Explain a population death phase
A death phase occurs when recorded population size declines; deaths exceed additions over the interval, often as conditions become less favourable.
Extended only (Supplement); 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
What characterises the death phase on a population-growth graph?
What you will learn
- Explain a declining population phase
- Relate the decline to limiting factors without assuming one cause
Death phase: population size falls
In a death phase, the graph shows declining population numbers. Over the interval, losses exceed additions. In a limited-resource environment, resources may become insufficient, competition may increase, or disease or predation may raise losses. Use only causes supported by the supplied context.
The phase name describes the population trend, not proof that every organism dies. A population can decline and still remain present. The factors listed in the Core syllabus can contribute in context, but a graph alone cannot identify which factor acted.
Connect changes to evidence
Compare the population at the start and end of the declining interval, preserve the time scale and state the amount or rate only if the graph supplies enough values. A change in one factor may coincide with the decline without being its only cause.
A death phase is not an inevitable next stage in every environment. It is one phase shown by a particular population record. If resources recover or conditions change, later population trends may differ.
Pause and explain
A population declines when food is low. Which explanation is most careful?
Worked example
A population falls from 820 to 500 organisms after food becomes scarce. Explain the observed phase and give a cautious mechanism.
Show the worked solution
- The count declines, so it is a death phase.
- Food shortage may reduce survival or reproduction.
Answer The record shows a death phase because population size declines. Scarce food may reduce survival or reproduction, but the data given do not show that food shortage was the only cause.
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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 graph falls from 1,200 to 840 organisms over six weeks. Identify the phase and state one supported conclusion.
Give me a hint
Read the downward trend and retain the interval.
Compare my reasoning
- Population size decreases by 360 in six weeks.
- A declining segment is the death phase.
The segment is a death phase; the recorded population fell by 360 organisms over six weeks. This is a population decline, not evidence that all individuals died.
Look for these in your work
- Phase named
- Change quantified and bounded
After a predator population rises, prey numbers decline. Give a plausible explanation and a limitation.
Give me a hint
Use predation as a possible factor.
Compare my reasoning
- More predation may increase prey losses.
- Other factors and causal evidence must be considered.
Increased predation may contribute to prey decline, but the timing alone does not prove it was the only cause; food, competition and disease may also matter.
Look for these in your work
- Mechanism plausible
- Causal limit retained
The population begins to recover after food becomes more available. Does this contradict the earlier death phase? Explain.
Give me a hint
A phase describes a time interval, not a permanent outcome.
Compare my reasoning
- The earlier interval had a decline.
- Later conditions can change the trend.
No. The earlier data show a death phase during that interval; improved food availability can be followed by a different trend later.
Look for these in your work
- Time intervals separated
- Conditions linked
Common mistakes
- Saying every organism died
- Assuming death phase is inevitable
- Treating coincidence as proof of cause
What can you explain now?
What characterises the death phase on a population-growth graph?
Compare with the explanation
Recorded population size declines
The curve slopes down during the interval.
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Make it stick
Explain what the death phase shows and how you would connect it to a possible limiting factor without overclaiming.
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.