Lesson Notes By Weeks and Term v5 - Grade 7

Lesson Video

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Performance objectives

• Define and explain food chains and food webs.
• Identify different trophic levels (producers, consumers, decomposers).
• Analyze the impact of human activities on food webs.
• Explain interdependence and symbiotic relationships.
• Construct and interpret simple food webs.

Lesson summary

This week, we delve deeper into the fascinating world of living organisms and their environments, focusing specifically on how living things interact with each other and their surroundings. Understanding these interactions is crucial, especially in South Africa, as it helps us appreciate the delicate balance of our ecosystems and the impact human activities have on our natural resources. We will explore concepts like food webs, interdependence, and how changes in one part of an ecosystem can affect everything else. This knowledge is vital for responsible environmental stewardship and sustainable living in our beautiful country.

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 who eats whom in a particular environment. The arrow in a food chain represents the flow of energy.

For example: Grass -> Grasshopper -> Mouse -> Snake -> Eagle This food chain shows how energy from the sun, captured by the grass, is transferred to the grasshopper when it eats the grass. Then, the energy is transferred to the mouse when it eats the grasshopper, and so on. A food web, on the other hand, is a more complex network of interconnected food chains. In reality, organisms often eat more than one type of food, and they may be eaten by more than one type of predator. A food web represents these complex feeding relationships within an ecosystem.

Example (South African context): Imagine a grassland ecosystem in the Free State.

A food web might include: Producers: Grasses, shrubs, wildflowers Primary Consumers (Herbivores): Springbok, zebras, grasshoppers, caterpillars Secondary Consumers (Carnivores/Omnivores): Meerkats, snakes, birds of prey (e.g., eagles, falcons), spiders Decomposers: Bacteria, fungi, earthworms The springbok might eat grass, and the zebra might eat shrubs. The meerkat might eat grasshoppers and spiders. The eagle might eat snakes and meerkats. Decomposers break down dead organisms and waste, returning nutrients to the soil, which are then used by the producers. Trophic Levels Trophic levels describe the position an organism occupies in a food chain or food web. There are typically four main trophic levels: Producers (Autotrophs): These are organisms that make their own food through photosynthesis (using sunlight, water, and carbon dioxide) or chemosynthesis. Plants, algae, and some bacteria are producers. They form the base of the food chain.

Primary Consumers (Herbivores): These are animals that eat producers. Examples include springbok, zebras, caterpillars, and grasshoppers. Secondary Consumers (Carnivores/Omnivores): These are animals that eat primary consumers. Carnivores eat only animals, while omnivores eat both plants and animals. Examples include meerkats, snakes, spiders, and some birds.

Tertiary Consumers (Top Predators): These are animals that eat secondary consumers. They are often at the top of the food chain and are not preyed upon by other animals. Examples include eagles, lions (although lions can also be secondary consumers depending on what they eat).

Decomposers (Detritivores): These organisms break down dead plants and animals, returning essential nutrients to the soil. Examples include bacteria, fungi, and earthworms. They play a vital role in nutrient cycling.

Worked example

Consider the following organisms: Lion, Grass, Zebra, Hyena, Wildebeest, Fungi

Classify each organism into its correct trophic level:

Lion: Tertiary Consumer (can also be secondary, depending on the prey)

Grass: Producer

Zebra: Primary Consumer

Hyena: Secondary/Tertiary Consumer (Scavenger)

Wildebeest: Primary Consumer

Fungi: Decomposer

Interdependence and Symbiotic Relationships

Interdependence refers to the reliance of organisms on each other for survival. In an ecosystem, all organisms are interconnected, and changes in one population can affect other populations.

Symbiosis is a close and long-term interaction between two different species. There are three main types of symbiotic relationships:

Mutualism: Both species benefit from the interaction. For example, bees pollinating flowers. The bee gets nectar (food), and the flower gets pollinated (reproduction). Another example, relevant to South Africa, is the relationship between oxpeckers and zebras/rhinos. The oxpecker eats ticks and other parasites off the zebra/rhino, providing the zebra/rhino with relief and the oxpecker with a food source.

Commensalism: One species benefits, and the other is neither harmed nor helped. An example would be birds nesting in trees. The bird gets shelter, and the tree is unaffected.

Parasitism: One species (the parasite) benefits, and the other species (the host) is harmed. An example would be ticks feeding on a dog. The tick gets food, and the dog loses blood and may contract diseases. Another example is the Dodder plant (a parasitic plant) that extracts nutrients from host plants, harming them.

Impact of Human Activities