A population of rabbits colonises a new area of grassland. At first the population grows slowly, then very rapidly, before eventually levelling off at a steady size. Describe the changes in birth rate and death rate that account for each of the three phases of population growth shown by this population.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
During the lag phase the birth rate is only slightly higher than the death rate, so the population increases only slowly.
During the log phase the birth rate is much greater than the death rate, giving rapid exponential growth.
During the stationary phase the birth rate equals the death rate, so the population size remains constant at carrying capacity, which is limited by a factor such as food, disease, predation or competition.
During the log phase the birth rate is much greater than the death rate, giving rapid exponential growth.
During the stationary phase the birth rate equals the death rate, so the population size remains constant at carrying capacity, which is limited by a factor such as food, disease, predation or competition.
Examiner tips
- Use the terms ‘lag’, ‘log’ and ‘stationary’ phases. State the relationship between birth and death rates for each phase. Mention a limiting factor for the stationary phase.
Common mistakes
- Confusing the lag phase with the stationary phase. Forgetting to mention the limiting factor that causes the population to level off.
Mark scheme (4 marks)
- During the lag (slow growth) phase, birth rate only slightly exceeds death rate / both rates are similar / population numbers increase slowly
- During the log (rapid/exponential growth) phase, birth rate greatly/significantly exceeds death rate
- During the stationary phase, birth rate equals death rate / population size remains constant / carrying capacity is reached
- A limiting factor (e.g. food supply / disease / predation / competition) causes the levelling off / keeps the population at carrying capacity
Key terms in this question
population · birth rate · death rate
Related
- All Cambridge International IGCSE Biology (0610) revision notes →
- How to answer a "Describe" question →
- Decode the mark scheme abbreviations →
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