The shape of the lesson
By the end of the lesson, learners can
- 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.
- Populations, communities and ecosystems: Ecologists study life at several levels.
- Explaining each phase: Extended.
- Population: one species, same area, same time. Community: all the populations. Ecosystem: community plus environment, interacting.
- Growth rate depends on food supply, competition, predation and disease.
- Phases: lag, exponential (log), stationary (births = deaths), death (deaths > births).
- Extended: name the limiting factor and link it to the birth and death rates.
One picture every idea returns to
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.
Forty-five minutes, phase by phase
| Time | Phase | What happens in the room | Grouping |
|---|---|---|---|
| 0 to 5 min | Hook: predict | Open with the question in the title, Slow start, boom, limit, crash.. Put a few yeast cells into a flask of sugar solution and count them over the next day and a half. Put the figure on the board: it is the one picture every idea in the lesson returns to. | Whole class, then pairs |
| 5 to 13 min | Teach and check: 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. Project tab 7 of the lab (Population growth of yeast in a flask) and let a student drive it while the class predicts. | Whole class, then pairs |
| 13 to 21 min | Teach and check: Populations, communities and ecosystems | Ecologists study life at several levels. | Pairs |
| 21 to 30 min | Teach and check: Explaining each phase | Extended. | Jigsaw |
| 30 to 36 min | Check | The four Stage 1 questions from the student lesson, answered on mini whiteboards or by show of fingers. Re-teach the one most get wrong. | Individual, then whole class |
| 36 to 43 min | Plenary | The exam-style questions from Stage 2, one at a time: thirty seconds alone, then a random call. The feedback lines in the key give the examiner's wording. | Individual, random call |
| 43 to 45 min | Exit and homework | Set the worksheet below (6 questions, 21 marks). Learners self-assess against the objectives before they leave. | Individual |
Protect the parts that carry the learning
The sequence carries about fifty minutes of material, so the lab is driven by one student while the class predicts rather than explored freely. Times are a guide.
Protected: the check at 30 minutes. It is the only individual read on whether the key idea landed, and it is not cut.
Cut first if the build overruns: the last check in Explaining each phase, never the check or the plenary.
In a 60 minute block: run worksheet questions 1 to 3 in class as a Rally Coach set and model both the hardest worked example and its check live.
Jigsaw
Split the content into expert pieces. Expert groups master one piece each, then regroup so every home team has one expert per piece, and each expert teaches their piece to the team.
The full structure, with its variations and the other twelve, is on the cooperative-learning page.
What to head off, and how
| Trap learners fall into | Teaching move that pre-empts it |
|---|---|
| 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. |
| 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. |
Populations: worksheet
6 questions, 21 marks. Written in the style of the papers; no past paper question is reproduced.
Define the terms population, community and ecosystem.
State four factors that affect the rate of growth of a population and, for each, say whether an increase in the factor speeds up or slows down growth.
The number of yeast cells in a culture was counted every 4 hours.
| Time / h | 0 | 4 | 8 | 12 | 16 | 20 | 24 | 28 |
|---|---|---|---|---|---|---|---|---|
| Cells / million per cm3 | 2 | 3 | 10 | 40 | 110 | 150 | 152 | 150 |
(a) Identify the times of the lag phase and of the stationary phase. (b) Calculate the mean rate of increase between 8 and 16 hours.
Extended. Explain the shape of the curve for the yeast culture in the previous question between 0 and 28 hours, referring to limiting factors.
Extended. Predict how the yeast curve would change if the culture were continued for another 48 hours without adding any more sugar. Explain your prediction.
A small number of rabbits is released onto an island with plenty of grass and no predators. Describe and explain how the rabbit population is likely to change over the following years.
Answer key and marking notes
- Population: a group of organisms of one species, living in the same area, at the same time.
- Community: all of the populations of different species in an ecosystem.
- Ecosystem: a unit containing the community of organisms and their environment, interacting together.
- Food supply: more food speeds up growth.
- Competition: more competition slows growth.
- Predation: more predators slow growth.
- Disease: more disease slows growth.
- (a) Lag phase: 0 to about 4 hours (little increase).
- Stationary phase: about 20 to 28 hours (numbers level at about 150 million per cm3).
- (b) Increase = 110 − 10 = 100 million per cm3 in 8 hours.
- Mean rate = 100 ÷ 8 = 12.5 million cells per cm3 per hour.
- Lag (0-4 h): few cells, adjusting to new conditions and making enzymes, so few divisions.
- Exponential (4-16 h): plenty of sugar and no limiting factors, so the birth rate is far greater than the death rate and numbers double regularly.
- Slowing then stationary (16-28 h): sugar (food) becomes a limiting factor and competition increases; ethanol starts to build up.
- So the birth rate falls until it equals the death rate and the population stays constant.
- The population would enter the death phase and fall.
- The sugar would run out, so cells could not respire and reproduce.
- Toxic waste (ethanol) would build up, so the death rate would be greater than the birth rate.
- At first the population grows slowly (lag), because there are few rabbits and they must mature and find mates.
- It then grows rapidly (exponential), because food is plentiful and there is no predation.
- Eventually food runs short and competition (and disease) increase, so growth slows and the population levels off at the size the island can support.
8 multiple-choice questions on this lesson
From the topic question bank. Assign them in the app for a marked score in your gradebook, or run the topic as a class game.
Which is an example of a population?
- AAll the limpets on one rocky shore this summer
- BAll the animals and plants on one rocky shore
- CThe rocky shore with its organisms, water and rocks
- DAll the limpets living anywhere in the world
Which term describes all the populations of different species living in a pond?
- AA community
- BA population
- CAn ecosystem
- DA trophic level
The graph shows the growth of a population of bacteria in a flask of nutrient broth. Which region is the exponential (log) phase?
- AX
- BW
- CY
- DZ
Use the graph. What is the mean rate of increase of the bacteria between 4 and 6 hours?
- A25 million per hour
- B50 million per hour
- C35 million per hour
- D12.5 million per hour
A disease spreads quickly through a crowded population of rabbits. What is the most likely effect on the rate of population growth?
- AIt decreases, because the death rate rises
- BIt increases, because there is less competition
- CIt is unchanged, because disease affects only plants
- DIt increases, because the birth rate rises
Explain region Y on the graph.
- AA limiting factor such as nutrients makes the birth rate equal the death rate
- BThere are no limiting factors, so births exceed deaths
- CThe bacteria are adjusting and making the enzymes they need
- DToxic waste has built up so deaths exceed births
At 8 hours, fresh nutrient broth is added to the flask and the waste is removed. What would you expect to happen next?
- AThe population rises again to a higher stationary level
- BThe population falls immediately into the death phase
- CThe population returns to a long lag phase at zero
- DThe population stays at exactly 95 million for ever
Rabbits on an island are in the stationary phase. A predator of rabbits is introduced. Which is the best explanation of the likely effect?
- APredation raises the death rate, so the rabbit population falls to a lower level
- BPredation lowers the birth rate to zero, so all rabbits die
- CPredation has no effect, because the population is already stationary
- DPredation removes competition, so the rabbits enter an exponential phase
Quiz key
Limpets of one species on one shore at one time form a population. All the animals and plants together are a community. The shore with its organisms and their non-living environment interacting is an ecosystem. Limpets across the world are not in the same area, so they are not one population.
All the populations of different species in an ecosystem form a community. A population is just one species. An ecosystem includes the non-living environment as well. A trophic level is a feeding position in a food chain, not a group of all species.
Region X is where the population rises fastest, doubling at regular intervals: the exponential phase. W is the lag phase, with little increase. Y is the stationary phase, where the curve is level. Z is the death phase, where the population falls.
At 4 hours there are 20 million and at 6 hours 70 million: (70 − 20) ÷ 2 = 25 million per hour. 50 million is the total increase, not divided by the time. 35 million divides the 70 million at 6 hours by 2 without subtracting the starting value. 12.5 million divides the increase by 4 hours instead of 2.
Disease kills individuals, so the death rate rises and the growth rate falls. Although survivors may later face less competition, the immediate effect of disease is fewer rabbits. Animals suffer from disease as well as plants. Disease does not raise the birth rate.
In the stationary phase nutrients (or space or oxygen) become limiting and competition increases, so the birth rate falls until it equals the death rate. No limiting factors with births exceeding deaths describes the exponential phase X. Adjusting and making enzymes describes the lag phase W. Deaths exceeding births because of toxic waste describes the death phase Z.
Adding nutrients and removing waste lifts the limiting factor, so the birth rate exceeds the death rate again and the population grows until a new limit is reached. Removing the limit does not cause a fall. The cells are already active, so there is no return to a lag at zero. A population cannot stay fixed for ever once its resources change.
Predators kill rabbits, raising the death rate above the birth rate, so the population falls until a new balance is reached at a lower level. Predators do not stop rabbits breeding, and they rarely eat every prey animal. A stationary population still responds to a new factor. Fewer rabbits does mean less competition later, but the predator keeps the death rate high, so an exponential phase is unlikely.
What to project
The student lesson embeds these tabs. Open them full screen on the projector; every slider has a drag handle on the stage.
Slides for this lesson
A projectable deck for every pack is on the way. Until then, present the student lesson full screen: its sections, figure and lab tabs follow the same order as the plan above.