Revisit the paper-chain food chains from Lesson 2. Model how two chains can link: "This bird eats insects and fruit, so it belongs in two chains at once." Introduce the term food web — many food chains joined together — and explain that this is a more accurate picture of nature, because most living things eat more than one food and are eaten by more than one animal.
Show the fruit-decomposition time-lapse. Ask what students notice as the fruit rots. Introduce decomposers (fungi, worms, bacteria): they break down dead plants and animals and return nutrients to the soil, which producers use to grow. Model on the board that decomposers connect back to every living thing in the web — the arrows lead back to them, so energy and nutrients cycle.
Run the string food-web activity. Give students role cards (producers, consumers, decomposers) and stand them in a circle. Pass a ball of string from one living thing to something that eats it (or that it feeds), and so on, until a web of string connects the group. Then remove one living thing (they drop their strings) and ask the class to observe how many connections go slack — a vivid model of flow-on effects.
Next, students build a T-chart of indigenous vs introduced WA species using the card set, and discuss: write a simple chain (grass → rock wallaby → wedge-tailed eagle) and ask how it could be broken (e.g. weeds crowding out the grass; foxes hunting wallabies) and trace the impact along the web.
Students complete the Food Webs and Decomposers worksheet: joining several chains into a web, adding a decomposer with arrows leading back to it, sorting indigenous and introduced species, and answering a short "what happens if this link is broken?" prompt.
Exit ticket: "Why are decomposers important to an ecosystem?" answered in one sentence.