snails. Zooplankton and snails feed small fish. Small fish feed large fish and herons. Decomposers receive dead matter from all organism...

Question 3

In a food web, an arrow from grass to rabbit shows:

The rabbit gives energy to the grass
Energy moves from grass to rabbit because the rabbit eats the grass
The rabbit and grass live in the same habitat
The grass eats the rabbit

Question 4

Match the term to its role in energy flow:

Producer

Primary consumer

Secondary consumer

Decomposer

4
Explain: energy pyramid and energy loss
                            ~5 minutes                        

Why there are fewer top predators

Organisms use energy to stay alive: moving, staying warm, growing, and reproducing. Much of that energy becomes heat during respiration and cannot be used by the next trophic level.

A common rule of thumb is that only about 10 percent of energy stored as biomass transfers to the next trophic level. The exact percent varies, but the pattern is consistent: less energy is available higher up.

Food Webs and Energy Pyramids: Bedrocks of Biodiversity (Amoeba Sisters)

Energy pyramid

  Energy pyramid and the 10 percent rule
  Four-level energy pyramid showing producers at the base with about 10000 energy units, then primary consumers with about 1000, secondary consumers with about 100, and tertiary consumers with about 10. Notes indicate most energy is lost as heat at each step, so less is available higher up.

  Energy pyramid (approximate)
  Only a small fraction of energy becomes biomass for the next level.

    Producers
    ~10000 units

    Primary consumers
    ~1000 units

    Secondary consumers
    ~100 units

    Tertiary consumers
    ~10 units
  
  less energy
  available

    Why so much loss?
    Respiration + movement
    + waste = heat
  
  Rule of thumb: about 10 percent transfers to the next trophic level (varies by ecosystem).
            
Four-level energy pyramid showing producers at the base with about 10000 energy units, then primary consumers with about 1000, secondary consumers with about 100, and tertiary consumers with about 10....

5
Wrap: decomposers and exit ticket
                            ~4 minutes                        

By the end, you should be able to

Identify the main source of energy for most ecosystems (sunlight).
Explain the role of producers, consumers, and decomposers in energy transfer.
Read a food web arrow as energy moving from food to eater.
Explain why energy decreases at higher trophic levels.

Question 5

Explain why energy pyramids get narrower toward the top (why there is less energy for top predators). Use the words heat and respiration in your answer.

Expected length: 35-110 words