Lesson Notes By Weeks and Term v5 - Grade 7

Lesson Video

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Performance objectives

• Define producers, consumers, and decomposers.
• Construct simple food chains and food webs.
• Explain the sun's role as the primary energy source.
• Describe the impact of removing organisms from a food web.
• Explain why decomposers are essential for nutrient recycling.

Lesson summary

This week, we delve deeper into the fascinating world of living organisms and how they interact with their environments. Understanding these interactions is crucial, especially in South Africa, where we have unique and diverse ecosystems facing various challenges like climate change, pollution, and habitat destruction. Knowing how organisms depend on each other and their surroundings will help us appreciate the importance of conservation and sustainable living in our communities and country. We'll focus specifically on food chains, food webs, and the crucial roles of producers, consumers, and decomposers.

Lesson notes

2.1 Ecosystems: A Web of Life An ecosystem is a community of living organisms (plants, animals, and microorganisms) that interact with each other and their physical environment (soil, water, air, sunlight). Ecosystems can be as small as a puddle or as large as the Kruger National Park. The key is the interaction between living (biotic) and non-living (abiotic) factors. 2.2 Producers: The Energy Starters Producers are organisms that make their own food through a process called photosynthesis. They convert sunlight, water, and carbon dioxide into glucose (sugar), which is their source of energy. Plants, algae, and some bacteria are producers. In a South African context, think of the grasses in the grasslands, the trees in the forests, and the algae in our rivers and oceans. These producers form the base of most food chains. 2.3 Consumers: Eating to Survive Consumers are organisms that cannot make their own food and must obtain energy by eating other organisms.

There are different types of consumers: Herbivores: These eat only plants (e.g., cows, sheep, springbok, caterpillars).

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

Omnivores: These eat both plants and animals (e.g., humans, chickens, pigs, some monkeys). Consumers are classified further by their trophic level (feeding level). For example, a primary consumer eats a producer (a herbivore). A secondary consumer eats a primary consumer (often a carnivore eating a herbivore). A tertiary consumer eats a secondary consumer (a carnivore eating another carnivore). 2.4 Decomposers: The Clean-Up Crew Decomposers are organisms that break down dead plants and animals and waste materials, releasing nutrients back into the environment. Bacteria and fungi are the main decomposers. They play a vital role in recycling nutrients, making them available for producers to use again. Imagine a dead tree falling in a forest; decomposers break it down, releasing minerals back into the soil that other plants can use. 2.5 Food Chains: A Simple Energy Pathway A food chain is a linear sequence of organisms through which nutrients and energy pass as one organism eats another. It shows who eats whom. It always starts with a producer.

For example: Grass → Springbok → Cheetah In this food chain: Grass is the producer. Springbok is the primary consumer (herbivore). Cheetah is the secondary consumer (carnivore). 2.6 Food Webs: Interconnected Food Chains A food web is a complex network of interconnected food chains. It shows how different food chains are linked together in an ecosystem. This is a more realistic representation of what happens in nature, as most organisms eat more than one type of food. For example, a springbok might eat different types of grasses and a cheetah might also hunt other animals besides springboks. 2.7 The Sun: The Ultimate Energy Source The sun is the primary source of energy for almost all ecosystems. Producers use sunlight to make food through photosynthesis. This energy is then passed on to consumers when they eat producers or other consumers. Each time energy is transferred from one organism to another, some of it is lost as heat. This is why food chains and food webs usually have a limited number of trophic levels (typically 3-5). 2.8 Impact of Removing Organisms: Removing organisms from a food chain/web can have significant and often unpredictable consequences. For example, the overhunting of sharks (apex predators) off the South African coast can lead to an increase in the populations of their prey, which can then overgraze kelp forests, damaging these vital marine ecosystems. Similarly, the spread of alien invasive plants like wattle can outcompete native plants, reducing biodiversity and affecting the animals that depend on those native plants. 2.9 Decomposers and Nutrient Cycling: Decomposers are vital for nutrient cycling. When organisms die, decomposers break down their remains, releasing essential nutrients like nitrogen and phosphorus back into the soil. These nutrients are then taken up by plants, completing the cycle and ensuring the continued health of the ecosystem. Without decomposers, nutrients would be locked up in dead organisms, and the ecosystem would eventually collapse. Guided Practice (With Solutions)

Question 1: Identify the producer, primary consumer, and secondary consumer in the following food chain: Karee tree leaves → Kudu → Leopard Solution: Producer: Karee tree leaves Primary Consumer: Kudu (herbivore that eats the Karee tree leaves)

Secondary Consumer: Leopard (carnivore that eats the Kudu)

Commentary: This question tests the basic understanding of the roles of different organisms in a food chain. It emphasizes identifying the flow of energy from producer to consumer.

Question 2: Draw a simple food chain involving the following organisms: Sun, Grass, Zebra, Lion. Indicate the direction of energy flow using arrows.