Population Growth IGCSE Biology 0610

Population Growth and Limiting Factors: Ecology of Populations

Biology editorial review is supported by the named review panel. Reviewer participation may vary by guide. Checked against Cambridge IGCSE Biology 0610 (2026–2028); last reviewed August 2026.

Key concept

Populations grow when birth rate exceeds death rate. Growth is limited by food supply, predation, disease, competition and available space. A population growth curve shows lag, exponential growth, and a plateau where limiting factors balance growth.

Population ecology questions test graph interpretation and the ability to explain why populations do not grow indefinitely. Students must identify limiting factors and explain how predator and prey populations are linked.

The population growth curve

When a small number of organisms colonise a new habitat, the population typically follows an S-shaped (sigmoid) growth curve with three phases:

Lag phase: The population is small and growing slowly as organisms adapt and begin reproducing. Exponential (log) phase: With abundant resources and few limiting factors, the population grows rapidly. Each generation is larger than the last. Stationary (plateau) phase: Growth slows and the population stabilises as limiting factors take effect. Birth rate approximately equals death rate.

Limiting factors

Factors that prevent unlimited population growth include:

Food and water supply: As the population grows, competition for food increases, and some individuals cannot get enough to survive or reproduce. Predation: More individuals attract more predators, increasing the death rate. Disease: Dense populations allow diseases to spread more easily. Competition: Individuals compete for resources including food, water, light (in plants), territory and mates. Accumulation of waste: Toxic waste products can build up and reduce survival.

Predator-prey relationships

Predator and prey populations are linked in a cyclical pattern. When prey numbers increase, there is more food for predators, so predator numbers increase after a time lag. As predator numbers rise, more prey are eaten, so prey numbers decrease. With less food, predator numbers then decrease, allowing prey to recover, and the cycle repeats.

On a graph, the predator curve follows the prey curve with a time lag. The predator peaks always come after the prey peaks. Students must be able to read these graphs and explain the relationship at each stage.

Competition within and between species

Intraspecific competition (within the same species) is usually more intense because individuals need exactly the same resources. This is the main factor that limits population size when other factors are not limiting.

Interspecific competition (between different species) occurs when two species use the same resources. If one species is better adapted, it may outcompete the other, reducing its population. Two species with identical niches cannot coexist in the same habitat indefinitely.

Try a focused question (3 marks)

A graph shows that when the number of prey organisms increases, the number of predators increases shortly after. Explain this relationship.

Show the answer and marking guidance

More prey provides more food for predators.
With more food, predators survive longer and reproduce more, so the predator population increases.
There is a time lag because it takes time for predators to reproduce and their population to grow.

Original Cambridge-style practice written for this site. It is not an official Cambridge past-paper question.

Common mix-ups to avoid

Check these points against your own answer before moving on.

Saying the predator population increases at exactly the same time as the prey (there is a time lag).
Forgetting that predator-prey graphs are cyclical, not a single rise and fall.
Listing limiting factors without explaining how each one restricts population growth.
Confusing intraspecific and interspecific competition.