الأسبوع 5 · المحور الأول «الأنظمة»، الوحدة الأولى «العلاقات الغذائية بين الكائنات الحية»، المفهوم 1.2 «انتقال الطاقة في النظام البيئي» (ج) والمفهوم 1.3 «التغيرات في الشبكات الغذائية» (أ): تفسير الطاقة وشبكات تتغير · 3 دروس
2
A dried-up lake
Discovery Education Science P5: Term 1, Unit 1, pp. 72–78
GoalExplain why a dried-up lake is not a healthy ecosystem, and predict how a food web changes when its surroundings change.
New words
population · all the living things of one kind in a placedrought · a long time with very little rainconservation · protecting and using nature wisely
Explanation
Think of a lake that has dried up in a long drought. Water is a nonliving part of its ecosystem. Without water, water plants die, fish cannot live, and birds that fed on the fish must leave. So it is not a healthy ecosystem. In a food web every living thing depends on others, so one change can spread. We can think in “if … then …” sentences. If there is a drought, then grass may die, so the population of hares may fall, and the foxes that hunt them have less food. Gentle rain helps plants grow, but a very heavy rain may wash plants away. People protect ecosystems by conservation, for example by not taking too many fish from a reef.
If …
then …
there is gentle rain in the desert
plants grow and there is more food for hares and rodents
there is a very heavy rain
a flood may wash away plants and burrows
a drought kills the grass
hares have less food, their population falls, foxes have less food
Worked examples
A lake near an oasis dries up in a hot summer with no rain. The water is gone, so the fish die and the birds have to leave.
A sea food web. Seaweed is the producer. If all the shrimp disappeared, the octopus would lose one of its foods, and the shark would have fewer octopuses to eat.
Salma writes a prediction for the sea web: “If the seaweed disappears, then the shrimp and small fish lose their food, because they eat seaweed.” She names what changes and says why.
Class activity
If–then cards. Copy these four starters on cards: “If there is a drought …”, “If a flood washes away plants …”, “If there are many more foxes …”, “If the water in a pond dries up …”. Finish each with “then … because …”. Swap cards with a partner and say whether their reasons are sensible.
Practice
1
Why is a dried-up lake not a healthy ecosystem?
The water is gone, so water plants and fish die and other animals lose their food and must leave.
2
Finish the sentence: “If a drought kills the grass, then the hares …”
… have less food, so their population falls, because they eat grass.
3
In the sea web, name two animals that would lose food if the seaweed disappeared.
The shrimp and the small fish eat the seaweed.
4
What does “population” mean?
All the living things of one kind in a place, such as all the hares in a valley.
