19.4 Populations
- Syllabus
- 0610–2026–2027
- Topic
- 19.4
- Level
- —
Learning objectives
19.4.1Population as a group of organisms• Describe a population as a group of organisms of one species, living in the same area, at the same time19.4.2Community as all of the populations• Describe a community as all of the populations of different species in an ecosystem19.4.3Ecosystem as a unit containing• Describe an ecosystem as a unit containing the community of organisms and their environment, interacting together19.4.4Factors affecting the rate• Identify and state the factors affecting the rate of population growth for a population of an organism, limited to food supply, competition, predation and disease19.4.5Lag, exponential (log), stationary• Identify the lag, exponential (log), stationary and death phases in the sigmoid curve of population growth for a population growing in an environment with limited resources19.4.6Interpret graphs and diagrams• Interpret graphs and diagrams of population growth19.4.7Factors that lead to each phase• Explain the factors that lead to each phase in the sigmoid curve of population growth, making reference, where appropriate, to the role of limiting factors
Define a population with all three conditions
A population is a group of organisms of one species living in the same area at the same time.
Check all three conditions: same species, same area and same time. If any condition is missing, the group is not fully described as a population.
All organisms in an area may include several species, so they form a community rather than one population.
Distinguish a community from a population
A community is all the populations of different species in an ecosystem.
One species in an area forms a population. Several populations of different species together form the community.
A community contains living populations; it does not by itself include the non-living environment.
Define an ecosystem as an interacting unit
An ecosystem is a unit containing a community of organisms and their environment, interacting together.
The community is the living part. Environmental conditions such as temperature, water or oxygen are non-living parts. Their interactions make the whole unit an ecosystem.
A list of species describes a community. It becomes an ecosystem description only when the environment and interactions are included.
Link four limiting factors to population growth
Population growth rate changes when food supply, competition, predation or disease changes.
| Factor | How it can limit population growth |
|---|---|
| food supply | less food increases competition and can reduce survival or reproduction |
| competition | organisms obtain fewer limited resources |
| predation | more individuals are killed and removed from the population |
| disease | illness or death lowers survival and may reduce reproduction |
More available food can increase growth; stronger competition, predation or disease usually decreases growth. State the causal link, not just the factor name.
These are the four required factors. Do not replace them with an unrequested list of environmental variables.
Identify four phases from curve shape
Identify each phase from the direction and steepness of the population curve, not from its letter or position alone.
| Phase | Graph feature | Birth and death relationship |
|---|---|---|
| lag | low population; little increase | growth is slow |
| exponential (log) | steepest rising section | birth rate is greater than death rate |
| stationary | plateau or small fluctuations around a level | birth rate is approximately equal to death rate |
| death | falling section | death rate is greater than birth rate |
A sigmoid curve describes growth under limited resources. The death phase is the falling section, not simply any point where the population is small.
Interpret a population growth graph
Read the axes and units first. Then describe whether population size rises, falls or stays approximately constant, and use the slope to compare rates of change.
Support a description with labelled regions, times or values from the graph. A steeper upward slope means a faster increase; a horizontal section means no net change; a downward slope means a decrease.
Translate the graph into population balance: rising means births exceed deaths, a plateau means births approximately equal deaths, and falling means deaths exceed births.
A curve can fluctuate around a stable level. Do not describe every small rise or fall as a new exponential or death phase without using the overall trend.
Explain the causes of each sigmoid phase
The sigmoid phases result from changes in reproduction, death and limiting factors as population density increases.
| Phase | Explanation |
|---|---|
| lag | the population is small, so the birth rate and total increase are low |
| exponential (log) | resources are abundant and competition, predation or disease have little limiting effect; births exceed deaths |
| stationary | limiting factors prevent further net growth; births approximately equal deaths around the carrying capacity |
| death | resources become insufficient or disease, predation or harmful waste effects increase; deaths exceed births |
Carrying capacity is the population size the environment can support under its current limiting factors. It explains the stable level, not an absolute maximum that can never fluctuate.
Name the phase, describe the curve and connect a relevant limiting factor to the birth–death balance. A factor name without its effect is not a complete explanation.