Education Always

Level: JHS, Basic 8  |  Subject: Integrated Science  |  Strand: Systems  |  Topic: Ecosystem (Energy Flow, Food Chains and Food Webs)

Curriculum reference: B8.3.3.1 Understand energy flow in an ecosystem through food chains and food webs. Indicator B8.3.3.1.1: Construct food chains and food webs to show energy flow. All the exemplars are merged into one set of notes.

1. Roles of Living Things in an Ecosystem

Role	Meaning	Examples
Producers	Green plants that make their own food by photosynthesis	Grass, maize, algae
Primary consumers	Herbivores that eat producers	Grasscutter, goat, grasshopper
Secondary consumers	Carnivores or omnivores that eat primary consumers	Snake, frog, cat
Tertiary consumers	Eat secondary consumers	Eagle, hawk, leopard
Decomposers	Break down dead plants and animals and return nutrients to the soil	Bacteria, mushrooms and other fungi

2. Food Chains

A food chain shows who eats whom in a straight line. It starts with a producer. The arrow points to the organism that eats, so it shows the direction of energy flow.

Examples from Ghana:

• Grass, then grasshopper, then frog, then snake, then eagle.
• Maize, then rat, then snake, then hawk.
• Algae, then tilapia, then catfish, then fish eagle (pond ecosystem).

Each step in a food chain is called a trophic level.

3. Food Webs

In nature, an animal usually eats more than one kind of food, and is eaten by more than one kind of animal. Many food chains joined together make a food web. A food web gives a more true picture of feeding in a habitat. If one organism in the web disappears, other organisms are affected.

Example: In a savanna, grass is eaten by grasshoppers and grasscutters. Grasshoppers are eaten by frogs and birds. Grasscutters are eaten by snakes and hawks. Frogs are eaten by snakes. Snakes are eaten by hawks. When these food chains are joined, we get a food web.

4. Energy Flow

• The Sun is the main source of energy. Producers capture it through photosynthesis.
• Energy moves from one level to the next when one organism eats another.
• Only about 10% of the energy is passed on to the next level. The rest is used for life activities or lost as heat.
• So energy decreases along the chain, and food chains are short (usually 4 or 5 levels).
• Energy flows in one direction. It is not recycled, but nutrients are.

5. Pyramid of Numbers

There are many producers, fewer herbivores and still fewer carnivores. So the pyramid is wide at the bottom and narrow at the top.

6. Effects of Removing an Organism

If all the snakes were killed, the rats would increase and eat more crops. If the producers are destroyed by bush fires, the animals that depend on them will starve. This shows why we must protect every living thing in a habitat.

Remember: Arrows in a food chain point from the food to the eater, in the direction of energy flow.

Education Always (education.inspiringalways.com). Original notes written to follow the NaCCA Basic 8 Integrated Science curriculum.