ACARA v9 CONTENT DESCRIPTION “use models, including food webs, to represent matter and energy flow in ecosystems and predict the impact of changing abiotic and biotic factors on populations”
Builds on earlier work grouping living things and observing what they need to survive. A food web turns those needs into a model: a map of who eats whom, and which way energy and matter travel through a community.
A food web is a model of energy flow
Every living thing needs energy. Plants, the producers, capture it from sunlight; animals get it by eating other living things. A food web draws these feeding links as arrows, and each arrow points the way the energy travels: from the organism that is eaten to the one that eats it. Reading the web shows how every species is connected.
Energy flows through a food web
Each arrow points from the living thing eaten to the one that eats it. Remove a species to predict what it affects.
Arrows show the direction energy travels: from grass to the animals that eat it, and on up to the hawk. Select a species to remove it and see which others depend on it.
Living and non-living factors both matter
Two kinds of change can shift the populations in an ecosystem. Biotic factors are living: removing a predator or a food source. Abiotic factors are non-living: rainfall, temperature or sunlight. Because every species is linked, a change to one factor rarely stays in one place; it ripples through the web.
An abiotic change ripples outward
Rainfall is an abiotic factor. Less rain means less grass, and that shortage travels up the web.
In a normal year the producers support the largest population, and each level above is smaller. Switch to drought to follow the shortage up the web.
Energy is lost at every step
Energy enters the web when producers capture sunlight, and it moves up as one living thing eats another. But energy is not passed on perfectly. At each step only about a tenth of the energy reaches the level above; the rest is used for living and lost as heat. That is why there are far more plants than plant-eaters, and only a few top predators.
Energy shrinks up the pyramid
Producers capture the most energy. Reveal each level above and see how much energy is left for it.
With 2 levels shown, the highest level here holds about 1000 of the 10000 energy units the producers captured. Only about a tenth of the energy passes to the next level up; the rest is used for living and lost as heat. That is why food chains rarely have more than four or five links.
Every organism has a role
In an ecosystem each living thing plays one of three roles. Producers, such as plants, make their own food from sunlight. Consumers eat other living things to get their energy. Decomposers, such as fungi and worms, break down dead material and return its nutrients to the soil, where producers can use them again. Together these roles keep matter cycling.
Producer, consumer or decomposer
Pick an organism, then tag its role. A green mark means the role is right; a gold mark means try again.
You have tagged 0 of 6 organisms and 0 are correct. Producers such as plants make their own food from sunlight, consumers eat other living things, and decomposers such as fungi and worms break down the dead and return nutrients to the soil.
Predicting the impact of a change
Because every species is linked, changing one part of a community shifts the others. Adding a new predator can thin out the animals it hunts, which lets their own prey increase. Removing the producers takes away the energy source the whole community depends on. A model lets you predict the direction of each change before it happens.
Introduce a change
Apply one change to a grass, rabbit and fox community and predict how each population responds.
This is the starting balance: plenty of grass supports the rabbits, and the rabbits support a smaller number of foxes. Apply a change to predict how the populations respond.
Why this matters
Food webs let scientists predict, rather than just describe. Before a species is removed or a habitat changes, a web shows which populations are likely to rise or fall. That makes it a powerful tool for protecting ecosystems and understanding our own effect on them.
Quick self-check
1. In a food web, what does an arrow point toward?
2. Which organisms are the original source of energy in this web?
3. If the grasshoppers disappeared, which is most directly affected?
4. Rainfall is an example of which kind of factor?
5. Why does a drought reduce the number of hawks, even though hawks do not eat grass?