Populations and population growth
The phases of a population growth curve and what limits size.
Review these first
Learning objectives
What you need to be able to do
Teacher-mapped phrasing — check against the official Cambridge syllabus for exact wording.
- 19.3.1Describe the phases of a population growth curve and the factors affecting population size.
5 minute read
Populations and population growth
Definitions
- Population — all the organisms of one species in an area at one time.
- Community — all the populations of different species in that area.
- Ecosystem — the community plus the non-living environment it interacts with.
The population growth curve
Four phases, each with a cause:
- Lag phase — slow growth. Few individuals are present and they are adapting to the conditions.
- Exponential (log) phase — rapid growth. Resources are plentiful, so the population roughly doubles in each equal time period.
- Stationary phase — growth levels off. Birth rate equals death rate as resources become limited. The population has reached the carrying capacity of the environment.
- Death phase — the population falls, as food runs out and toxic waste accumulates.
What limits population size
- Food supply and water
- Predation
- Disease, which spreads faster in dense populations
- Competition for space, light or mates
- Accumulation of waste
These are density-dependent — their effect increases as the population grows, which is exactly why growth slows rather than continuing forever.
Human population
The human population shows exponential growth, driven by improved food supply, better medical care and sanitation reducing death rates. Whether a stationary phase follows depends on whether resources can keep pace — the same biological limits apply to us as to any other species.
Think of it like this
The growth curve is a party in a small flat: slow at first, then filling rapidly, then jammed to the doors because the space simply cannot hold more. The carrying capacity is how many the flat holds, not how many wanted to come.
Worked examples
Method, step by step
Explain why a population of bacteria in a closed flask eventually stops increasing and then falls.
- 1During exponential growth, nutrients are plentiful and the population increases rapidly.
- 2As numbers rise, nutrients are used up and competition increases, so the birth rate falls until it equals the death rate — the stationary phase.
- 3In a closed flask no new nutrients arrive and toxic waste products accumulate.
- 4The death rate then exceeds the birth rate, so the population declines.
Nutrients become limiting and waste accumulates, so birth rate first equals then falls below death rate, giving a stationary phase followed by a death phase.
Common misconceptions
- Thinking a population stops growing because reproduction ceases. Birth rate simply comes to **equal** death rate.
- Confusing population and community — a population is one species only.
- Assuming exponential growth can continue indefinitely. Resource limits always intervene.
In the exam
- Name all four phases in order and give a **cause** for each — the causes are the marks, not the labels.
- Use the term "carrying capacity" when explaining the stationary phase; it is a specific marking point.