Identify phases on a sigmoid growth curve
A sigmoid curve in a limited-resource environment can show lag, exponential (log), stationary and death phases.
Core and Extended; Biology 0610, 2026–2028. Use the stated specimen features and evidence. Diagrams are schematic. Written practice is self-checked
Before you begin
- Know population, community and ecosystem terms and read the axis labels on a graph.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which phase is shown by a rapid upward increase in population size?
What you will learn
- Identify all four phases by their curve trends
- Distinguish exponential growth from a stationary population
Read the phases from the population trend
On a sigmoid population-growth graph, the lag phase begins with a slow increase. The exponential (log) phase has a rapid increase. During the stationary phase, the curve levels off and population size changes little overall. In a death phase, the recorded population declines.
Identify phases from the curve and axes, not from equal time intervals or a fixed phase length. The death phase is shown when the stated population falls; not every population record must include all four phases. The syllabus context is growth in an environment with limited resources.
Describe the curve, not the cause yet
This Core objective asks you to identify the phases. A low slope indicates slower change; a steep upward slope indicates rapid increase; a near-horizontal segment indicates little net change; a downward slope indicates a decline. Explanations for why each phase occurs are addressed in the Supplement objective.
Do not call a stationary curve “no organisms” or a death phase “all organisms dead.” The graph represents population size at sampled times, and the plotted observations determine what can be concluded.
Pause and explain
What does a stationary phase show on the graph?
Worked example
A curve rises slowly, then steeply, levels off, and finally declines. Name the phases in order.
Show the worked solution
- Slow initial rise is lag.
- Steep rise is exponential.
- Level section is stationary.
- Decline is death phase.
Answer Lag → exponential (log) → stationary → death.
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Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
A population curve is nearly horizontal between weeks 12 and 18. Identify the phase and describe the trend.
Give me a hint
Read the slope and population level.
Compare my reasoning
- A nearly horizontal curve is stationary.
- Population size changes little overall in that interval.
This is the stationary phase; the population size changes little overall between weeks 12 and 18.
Look for these in your work
- Phase identified
- Trend described
A population curve falls from 900 to 540 individuals after week 16. Identify the phase and state what the graph shows.
Give me a hint
A downward trend is a decline.
Compare my reasoning
- The number falls over the interval.
- The death phase is the declining segment.
The segment shows the death phase: recorded population size declines from 900 to 540 individuals. It does not show that every individual died.
Look for these in your work
- Phase identified
- Evidence stated
A graph shows a short lag phase and a long exponential phase. Is that impossible because phases should be equal lengths? Explain.
Give me a hint
Phase names describe trends, not fixed durations.
Compare my reasoning
- The duration depends on the population and conditions.
- No equal-time rule is stated.
No. Phase lengths need not be equal; identify each from its trend and the graph’s time axis.
Look for these in your work
- No false duration rule
- Trend criterion used
Common mistakes
- Calling a steep decline “exponential growth”
- Treating stationary as zero population
- Assuming all four phases last the same time
What can you explain now?
Which phase is shown by a rapid upward increase in population size?
Compare with the explanation
Exponential (log) phase
The curve rises steeply during this phase.
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Make it stick
Name the four population-growth phases in order and describe the shape of each segment.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.