Put a few yeast cells into a flask of sugar solution and count them over the next day and a half. The numbers trace a characteristic S-shaped curve. Food supply, competition, predation and disease decide how fast a population grows; explaining each phase with limiting factors is Extended content.
A population is a group of organisms of one species, living in the same area, at the same time. A community is all of the populations of different species in an ecosystem, and an ecosystem is a unit containing the community of organisms and their environment, interacting together. The rate of population growth depends on food supply, competition, predation and disease. In an environment with limited resources a population follows a sigmoid curve with lag, exponential (log), stationary and death phases.
Fig. 1: the sigmoid growth curve of a population with limited resources, showing the four phases
Section 01
The sigmoid growth curve.
Press Start: five yeast cells go into a flask of sugar solution and the graph plots the number of cells against time in hours. Watch the PHASE readout change. Then start again and try Add more food or Disease strikes part of the way through.
Phase
What the curve does
Births and deaths
Lag
almost flat at first
few births: the cells are adjusting to new conditions
Exponential (log)
rises steeply, doubling at regular intervals
birth rate much greater than death rate
Stationary
levels off
birth rate equals death rate
Death
falls
death rate greater than birth rate
Add more food: raising the food supply lets the population grow again to a new, higher level.
Disease strikes: the population suddenly drops; if resources are still available it can grow back.
Interpreting graphs: name the phase, quote values from the axes with units, and link each change in the slope to births and deaths.
Examiner trap
In the stationary phase organisms are still being born and still dying. The population stays level because the two rates are equal, not because reproduction has stopped.
Stage 1 · Learn
Check what the sim just showed you
Four quick checks. Each correct answer earns XP and records this skill on your profile.
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In the yeast simulation, what does the population do in the stationary phase?
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Why are there few new yeast cells in the lag phase shown in the sim?
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In the sim, what happens when you press Add more food during the stationary phase?
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What causes the death phase in the yeast flask?
Section 02
Populations, communities and ecosystems.
Ecologists study life at several levels. The yeast in the flask are one population. A pond contains many populations that together make a community, and the pond as a whole, living and non-living parts together, is an ecosystem.
Population: a group of organisms of one species, living in the same area, at the same time (all the frogs in one pond this spring).
Community: all of the populations of different species in an ecosystem (frogs, pondweed, water fleas, fish and so on).
Ecosystem: a unit containing the community of organisms and their environment, interacting together (the pond with its water, mud, light and temperature).
Factors affecting growth rate:food supply (more food, faster growth), competition for food, space or mates, predation (more predators, more deaths) and disease (spreads faster in crowded populations).
Section 03
Explaining each phase.
Extended. A limiting factor is something in short supply that stops a population growing any faster. Each phase of the sigmoid curve can be explained by what is, or is not, limiting.
Lag phase: there are only a few organisms and they are adjusting to the new conditions (yeast are making the enzymes they need), and sexually reproducing organisms must mature and find mates. So there are few births.
Exponential (log) phase: food and space are plentiful and there are no limiting factors, so every individual can reproduce. Births far exceed deaths and the population doubles at regular intervals.
Stationary phase: food (or space, or oxygen) has become a limiting factor. Competition increases, and predation and disease rise in the crowded population. The birth rate falls to equal the death rate.
Death phase: food runs out and toxic waste builds up (ethanol, in the yeast flask). The death rate exceeds the birth rate and the population falls.
In the wild: a population usually stays in the stationary phase, rising and falling around the level its resources can support; a death phase follows when resources are used up or the habitat changes.
Examiner trap
Do not say the population levels off "because it is full". Name the limiting factor (food, space, oxygen) and say that it makes the birth rate fall to equal the death rate.
Stage 2 · Exam
Exam-style questions
Unlocks once the four checks above are done. Worth more XP, written to IGCSE Paper 2 and Paper 4 standard.
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Which is the best description of a community?
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Which factor would increase the rate of growth of a population of rabbits?
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Extended: in the exponential phase of a sigmoid curve the population grows rapidly because:
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Extended: a population of bacteria stops increasing even though there is still plenty of glucose. Which limiting factor is most likely?
Skill unlocked
Populations, mastered.
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