Population

A population is a group of individuals of the same species in one area, and density is number divided by living space. In theory growth is J-shaped, but environmental resistance bends the real curve into an S-shape that approaches carrying capacity K. There are also survivorship curve types and interactions such as territory and ranking within the population. Slide carrying capacity K and watch how the growth curve bends.

Same species gather into one unit

🐭 What is a population?

①Population = a group of individuals of the same species in one area

②Density = number of individuals ÷ living-space area (or volume)

③As numbers rise, food and space run short and waste builds up → growth cannot continue forever

Growth curves — J-shaped vs S-shaped

Carrying capacity K70

📈 Environmental resistance bends the curve

①Theoretical (J): assumes unlimited resources → explodes exponentially

②Actual (S): slows down due to environmental resistance (food shortage, space competition, predators, disease)

③Carrying capacity K: the maximum population the environment can sustain → the S-curve converges to K

Survival curves — timing of death differs by species

Survival curves

ChartThree types of survival curve

Type	Trait	Example
Type I	low early mortality, deaths late in life	humans, large mammals (few young, well cared for)
Type II	mortality roughly constant at all ages	squirrels, small birds, hydra
Type III	very high early mortality	oysters, fish (many young, no care)

Interactions within a population

Intraspecific interactions

Interactions among the same species

From concept to problem

Example 1 — density

A 200 m² grassland holds 60 field mice. Find the density of this population.

Density is the number of individuals divided by living-space area.

D = NS

Substitute the values.

D = 60200 = 0.3

▸ 0.3 mice/m²

Density shows how crowded individuals of one species are. For the same count, a larger area means lower density.

Example 2 — reading a growth curve

A population shows an actual S-shaped growth curve. In which region is the effect of environmental resistance on growth the greatest?

The gap between the theoretical (J) and actual (S) curve is exactly the environmental resistance.

That gap widens as the population approaches K.

▸ the later region, as numbers approach the carrying capacity K

Early on, resources are ample so resistance is small; near K, competition intensifies and resistance grows.

Summary

Population density

D = NS

D=density, N=number of individuals, S=living-space area (volume)

Actual growth curve (S-shaped)

numbers ↑ → resistance ↑ → growth slows → converges to K

environmental resistance = gap between theoretical (J) and actual (S) curve

CSAT-style (adapted, Biology I)

A figure shows a population's theoretical growth curve ㉠ and actual growth curve ㉡ (㉠ is J-shaped, ㉡ is S-shaped and converges to K). Choose the correct statement.

①㉠ is the curve that accounts for environmental resistance.

②In ㉡ the population greatly exceeds K and keeps rising.

③The gap between ㉠ and ㉡ corresponds to environmental resistance.

④Environmental resistance is larger when the population is smaller.

⑤An S-shaped curve has no carrying capacity.

▸ ③ The gap between ㉠ and ㉡ corresponds to environmental resistance.

㉠ (theoretical, J-shaped) assumes unlimited resources, so it has no resistance → ①, ⑤ wrong.

The actual curve ㉡ converges to K and cannot greatly exceed it → ② wrong.

Resistance grows as numbers rise (closer to K), so ④ is wrong and ③ is correct.

🎯 Exam Points

①Density = individuals ÷ living space

②J-shaped (theoretical, unlimited) vs S-shaped (actual, with resistance)