الأسبوع 4 · المحور الأول «الأنظمة»، الوحدة الأولى «العلاقات الغذائية بين الكائنات الحية»، المفهوم 1.2 «انتقال الطاقة في النظام البيئي» (ب): السلاسل والشبكات الغذائية · 3 دروس
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Food webs and clean-up crews
Discovery Education Science P5: Term 1, Unit 1, pp. 57–66
GoalShow food chains joining into a food web, and explain what scavengers and decomposers do.
New words
food web · many food chains that cross and join in one ecosystemscavenger · an animal that eats the remains of dead animalsdecomposer · a living thing, such as a fungus, that breaks dead matter into nutrients
Explanation
Most animals eat more than one kind of food, and most are eaten by more than one kind of animal. So food chains cross over and join. Many joined chains make a food web. Arrows in a web also point from the food to the eater. A web is a better model than one chain because it shows all the feeding links. When plants and animals die, scavengers such as vultures, crabs and flies eat the remains and break them into small pieces. Then decomposers, such as fungi, bacteria and worms, finish the job and turn them into nutrients. The nutrients return to the soil, and plants use them again. Decomposition is nature’s recycling.
Worked examples
In the web above there are several chains: grass, mouse, snake; grass, mouse, fox; and grass, hare, eagle. The mouse is food for both the snake and the fox.
A fallen leaf on the ground is eaten by snails and worms and broken down by fungi and bacteria. The nutrients go into the soil and a new plant uses them.
Group
Examples
What they do
scavengers
vultures, crabs, flies
eat remains of dead animals, break them into pieces
decomposers
fungi, bacteria, worms
break dead matter into nutrients for the soil
This table compares scavengers and decomposers.
Class activity
Neighbourhood web (with a grown-up). Walk slowly outside near school or home and write down five or more living things you see: plants, insects, birds, cats. Do not touch or disturb them. At home draw each as a name in a box and add arrows from each food to its eater. If you did not see who eats what, look it up or ask a grown-up. Add one decomposer, such as a mushroom or a worm in the soil.
Practice
1
Why is a food web a better model than a single food chain?
Because most animals eat and are eaten by more than one kind of living thing, and a web shows all the links.
2
In the web, which animals eat the mouse? Name two.
The snake and the fox.
3
What do decomposers do, and why does this matter for plants?
They break dead plants and animals into nutrients that go back into the soil, where plants use them.
4
Name one scavenger and one decomposer.
Scavenger: for example a vulture, crab or fly. Decomposer: for example a fungus, bacterium or worm.
