GoalDecide when a change helps (benefit) or hurts (harm) a habitat, read goat-game data, and plan a brochure about a problem.
New words
benefit · something that helpsharm · something that hurts or damageswater hyacinth · a floating plant (ward el-Nil in Arabic) that spreads fast on waterbrochure · a folded paper that gives information about a topic
Explanation
Plants and animals can change a place. A change that helps the habitat is a benefit. A change that hurts it is a harm. Many changes do both. Goats eating grass are a good example. A few goats eat weeds, and their droppings feed the soil. But if there are too many goats, they eat all the plants and the soil is left bare. Water hyacinth, called ward el-Nil, is a floating plant with pretty flowers, and some people collect it for crafts. But it spreads quickly over a river or a canal. It can block boats and shut out the light that water plants and fish need. So it can both benefit and harm. We look for ways to manage it, and we do not blame anyone. The goat game shows how numbers change. When there is plenty of food the herd grows. If the herd becomes too big, food runs short and the next year the herd is smaller. Then the plants grow back and the herd grows again. So the number of goats goes up and down and never stays the same. To tell people about a problem, a brochure plan has five boxes: the problem, its effect, its cause, possible solutions and where to find more information.
Benefit
Harm
Goats
a few eat weeds; droppings feed the soil
too many eat all the plants
Water hyacinth
pretty flowers; used for crafts
covers the water; blocks boats and light
Worked examples
Change
Benefit or harm?
bees carry pollen between flowers
benefit
litter dropped in a canal
harm
planting trees
benefit
cutting down all the trees on a hillside
harm
Benefit or harm? Bees carrying pollen between flowers: benefit. Litter dropped in a canal: harm. Planting trees: benefit. Cutting down all the trees on a hillside: harm, because the soil can wash away.
The goat game. Rule: if the herd has 20 or more goats at the start of a year, food runs short and 6 goats leave. If it has fewer than 20, 5 kids are born. The class starts with 10 goats and writes the number at the end of each year: 15, 20, 14, 19, 24, 18. The most goats was 24 in year 5. In year 6 the herd fell by 24 − 18 = 6, because food ran short.
Box
Our plan
Problem
a floating plant covers the water in a canal
Effect
boats get stuck and less light reaches the water
Cause
the plant spreads very fast
Solutions
grown-ups remove it and stop it spreading
More information
a teacher or a science book
A brochure plan about water hyacinth.
Class activity
Play the goat game with paper. Start with 10 paper goats. Each round is one year, and a grown-up helps you keep the rule: with 20 or more goats at the start, take away 6; with fewer than 20, add 5. Play ten years. Write the start and end numbers in a table, then draw a bar graph. Answer: what happened after the biggest herd? Why did the number never stay the same? Then plan a brochure about a problem you know, such as litter, using the five boxes.
Practice
1
Give one benefit and one harm of goats eating grass.
Benefit: a few goats eat weeds and feed the soil. Harm: too many goats eat all the plants.
2
Why does the number of goats never stay the same in the game?
A big herd uses up the food, so it shrinks. A small herd lets the plants grow back, so it grows again.
3
In which year did the herd get smaller, and by how many goats?
Year 3: from 22 to 16, which is 22 − 16 = 6 fewer goats.
4
Is water hyacinth a benefit or a harm? Explain.
Both. It has flowers and can be used for crafts, but it can cover the water, block boats and shut out light.
