9 min readPathway sheetSection A MCQ (1 mark), objective (2 marks, two parts) or passage (5 marks); Section B very short answer (2 marks) or short answer (3 marks) with a sketch

Population growth, environmental resistance and growth curves

A population grows when births and immigration exceed deaths and emigration. Environmental resistance limits growth, giving an S-shaped curve that levels off, or a J-shaped curve that crashes.

Growth rateBirth rate minus death rate; when births exceed deaths, the population density rises

Natality and mortalityNumber of live births (natality) or of deaths (mortality) per thousand individuals per unit time

Environmental resistanceThe physical and biological factors (space, food, temperature, natural enemies, disease) that stop a species from breeding at its maximum rate

Carrying capacityThe largest population an environment can support for an unlimited time

What decides the size of a population

A population never stays still; its numbers keep changing. Population growth means the rise or fall in the number of individuals over a given time. The size depends on density, natality (birth rate), mortality (death rate), dispersal, age distribution and the environmental resistance it faces. An area has limited food, water and space, so only a certain number can live there in comfort. The growth rate is the birth rate minus the death rate: when births are more than deaths, the population rises.

Emigration vs immigration

EmigrationThe permanent outward movement of individuals from a population. It decreases the local population.

ImmigrationThe permanent inward movement of individuals from outside into a population. It increases the local population.

Both are kinds of population dispersal: movement by which organisms leave one place and settle in new areas.

The S-shaped (sigmoid) growth curve

Seen when a few organisms enter an area that nobody occupied before.

1
Lag phase: growth is slow while the organisms adapt to the new place and settle in. Breeding starts only after some time; births and deaths are both few and steady.
2★
Growth phase: with plenty of food and little competition, births rise while deaths stay low, so numbers rise fast.
3
Limits appear: food, water or resting places run short, and predators or disease kill more. Plants may lack water, nutrients or sunlight. Deaths increase.
4★
Stable phase: individuals entering equal those leaving by death or migration. Births and deaths are about equal, and the size levels off near the carrying capacity.

★ key step

The J-shaped growth curve

In a J-shaped pattern, the population grows faster and faster (exponentially) with no check. Then environmental resistance acts all at once: competition for survival becomes fierce, growth stops abruptly, and deaths rise suddenly. This sudden fall is a population crash. The S curve levels off gently; the J curve shoots up and then falls.

Worked example: is the population growing?

Problem

A village pond has 2,000 frogs. In one year there are 300 births and 120 deaths; 30 frogs move in and 60 move out. Find the birth rate and death rate per thousand, and the new size of the population.

Working

Birth rate = 300 ÷ 2,000 × 1,000 = 150 per thousand per year.
Death rate = 120 ÷ 2,000 × 1,000 = 60 per thousand per year.
Change from births and deaths = 300 − 120 = +180.
Change from dispersal = 30 in − 60 out = −30.
Total change = 180 − 30 = +150, so the new size = 2,000 + 150 = 2,150.

Answer

Birth rate 150 and death rate 60 per thousand per year; the population grows to 2,150 frogs

How to draw the two growth curves

Draw two axes: time along the bottom and number of individuals up the side.
S curve: start almost flat (lag phase), rise steeply in the middle (growth phase), then flatten at the top (stable phase).
Draw a dashed horizontal line where the S curve levels off, labelled carrying capacity.
J curve (on a second graph): rise slowly, then very steeply, then draw a sharp drop, labelled crash.
Label each phase, and give each graph a title.

In real life

Mosquitoes in a stagnant drain multiply very fast in the monsoon and then crash when the water dries up, much like a J-shaped curve. Human numbers also press on land, water and forests, which lesson 30 looks at.

Exam angle

1 marks: “What is carrying capacity?” The largest population that an environment can support for an unlimited time.

2 marks: “What is environmental resistance? Give one abiotic and one biotic example.” The factors that stop a species from breeding at its maximum rate. Abiotic: shortage of space or extreme temperature. Biotic: natural enemies or disease.

3 marks: “Sketch the S-shaped growth curve, label its three phases and say what happens to births and deaths in each.” Lag: slow growth, few births and deaths. Growth: births high, deaths low, fast rise. Stable: births about equal to deaths, size levels off at the carrying capacity. Sketch with time and number on the axes.

Remember it

S settles, J jumps and crashes. Lag, Grow, Stay: the three steps of the S.