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Detailed notes on Organisms and their Environment for Cambridge IGCSE Biology, covering key concepts, explanations, examples, and exam-focused revision points.
How populations grow and what stops them. Sigmoid curve has four phases. Limited by food, predators, disease, space.
Mapped to the Cambridge IGCSE 0610 syllabus (2026-2028).
Three nested terms: smallest to largest scale.
Population — all individuals of ONE species in a particular area.
Community — ALL populations of DIFFERENT species in an area.
Ecosystem — community PLUS the NON-LIVING (abiotic) environment.
Habitat — where a particular organism lives. Smaller scale.
Worked qualitative. A pond contains: 200 frogs, 50 fish, 1000 water beetles, plus weeds and bacteria. How many populations are present? What's the community?
Cambridge tip. Always be precise about scale. Population = one species. Community = many species. Ecosystem = community + environment.
Four phases: lag, exponential, stationary, decline. Standard pattern.
When a population is introduced to a new environment with abundant resources, it follows a typical S-shape (sigmoid) curve:
1. LAG phase.
2. EXPONENTIAL (LOG) phase.
3. STATIONARY phase.
4. DECLINE / DEATH phase.
Carrying capacity (K). The maximum population size an environment can sustain. Different environments have different K values for the same species.
Human population.
Worked qualitative. Why do BACTERIAL cultures eventually crash even with food still available?
Cambridge tip. Memorise the four phases. Cambridge often gives a graph and asks you to label.
Food, predators, disease, space, water. Climate and competition too.
Limiting factors — anything that prevents population from growing.
Density-dependent factors:
Density-independent factors:
Cambridge focus areas:
Food. Less per individual → some starve → fewer reproduce.
Predators. More predators → more prey eaten → population shrinks. Predator-prey populations often OSCILLATE.
Disease. Crowding → more infections → more deaths.
Space. Limited territory, breeding sites, sunlight (for plants).
Water. Especially in deserts.
Light. Plants need it for photosynthesis; competition for canopy.
Worked qualitative. Why do predator and prey populations OSCILLATE?
Worked qualitative. Why is HUMAN population still growing exponentially?
Cambridge tip. When asked about limiting factors, give 3-4 different ones. Cambridge marks each.
Verbatim phrases and definitions Cambridge mark schemes credit.
Populations appear Paper 4 (5-7 marks: define, label growth curve, identify factors). Examiner reports flag students mixing population and community.
Sources: Cambridge IGCSE Biology 0610 syllabus 2026-2028 (19.4); 0610/42 May/Jun 2024 — Q24 (populations); 0610 Examiner Reports 2022-2024. Last reviewed 2026-05-07.
Step-by-step solutions to past-paper-style questions on populations, written exactly the way a tutor would explain them at the board.
Question
Define population, community and ecosystem.
Step-by-step solution
Step 1
Population: a group of organisms of one species, living in the same area at the same time.
Step 2
Community: all of the populations of different species living together in an area.
Step 3
Ecosystem: the community of organisms plus the non-living environment they interact with.
Answer
Population = one species in an area. Community = all the different species together. Ecosystem = community + non-living environment.
Question
State four factors that affect the rate of growth of a population.
Step-by-step solution
Step 1
Food supply — more food allows more individuals to survive and reproduce.
Step 2
Competition — for food, space, mates, etc.
Step 3
Predation — more predators means fewer survive.
Step 4
Disease — spreads more easily and reduces numbers, especially in crowded populations.
Answer
Food supply, competition, predation and disease.
Examiner tip
The syllabus lists exactly these four factors: food supply, competition, predation, disease.
Question
Describe the four phases of the sigmoid (S-shaped) population growth curve.
Step-by-step solution
Step 1
Lag phase: the population grows slowly at first, as there are few individuals.
Step 2
Exponential (log) phase: the population grows rapidly, because there are plenty of resources and few limiting factors.
Step 3
Stationary phase: growth levels off and the population stays roughly constant; the birth rate equals the death rate.
Step 4
Death (decline) phase: the population falls, for example as resources run out, waste builds up, or disease spreads.
Answer
Lag (slow start) → exponential/log (rapid growth) → stationary (levels off, births ≈ deaths) → death/decline (falls).
Question
Explain how food supply and predation can limit the size of a population.
Step-by-step solution
Step 1
Food supply: as a population grows, there is more competition for food.
Step 2
When food runs short, some individuals starve or cannot reproduce, so the population stops increasing.
Step 3
Predation: as prey numbers rise, predators have more food, so predator numbers may rise too.
Step 4
More predators eat more prey, so the prey population is reduced — limiting its size.
Answer
Limited food causes competition and starvation, limiting growth. More predators eat more prey, reducing the prey population.
Question
Explain the factors that cause each phase of the sigmoid growth curve.
Step-by-step solution
Step 1
Lag phase: growth is slow because there are few individuals to reproduce, and they may need time to settle and (for some) find mates.
Step 2
Exponential phase: growth is rapid because there are plenty of resources (food, space) and few limiting factors, so the birth rate is much higher than the death rate.
Step 3
Stationary phase: growth levels off because limiting factors now act — food and space become short, predators increase and disease spreads — so the birth rate equals the death rate.
Step 4
Death phase: the population falls because resources are used up, waste accumulates, or disease spreads, so the death rate becomes greater than the birth rate.
Answer
Lag: few individuals. Exponential: plenty of resources, few limits. Stationary: limiting factors balance births and deaths. Death: resources exhausted/waste/disease so deaths exceed births.
Examiner tip
For each phase, link the rate of growth to the resources available and the limiting factors acting.
Question
Bacteria are grown in a closed flask of nutrient broth. Their numbers rise slowly, then very rapidly, then level off, and finally fall. Explain this pattern.
Step-by-step solution
Step 1
At first the numbers rise slowly (lag phase), as there are few bacteria and they are adjusting to the conditions.
Step 2
Then they rise very rapidly (exponential phase), because there is plenty of food in the broth and few limiting factors, so they divide quickly.
Step 3
The numbers level off (stationary phase) as the food begins to run out and toxic waste builds up, so the birth rate equals the death rate.
Step 4
Finally the numbers fall (death phase), because the food is used up and the waste products become harmful, so more bacteria die than are produced.
Answer
Slow (lag) → rapid (exponential, plenty of food) → level (stationary, food running out/waste building up) → fall (death, food gone/waste toxic). A closed flask cannot replace resources, so the population eventually declines.
Examiner tip
In a closed system, the death phase occurs because resources cannot be replaced and waste accumulates.
High-scoring sample answers for populations on the Cambridge IGCSE 0610 paper, with examiner-style notes mapping each response to the mark scheme and assessment objectives.
Define the term population. (1 mark)
Model answer
A population is a group of organisms of one species living in the same area at the same time.
Why this scores
One mark for 'one species, same area (same time)'.
State two factors that affect the rate of growth of a population. (2 marks)
Model answer
Any two of: food supply, competition, predation and disease.
Why this scores
Two of the four named factors in the syllabus.
Define the terms population, community and ecosystem. (3 marks)
Model answer
A population is a group of organisms of one species living in the same area at the same time. A community is all the populations of the different species living together in an area. An ecosystem is a community of organisms together with the non-living environment they interact with.
Why this scores
Three marks: population (one species); community (all species together); ecosystem (community + environment).
Describe the four phases of a sigmoid population growth curve. (4 marks)
Model answer
In the lag phase, the population grows slowly because there are few individuals. In the exponential (log) phase, the population grows rapidly, because resources are plentiful. In the stationary phase, the population stays roughly constant, as the birth rate equals the death rate. In the death (decline) phase, the population falls, for example as resources run out or disease spreads.
Why this scores
Four marks: lag (slow); exponential (rapid); stationary (constant/births = deaths); death (falls).
Explain the factors that cause the lag, exponential and stationary phases of a population growth curve. (5 marks)
Model answer
In the lag phase, growth is slow because there are only a few individuals to reproduce (and they may be settling into the new conditions). In the exponential phase, growth is rapid because there are plenty of resources such as food and space and few limiting factors, so the birth rate is much higher than the death rate. In the stationary phase, growth levels off because limiting factors now act — food and space become short, predators increase, and disease spreads more easily in the larger population — so the birth rate becomes equal to the death rate and the population stays roughly constant.
Why this scores
Five marks: lag = few individuals; exponential = plenty of resources/few limits/births > deaths; stationary = limiting factors act (food/space/predators/disease); births = deaths.
A population of animals is introduced to a new area with abundant food. Explain why its growth follows a sigmoid (S-shaped) curve and what could cause the population to decline. (6 marks)
Model answer
At first the population grows slowly (lag phase) because there are only a few individuals. It then grows rapidly (exponential phase) because there is plenty of food and space and few limiting factors, so the birth rate is much greater than the death rate. As the population becomes large, limiting factors begin to act: there is increased competition for food and space, predators may increase, and disease spreads more easily. These factors slow the growth until the birth rate equals the death rate, so the population levels off at the stationary phase, giving the S-shaped (sigmoid) curve. The population could decline if the food supply runs out, if a disease or predator greatly increases, or if waste products build up — so that the death rate becomes greater than the birth rate.
Why this scores
Up to 6 marks: lag (few individuals); exponential (plenty of resources/few limits); limiting factors act as it grows (competition/predators/disease); stationary (births = deaths) giving the S-shape; decline if food runs out / disease / predation / waste (death rate > birth rate).
Definitions to memorise and the exact keywords mark schemes credit for populations answers — sharpened from recent examiner reports for the 2026 0610 sitting.
A group of organisms of one species living in the same area at the same time.
All the populations of the different species living together in an ecosystem.
A unit containing the community of organisms and their non-living environment, interacting together.
A factor (such as food supply, competition, predation or disease) that prevents a population from growing further.
The traps other students keep falling into on populations questions — taken from recent Cambridge IGCSE 0610 examiner reports and mark schemes — and how to avoid them.
Why it happens
Confusing the scope of the terms.
How to avoid it
A population is ONE species only. All the species together = community. Community + environment = ecosystem.
Why it happens
The population is not changing.
How to avoid it
In the stationary phase, births still happen and deaths still happen — they are just BALANCED (birth rate = death rate), so the population stays constant.
Why it happens
The rapid growth looks unstoppable.
How to avoid it
Limiting factors (food, space, predators, disease) eventually slow the growth, so it levels off — giving the S-shaped curve.
The things students keep getting wrong in this sub-topic, answered.