الأسبوع 12 · الدرس الثاني «الانتخاب الطبيعي والانتخاب الصناعي»: الجماعات الحيوية ونموها والسعة التحملية والعوامل البيئية والوراثية والانتخاب الطبيعي والصناعي وتقنياته · 3 دروس
1
How populations grow, and where growth stops
Science Prep 3 Term 1, Unit 3: pp. 75–83
GoalDescribe how a population grows, what carrying capacity is, and how environmental resistance limits growth.
New words
population · all the individuals of one species living in the same placecarrying capacity · the largest number of individuals that a place can support for a long timeenvironmental resistance · the factors that limit growth, such as food, water, space, disease and predators
Explanation
A population is all the individuals of one species in one place. With plenty of food and space it can grow very fast, doubling each round: 2, 4, 8, 16 and 32. This is exponential growth, and it makes a curve that bends upwards. But no place has unlimited resources. Environmental resistance (limited food, water and space, disease, predators) slows the growth until the population levels off near the carrying capacity, the largest number that the place can support for a long time. The whole curve looks like an S. If a population rises far above the carrying capacity for a while, the food runs short and the numbers fall again. The carrying capacity is not fixed: a rainy year with more grass raises it. Scientists study populations to protect rare species and to use resources wisely.
Worked examples
Pond snails start with 2 and double every round: 2, 4, 8, 16, 32, 64. After 5 doublings there are 2 × 2⁵ = 64. In real ponds this does not go on, because food and space run out.
A field can feed at most 50 rabbits for a long time. Its carrying capacity is 50. With 80 rabbits, food runs short, so some rabbits move away or die and the number drops toward 50.
A dry year: the grass is half as much, so the carrying capacity of the field goes down. Environmental resistance depends on the conditions of each year.
Class activity
Bean growth game with a grown-up. Place 2 beans in the middle of a sheet. Each round, double the beans, but stop when the sheet’s “space” (a drawn circle that holds 20 beans) is full. Record the number each round and plot a graph on squared paper. Where does it bend? Then make the circle smaller and play again to see how the carrying capacity changes.
Practice
1
What does exponential growth mean?
The population multiplies by the same factor in each round, such as doubling.
2
What is the carrying capacity?
The largest number of individuals a place can support for a long time.
3
Name three parts of environmental resistance.
Food, water or space, disease, predators (any three).
4
2 insects double 4 times. How many are there?
2 × 2⁴ = 32.
5
What shape does a growth curve with a carrying capacity have?
