Lesson Notes By Weeks and Term v5 - Grade 8

Lesson Video

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Performance objectives

• Define an ecosystem and identify its biotic and abiotic components.
• Explain and illustrate food chains and food webs with South African examples.
• Describe different types of interactions between organisms.
• Explain the role of decomposers in nutrient cycling.
• Explain the flow of energy through an ecosystem using trophic levels.

Lesson summary

South Africa is a country blessed with incredible biodiversity, from the fynbos of the Western Cape to the savannahs of Kruger National Park. Understanding ecosystems and the interactions within them is crucial for managing and protecting this natural heritage. Think about the impact of drought on farmers' crops, or the effect of pollution on the fish in our rivers – these are all connected to the health of our ecosystems. This week, we will delve deeper into how living things interact with each other and their environment, and how these interactions influence the balance of nature around us.

Lesson notes

What is an Ecosystem? An ecosystem is a community of interacting organisms (plants, animals, and microorganisms) and their physical environment (soil, water, air). It's a dynamic system where living (biotic) and non-living (abiotic) components are interconnected.

Biotic Components: These are all the living organisms in an ecosystem.

Examples include: Producers (Autotrophs): These are organisms that make their own food through photosynthesis, like plants. In South Africa, examples include grasses in the savannah, trees in forests, and algae in rivers and oceans.

Consumers (Heterotrophs): These are organisms that eat other organisms.

We can further classify them as: Herbivores: Eat plants (e.g., zebras, cows, caterpillars).

Carnivores: Eat meat (e.g., lions, eagles, snakes).

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

Decomposers: These break down dead organisms and waste, returning nutrients to the soil (e.g., bacteria, fungi).

Abiotic Components: These are the non-living parts of an ecosystem.

Examples include: Sunlight: Provides energy for photosynthesis.

Water: Essential for all life processes.

Soil: Provides nutrients and support for plants.

Air: Provides oxygen for respiration and carbon dioxide for photosynthesis.

Temperature: Affects the rate of biological processes.

Rainfall: Influences the availability of water.

Food Chains and Food Webs Food Chain: A linear sequence of organisms through which nutrients and energy pass as one organism eats another.

For example: Grass → Zebra → Lion. This shows how energy flows from the grass (producer) to the zebra (herbivore) and then to the lion (carnivore). A typical South African food chain might look like this: Mielies (corn) -> Corn Earworm -> Hadeda Ibis.

Food Web: A more complex network of interconnected food chains. It represents the feeding relationships within an ecosystem more accurately because most organisms eat more than one type of food. For example, a grasshopper might be eaten by a bird, a lizard, or even a spider. A food web is made up of many food chains. Consider the Klaserie Private Nature Reserve near Kruger National Park. Their food web will be very complex, with many organisms depending on others for food. Interactions Between Organisms Organisms interact with each other in many ways, affecting their survival and reproduction.

Some common types of interactions are: Predation: One organism (the predator) kills and eats another organism (the prey).

Example: A leopard (predator) hunting an impala (prey).

Competition: Organisms compete for limited resources such as food, water, shelter, or mates.

Example: Two male kudus competing for a female. This can happen between different species (interspecific competition) or within the same species (intraspecific competition). Another example is various grasses competing for sunlight in the grasslands.

Mutualism: A relationship where both organisms benefit.

Example: Bees pollinating flowers. The bee gets nectar (food), and the flower gets pollinated (reproduction). Another local example is the relationship between oxpeckers and zebras. The oxpeckers eat ticks and other parasites off the zebra, benefitting both species.

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

Example: Epiphytes (plants that grow on other plants) like orchids growing on the branches of trees. The orchid gets support and access to sunlight, while the tree is unaffected.

Parasitism: A relationship where one organism (the parasite) benefits at the expense of the other organism (the host).

Example: A tick (parasite) feeding on the blood of a dog (host). The tick benefits, while the dog is harmed. Another example is Striga (witchweed), a parasitic plant that attacks the roots of maize crops in South Africa, causing significant yield losses. Decomposers and Nutrient Cycling Decomposers, like bacteria and fungi, play a vital role in breaking down dead organisms and waste products into simpler substances. This process releases nutrients back into the soil, which are then used by plants. This cyclical process is known as nutrient cycling and is essential for maintaining the health and productivity of ecosystems. Imagine a dead tree falling in a forest. Without decomposers, it would just lie there, locking up valuable nutrients. Decomposers release these nutrients, allowing new plants to grow. Energy Flow and Trophic Levels Energy flows through an ecosystem from the sun to producers, then to consumers. Each step in this flow is called a trophic level.

Trophic Level 1: Producers (plants) capture energy from the sun.

Trophic Level 2: Primary consumers (herbivores) eat producers.

Trophic Level 3: Secondary consumers (carnivores) eat primary consumers.

Trophic Level 4: Tertiary consumers (top predators) eat secondary consumers. Only about 10% of the energy is transferred from one trophic level to the next. The rest is lost as heat during metabolic processes.