Lesson Notes By Weeks and Term v5 - Grade 8

Lesson Video

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Performance objectives

• Define an ecosystem and its components (biotic & abiotic).
• Explain and construct food chains and food webs.
• Describe the different types of interactions between organisms.
• Explain the role of decomposers in nutrient cycling.
• Explain how changes in one part affect the whole ecosystem.

Lesson summary

This week, we delve into the fascinating world of ecosystems and the interactions that occur within them. Understanding ecosystems is crucial for South African learners because our country boasts incredible biodiversity, from the fynbos biome of the Western Cape to the savannahs of Limpopo. The health of these ecosystems directly impacts our livelihoods through agriculture, tourism, and access to clean water. Climate change, pollution, and unsustainable resource use all threaten these ecosystems, making it essential for us to understand how they function and how we can protect them.

Lesson notes

What is an Ecosystem? An ecosystem is a community of living organisms (biotic factors) interacting with each other and with their physical environment (abiotic factors). It's a dynamic system where energy flows and nutrients cycle.

Biotic Factors: These are the living components of an ecosystem, including plants (producers), animals (consumers), fungi, and bacteria (decomposers). In a South African context, think of the lions, zebras, and acacia trees of Kruger National Park, or the kelp forests and seals along the Cape coast.

Abiotic Factors: These are the non-living components of an ecosystem, such as sunlight, temperature, water, soil, and air. For example, the hot, dry climate of the Karoo influences the types of plants and animals that can survive there.

Food Chains and Food Webs Food Chain: A food chain is a linear sequence of organisms through which nutrients and energy pass as one organism eats another. Each organism occupies a specific trophic level.

Producers: These are organisms that make their own food through photosynthesis (e.g., plants, algae). In a South African grassland, grasses are primary producers.

Consumers: These are organisms that eat other organisms to obtain energy.

Primary Consumers (Herbivores): Eat producers (e.g., zebras eating grass). Secondary Consumers (Carnivores or Omnivores): Eat primary consumers (e.g., lions eating zebras).

Tertiary Consumers (Carnivores): Eat secondary consumers (e.g., a hawk eating a snake).

Decomposers: These are organisms that break down dead plants and animals, returning nutrients to the soil (e.g., bacteria, fungi). Think of mushrooms growing on decaying logs in a forest.

Example Food Chain: Grass → Zebra → Lion → Bacteria (Decomposers)

Food Web: A food web is a more complex network of interconnected food chains. It shows the feeding relationships between all the organisms in an ecosystem. It's more realistic than a food chain because organisms often eat a variety of things. A food web in the fynbos biome might include proteas (producer), sugarbirds (primary consumer), spiders (secondary consumer - eating insects which eat the protea), and eagles (tertiary consumer).

Interactions Between Organisms Predation: One organism (the predator) kills and eats another organism (the prey).

Example: A cheetah hunting a springbok.

Competition: Occurs when two or more organisms require the same limited resources, such as food, water, or shelter.

Example: Lions and hyenas competing for the same prey animals.

Mutualism: A relationship where both organisms benefit.

Example: Bees pollinating flowers; the bee gets nectar, and the flower gets pollinated. Another example is the mutualistic relationship between nitrogen-fixing bacteria and legume plants, crucial for soil fertility in South African agriculture.

Commensalism: A relationship where one organism benefits, and the other is neither harmed nor helped.

Example: Birds nesting in trees. The tree is unaffected, but the bird gets a safe place to live.

Parasitism: A relationship where one organism (the parasite) benefits by living on or in another organism (the host) and harms the host.

Example: Ticks feeding on a dog. Role of Decomposers and Nutrient Cycling Decomposers, like bacteria and fungi, break down dead organic matter (dead plants, animals, and waste products). This process releases nutrients, such as nitrogen and phosphorus, back into the soil. These nutrients are then used by plants to grow, completing the nutrient cycle. Without decomposers, nutrients would be locked up in dead organisms, and plant growth would be severely limited. Think about how compost heaps in South African gardens rely on decomposers to create nutrient-rich soil. Impact of Changes in Ecosystems Ecosystems are interconnected, so changes in one part of the ecosystem can have ripple effects throughout the entire system.

Example 1: Deforestation: Cutting down forests removes habitats for many animals, leading to population declines. It also reduces the amount of carbon dioxide absorbed by trees, contributing to climate change. Soil erosion also increases as the tree roots are no longer there to hold the soil.

Example 2: Invasive Species: Introducing a non-native species can disrupt the balance of an ecosystem. For example, the water hyacinth is an invasive plant in many South African rivers and dams. It grows rapidly, blocking sunlight and reducing oxygen levels, harming native aquatic plants and animals.

Example 3: Overfishing: Overfishing can decimate fish populations and disrupt the food web in marine ecosystems. It can also affect the livelihoods of people who depend on fishing for their food and income. Guided Practice (With Solutions)

Question 1: Identify the biotic and abiotic components of a freshwater ecosystem like a dam in South Africa.

Solution: Biotic: Fish, aquatic plants (e.g., water lilies, reeds), algae, insects (e.g., mosquitoes, dragonflies), frogs, bacteria, and fungi.