Lesson Notes By Weeks and Term v5 - Grade 7

Lesson Video

This page supports the lesson note with a companion video and a short classroom-ready summary.

For class groups and homework, share this lesson page so learners also get the summary, objectives, and full lesson context.

Performance objectives

• Define and give examples of biotic and abiotic factors.
• Describe the roles of producers, consumers, and decomposers.
• Construct a food web and explain energy flow.
• Explain how changes in an ecosystem have ripple effects.
• Describe the role of decomposers in nutrient cycling.

Lesson summary

This week, we delve deeper into understanding how living organisms interact with their environment. This is crucial because the health of our ecosystems directly impacts our lives in South Africa. From the food we eat (derived from healthy agricultural ecosystems) to the water we drink (filtered by healthy wetlands and rivers), and even the air we breathe (cleaned by forests), our well-being is intertwined with the well-being of our environment. Understanding these connections allows us to make informed decisions about how we interact with and protect our natural resources.

Lesson notes

Biotic and Abiotic Factors: An ecosystem is a community of interacting organisms (plants, animals, and microorganisms) and their physical environment. This environment includes both living and non-living components. Biotic factors are the living components of an ecosystem. These include all plants, animals, fungi, bacteria, and other organisms. They interact with each other in various ways, such as through feeding relationships (food chains and food webs), competition for resources, and symbiosis (close relationships between different species). Abiotic factors are the non-living components of an ecosystem. These include things like sunlight, water, temperature, soil, minerals, air, and climate. Abiotic factors influence the types of organisms that can survive and thrive in a particular ecosystem.

Example: Consider a South African veld.

Biotic Factors:* Grasses (various species), grazing animals like zebras and springbok, predators like lions and cheetahs, insects like grasshoppers and termites, bacteria and fungi in the soil.

Abiotic Factors:* Sunlight, rainfall, temperature fluctuations, soil type (sandy, clay), wind, availability of minerals like nitrogen and phosphorus in the soil. Producers, Consumers, and Decomposers: Organisms within an ecosystem obtain energy in different ways.

Producers: These are organisms that make their own food using energy from the sun through a process called photosynthesis. Plants are the primary producers in most ecosystems. They convert sunlight, water, and carbon dioxide into glucose (a type of sugar) and oxygen. In aquatic ecosystems, algae and phytoplankton are important producers.

Consumers: These are organisms that obtain energy by eating other organisms.

There are different types of consumers: Herbivores: These eat only plants (e.g., springbok grazing on grass).

Carnivores: These eat only animals (e.g., a lion eating a zebra).

Omnivores: These eat both plants and animals (e.g., a baboon eating fruits, insects, and small animals).

Scavengers: These feed on dead animals (e.g., vultures feeding on a carcass).

Decomposers: These are organisms that break down dead plants and animals and other organic matter. Decomposers, primarily bacteria and fungi, recycle nutrients back into the soil, making them available for producers to use. Without decomposers, nutrients would remain locked up in dead organisms, and the ecosystem would eventually collapse.

Food Chains and Food Webs: A food chain is a linear sequence of organisms in which each organism feeds on the one below it. It shows the flow of energy from one organism to another.

Example: Grass → Grasshopper → Frog → Snake → Eagle A food web is a more complex diagram that shows the interconnected food chains within an ecosystem. It represents the feeding relationships among all the organisms in a community. Food webs are more realistic than food chains because most organisms eat more than one type of food. Example of a simplified South African grassland food web: Producers:* Grasses, Wildflowers Herbivores:* Springbok, Zebras, Grasshoppers Carnivores:* Lions, Cheetahs, Wild Dogs, Snakes, Eagles Omnivores:* Baboons, Some Birds Decomposers:* Bacteria, Fungi in the soil Arrows in the food web indicate the flow of energy. The arrow points from the organism being eaten to the organism that is eating it.

Energy Flow and Nutrient Cycling: Energy flows through an ecosystem in a one-way direction. The initial source of energy is the sun. Producers capture this energy and convert it into chemical energy through photosynthesis. When consumers eat producers or other consumers, they obtain some of that energy.

However, at each level, some energy is lost as heat during metabolic processes. This is why food chains and food webs typically have only a few levels; there is not enough energy to support many more levels. Nutrients, on the other hand, are recycled within an ecosystem. Decomposers play a crucial role in this process. They break down dead organisms and waste products, releasing nutrients back into the soil. These nutrients are then absorbed by producers, completing the cycle. Key nutrients include nitrogen, phosphorus, and potassium, all vital for plant growth.

Impact of Changes in Ecosystems: Ecosystems are dynamic and constantly changing. Changes can be caused by natural events (e.g., droughts, floods, fires) or by human activities (e.g., deforestation, pollution, introduction of invasive species). Any change in one part of an ecosystem can have ripple effects throughout the entire system.

Example 1: A prolonged drought in a grassland can kill off many of the grasses, leading to a decline in the population of herbivores (like springbok) that depend on the grasses for food. This, in turn, can affect the predators (like lions) that prey on the herbivores.

Example 2: The introduction of an invasive plant species (e.g., lantana) can outcompete native plants, reducing the diversity of plant life in an area.