Lesson Notes By Weeks and Term v5 - Grade 7

Lesson Video

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Performance objectives

• Construct and interpret food chains and food webs using South African examples.
• Describe the roles of producers, consumers, and decomposers.
• Explain the flow of energy through a food chain/web.
• Identify the impact of human activities on local ecosystems.
• Differentiate between a food chain and a food web.

Lesson summary

This week, we delve deeper into the fascinating world of living organisms and their environments. Understanding how organisms interact with each other and their surroundings is crucial, especially in South Africa, where we have incredible biodiversity and are faced with unique environmental challenges like water scarcity, deforestation, and the impact of invasive species. Learning about these interactions helps us appreciate the delicate balance of nature and empowers us to make informed decisions about how to protect our environment for future generations.

Lesson notes

Food Chains and Food Webs: A food chain is a linear sequence of organisms through which nutrients and energy pass as one organism eats another. It illustrates a simple feeding relationship.

For example: Grass → Grasshopper → Mouse → Snake → Eagle Here, the grass is eaten by the grasshopper, which is then eaten by the mouse, and so on. Each step in the food chain is called a trophic level. A food web, on the other hand, is a more complex and realistic representation of feeding relationships in an ecosystem. It consists of interconnected food chains, showing that most organisms eat (and are eaten by) multiple species. Think of it like a spider web - lots of connections!

Why Food Webs are Important: Food webs are crucial for ecosystem stability. If a single food chain is disrupted, the entire ecosystem is less likely to be affected because organisms can rely on alternative food sources within the web. Producers, Consumers, and Decomposers: Every organism in an ecosystem plays a specific role: Producers: These are organisms that make their own food through photosynthesis. They are the foundation of all food chains and webs. Examples include grasses, trees (like the acacia tree in the savanna), algae in rivers and oceans, and maize plants in a mealie field. Producers are autotrophs (self-feeders).

Consumers: These are organisms that obtain their energy by eating other organisms. They are heterotrophs (other-feeders).

Consumers are further divided into: Herbivores: Eat only plants (e.g., grasshoppers, cows, elephants, springbok).

Carnivores: Eat only animals (e.g., lions, snakes, sharks, eagles).

Omnivores: Eat both plants and animals (e.g., humans, chickens, monkeys, baboons).

Decomposers: These are organisms that break down dead plants and animals, returning nutrients to the soil. This process is essential for recycling nutrients and supporting plant growth. Examples include bacteria, fungi (like mushrooms), and earthworms. Imagine if nothing ever decomposed! We'd be buried in dead leaves and animals! Energy Transfer in Food Chains and Food Webs: Energy flows through food chains and food webs from producers to consumers. The initial source of energy is usually the sun, which plants use to produce food through photosynthesis.

However, energy is lost at each trophic level. This is because organisms use energy for their own life processes (e.g., movement, growth, reproduction), and some energy is lost as heat. The 10% Rule: A useful (though simplified) rule of thumb is the "10% rule," which states that only about 10% of the energy stored in one trophic level is transferred to the next trophic level. The other 90% is used by the organism or lost as heat. This is why food chains typically have only 4 or 5 trophic levels – there isn't enough energy to support more!