Ecology Reading 2: How Energy Moves Through an Ecosystem (HS-LS2-4)

In every ecosystem, energy moves from one organism to another. This happens in a food chain or a food web. Energy starts with the producers. These are plants or algae that make their own food using sunlight. Producers are also called autotrophs.

Animals cannot make their own food. They are called consumers or heterotrophs. There are different kinds of consumers:

• Herbivores – eat only plants (like rabbits).

• Carnivores – eat only other animals (like lions).

• Omnivores – eat both plants and animals (like bears).

We can represent these feeding relationships in a food web. Arrows show the direction energy moves, from the food to the organism eating it. For example, in the food web below, sharks eat octopuses.

Algae has no arrows pointing toward it because it is a producer.

We can also show these organisms in trophic levels:

1. Producers (plants, algae)

2. Primary consumers (herbivores)

3. Secondary consumers (carnivores or omnivores that eat herbivores)

4. Tertiary consumers (carnivores or omnivores that eat secondary consumers)

5. Apex predators (top predators that are not eaten by others, like sharks or eagles)

Not all energy moves from one level to the next. Most energy (about 90%) is lost as heat or waste. Only about 10% of the energy moves to the next level. This is called the 10% rule. For example:

• If plants have 10,000 calories of energy,

• Herbivores get only about 1,000 calories,

• Carnivores that eat the herbivores get 100 calories,

• Carnivores that eat other carnivores get 10 calories,

• The apex predator gets only 1 calorie.

Hint: If you struggle with math, to find ~10% of a number, chop off the last digit!

10,000 —> 1,000 —> 100, etc.

This is why there are fewer animals at the top of the food chain. It takes a lot of plants to support just one lion! This is also why a decrease in the number of producers will cause a decrease in all species.

Because so much energy is lost, ecosystems need a constant input of energy from the sun (for plants) or from inorganic compounds (for some bacteria).

If one part of the food web changes, it affects the other parts:

• If the number of predators increases, the prey number will usually decrease.

• If producers increase, the herbivores can also increase.

We can use mathematical models (like energy pyramids or bar graphs) to show how energy moves and how many organisms can survive at each level.

Vocabulary List (with explanations)

1. Ecosystem – A place where living and non-living things live and work together.
   Example: A forest or a pond.

2. Food chain – A list that shows who eats whom.
   Example: Grass → rabbit → fox.

3. Food web – Many food chains connected together.
   Example: In a pond, frogs eat bugs, fish eat frogs, birds eat fish.

4. Trophic levels – Steps in a food chain that show where energy goes.
   Example: Plants are at level 1, herbivores are at level 2.

5. Producers – Living things that make their own food (plants, algae).
   They start the food chain.

6. Autotrophs – Another word for producers.
   They use sunlight or chemicals to make food.

7. Consumers – Living things that eat other living things.
   They cannot make their own food.

8. Heterotrophs – Another word for consumers.
   All animals are heterotrophs.

9. Herbivores – Animals that eat only plants.
   Example: Cows and rabbits.

10. Carnivores – Animals that eat only other animals.
    Example: Tigers and hawks.

11. Omnivores – Animals that eat both plants and animals.
    Example: Humans and bears.

12. Primary consumers – Herbivores that eat plants.
    Example: Grasshoppers.

13. Secondary consumers – Animals that eat herbivores.
    Example: Frogs that eat insects.

14. Tertiary consumers – Animals that eat secondary consumers.
    Example: Snakes that eat frogs.

15. Apex predators – Top predators that are not eaten by others.
    Example: Sharks, eagles.

16. 10% rule – Only about 10% of the energy moves to the next trophic level.
    This is why there are fewer predators than herbivores.

17. Mathematical model – A way to use math to show how energy moves.
    Example: An energy pyramid.

18. Energy pyramid – A picture that shows how energy gets smaller at each level.
    The biggest part is at the bottom (plants), and the smallest is at the top (apex predators).

19. Inorganic compounds – Non-living chemicals that some bacteria use for energy.
    Example: Some bacteria near volcano vents use sulfur.

20. Constant input of energy – New energy that comes all the time from sunlight or chemicals.
    Ecosystems need this to keep working.