seegongsik
Grade 10-11 (age 15-17)

high school Population

A population is a group of individuals of the same species living together in one area, and its density is the number of individuals divided by the area or volume they occupy. If food and space were unlimited, numbers would shoot up in a J shape, but in reality environmental resistance, such as food shortage, competition for space, predators and disease, makes them grow in an S shape and level off at the carrying capacity K. Populations also differ in the age at which most individuals die, giving different survivorship curves, and members of the same species form relationships such as territories and dominance hierarchies that reduce clashes. In the diagram you can change the carrying capacity K from 30 to 100 and see the level at which the S-shaped curve settles, along with the environmental resistance revealed by the gap between the J-shaped and S-shaped curves.

Individuals of One Species Form a Population

🐭 What Is a Population?
①A population is a group of individuals of the same species living together in one area
②Population density is the number of individuals divided by the area (or volume) of their living space
③As numbers grow, food and space run short and wastes build up, so a population cannot grow without limit

Growth Curves: J-Shaped and S-Shaped

70
📈 Environmental Resistance Bends the Curve
①The theoretical growth curve (J-shaped) assumes unlimited resources, so numbers keep multiplying at the same rate
②The actual growth curve (S-shaped) rises much like the J curve at first, but as numbers grow, environmental resistance increases and growth slows
③The carrying capacity K is the largest population the environment can support over time, and the S curve stops rising near K
④The gap between the two curves is the environmental resistance; lowering K bends the S curve sooner and widens the gap

Survivorship Curves: When Most Individuals Die

Survival curves

Three Survivorship Curves
TypeTraitExample
Type Ilow death rate when young; most die in old agehumans, large mammals (few young, well cared for)
Type IIabout the same death rate at every agesquirrels, small birds, hydras
Type IIIvery high death rate when youngoysters, many fish (many young, no care)

Interactions Within a Population

Intraspecific interactions

Relationships Among Members of One Species

From concept to problem

Example 1: Finding Population Density
Sixty field mice live in a 200 m² meadow. Find the density of this mouse population.
1
Density is the number of individuals divided by the area of the living space.
D = NS
2
Substitute the number of mice and the area.
D = 60200 = 0.3
▸ 0.3 mice/m²
Density shows how crowded the individuals are. The same 60 mice in a 400 m² meadow would have a density of only 0.15 mice/m².
Example 2: Reading a Growth Curve
A population's actual growth curve turns out S-shaped. In which stretch does environmental resistance hold back growth the most?
1
Environmental resistance is how far the theoretical (J) curve and the actual (S) curve are apart at the same moment.
2
The gap between the two curves widens as the population approaches K.
▸ the later stretch, as the population approaches the carrying capacity K
The lightly shaded band in the diagram grows wider toward the right. Early on, when numbers are small, resources are plentiful and resistance is weak; nearer K, competition for food and space becomes fierce.

Summary

Population Density
D = NS
D is density, N the number of individuals and S the area (or volume) of the living space
Actual Growth Curve (S-Shaped)
more individuals → more environmental resistance → slower growth → levels off at K
environmental resistance is how far the theoretical (J) curve and the actual (S) curve are apart at the same moment
CSAT-style problem (Life Science I), adapted
The figure shows a population's theoretical growth curve ㉠ and actual growth curve ㉡. ㉠ is J-shaped and ㉡ is S-shaped, approaching K. Choose the correct statement.
①㉠ is a curve that takes environmental resistance into account.
②In ㉡ the population goes far beyond K and keeps growing.
③The difference between ㉠ and ㉡ corresponds to environmental resistance.
④Environmental resistance is greater when the population is smaller.
⑤An S-shaped curve has no carrying capacity.
▸ ③ The difference between ㉠ and ㉡ corresponds to environmental resistance.
1
㉠ (J-shaped) assumes unlimited resources, so it leaves out environmental resistance; ① is wrong, and since the actual curve does have a carrying capacity, ⑤ is wrong too.
2
The actual curve ㉡ levels off near the carrying capacity K and does not keep growing far beyond it, so ② is wrong.
3
Environmental resistance grows as the population gets larger, so ④ is wrong, and the difference between ㉠ and ㉡ at the same moment is the environmental resistance, so ③ is correct.
🎯 Exam Points
①Population density = number of individuals ÷ area (volume) of living space
②The theoretical growth curve is J-shaped (unlimited resources); the actual growth curve is S-shaped (environmental resistance)
③The carrying capacity K is the largest population the environment can support, and the S curve levels off near K
④Environmental resistance is the difference between the J and S curves and grows as the population gets larger
⑤There are type Ⅰ, Ⅱ and Ⅲ survivorship curves, and interactions within a population include territoriality, dominance hierarchies, leadership, social life and family life
← Previous
Ecosystems & Energy Flow
Next →
Community

Last updated

Was this helpful? Support seegongsik