Biology · 19 Organisms and their environment

19.4 Populations

Before you start

What a species is: organisms of the same kind that can reproduce together.
From 19.2: producers, consumers and predators, and that organisms compete for food.
Reading a value off a line graph and off a table, including a graph with an uneven scale.
Working out a percentage with a calculator, and the idea of a change being an increase or a decrease.
That bacteria and other microorganisms reproduce very quickly when conditions are suitable.

Syllabus objectives covered (7)

Describe a population as a group of organisms of one species, living in the same area, at the same time
Describe a community as all of the populations of different species in an ecosystem
Describe an ecosystem as a unit containing the community of organisms and their environment, interacting together
Identify and state the factors affecting the rate of population growth for a population of an organism, limited to food supply, competition, predation and disease
Identify the lag, exponential (log), stationary and death phases in the sigmoid curve of population growth for a population growing in an environment with limited resources
Interpret graphs and diagrams of population growth
Explain the factors that lead to each phase in the sigmoid curve of population growth, making reference, where appropriate, to the role of limiting factors

Part 1 of 5

Three words that nest inside each other

Population, community and ecosystem are three definitions worth a mark each, and they are examined by being swapped for one another. Learn them in order of size, because each one contains the one before.
A population is a group of organisms of one species, living in the same area, at the same time. Three conditions, and each is doing work: one species, one area, one time.
The "at the same time" is the part people leave out, and it is often the difference between one mark and two. All the rabbits that have ever lived in a wood are not a population; the rabbits living there now are.
A community is all of the populations of different species in an ecosystem. So a community is populations added together — rabbits plus foxes plus grass plus earthworms plus the bacteria in the soil.
An ecosystem is a unit containing the community of organisms and their environment, interacting together. The extra ingredient is the non-living surroundings — the soil, water, air and light — and the word interacting.
So the ladder runs: one species is a population, all the populations together are a community, and the community plus its physical environment, interacting, is an ecosystem. If a question gives you an example, name which of the three it is and quote the defining phrase.

Part 2 of 5

What actually changes the rate of population growth

A population grows when more individuals are born than die, shrinks when more die than are born, and stays the same when the two rates are equal. Everything in this section is about what pushes those two rates around.
Your syllabus limits the factors to four: food supply, competition, predation and disease. That limit is helpful — it tells you exactly what to write, and it tells you when you have written enough.
Food supply. More food means more individuals survive and reproduce, so the population grows faster. Less food means more die of starvation and fewer offspring are produced.
Competition. When individuals compete for the same resource — food, water, space, light, shelter, mates — the ones that lose out do not survive or do not reproduce, so growth slows. Competition gets fiercer as the population gets denser, which is why it acts as a brake on a growing population.
Predation. More predators means more individuals eaten, so the population falls; fewer predators means the population grows.
Disease. Disease increases the death rate. It spreads more easily when individuals are crowded together, so like competition it bites hardest when a population is large.
Now the answering discipline that earns you marks in this whole subtopic. Never write "the population goes up" on its own. Always name the factor and say which way it acted: "the population increases because there is plenty of food and few predators". A change with no named factor is not an explanation.

Part 3 of 5

The sigmoid curve and its four phases

Grow a population in an environment with limited resources — bacteria in a flask of nutrient broth is the standard example — and plot numbers against time. The curve has a recognisable S shape, which is why it is called sigmoid, followed by a final drop.
Read it left to right in four named phases. You must be able to identify each one on a graph and give it its name, so learn the shape of each in words.
Phase one is the lag phase. The population is small and the curve is almost flat, rising very slowly.
Phase two is the exponential phase, also called the log phase. The curve rises steeply and gets steeper: the population is doubling in a fixed time, so the number added each hour keeps getting bigger.
Phase three is the stationary phase. The curve levels off into a horizontal line at the maximum population the environment can support. The population is not still growing here — it has stopped changing.
Phase four is the death phase. The curve falls, because more individuals are dying than are being produced.
When a question says "identify the phase between 4 and 6 hours", it is asking you to match a section of the curve to one of those four names. Use the shape: flat and low is lag, steeply rising is exponential, flat and high is stationary, falling is death.
Watch the axis on a population graph. Numbers often run into the millions, so a graph may use an uneven scale in which each step multiplies rather than adds. On a scale like that, the exponential phase appears as a straight line rather than a curve, and the flat top still marks the stationary phase.

Part 4 of 5

Explaining each phase, and the idea of a limiting factor

Identifying the phases is the Core skill. Explaining why each one happens is the Extended skill, and it is where the marks are. The tool for it is the idea of a limiting factor: the one resource in shortest supply, which is holding the growth rate back.
The lag phase. There are very few individuals to start with, so even though each is reproducing, few offspring are produced and the total rises slowly. The organisms are also adjusting to their new surroundings — for bacteria, taking up water and producing the enzymes they need — before they can begin dividing.
So the lag phase is not a pause in which nothing happens. Say that the number of reproducing individuals is small, and that the organisms are becoming adapted to the conditions.
The exponential phase. Now there are enough individuals to make the numbers climb quickly, and nothing yet is holding them back: food is plentiful, there is little competition, and there is no shortage of space. Each individual reproduces, and their offspring reproduce, so the population doubles repeatedly.
The phrase that earns the mark here is "there are no limiting factors", or more carefully, "no factor is yet limiting growth". Then name the resources that are still plentiful.
The stationary phase. As the population becomes large, resources run short. Food supply becomes limiting, competition for food and space increases, and disease spreads more easily through a dense population — and where there are predators, predation increases too.
State the balance point explicitly, because it is a mark of its own: the birth rate has fallen and the death rate has risen until the two are equal, so the population stays constant at the maximum the environment can support.
The death phase. The food supply is now exhausted, so individuals cannot survive or reproduce; the death rate exceeds the birth rate and the population falls.
Whenever the question uses the word "explain", make sure each sentence contains one of your four factors — food supply, competition, predation, disease — plus its effect on births or deaths. That structure is the mark scheme.

Part 5 of 5

Reading numbers off a population graph

Population questions carry small calculations, and Biology 0610 gives you no formula sheet — nothing is printed for you in any paper. There are two relations to recall, and both are ordinary arithmetic you already own.
The first is percentage change. Subtract to find the change, divide by the value you started from, and multiply by 100.
$percentage change = \frac{change in population}{original population} \times 100$
Two details decide the mark. Divide by the original value, not the new one — a rise from 200 to 300 is a 50% increase, not 33%. And say whether it is an increase or a decrease, because the sign is part of the answer.
The second is the rate of growth: the change in the number of individuals divided by the time it took. Its unit comes from the axes — individuals per hour, or per day — and you must give that unit.
$rate of population growth = \frac{change in number of individuals}{time taken}$
Work an example so the arithmetic is real. A population rises from $400$ to $2800$ in 4 hours. The change is $2800 - 400 = 2400$ , so the rate is $2400 \div 4 = 600$ individuals per hour.
For the same figures, the percentage increase is $2400 \div 400 \times 100 = 600$ , so 600%. A percentage change larger than 100 is perfectly possible whenever a population more than doubles — do not assume you have made a mistake.
One more habit: when a question asks you to describe what a graph shows, quote figures from it. "The population rose from 400 to 2800 between 2 and 6 hours" is worth more than "the population went up quickly", and it costs you nothing.

Worked examples

Example 1. A student grows bacteria in a flask of nutrient broth and counts them each hour. The numbers per cm³ are: 0 h, 200; 2 h, 240; 4 h, 1600; 6 h, 12 800; 8 h, 41 000; 10 h, 42 000; 12 h, 30 000. Identify the phase occurring in each part of this experiment, and explain what causes the population to stop rising after 8 hours.

Show the working

Work through the numbers looking at how much they change, not at how large they are. That is what tells you the phase.
From 0 to 2 hours the count rises only from 200 to 240 — barely any change. That is the lag phase: few individuals are present to reproduce, and the bacteria are still adjusting to the broth and producing the enzymes they need.
From 2 to 8 hours the count multiplies each time: 240, then 1600, then 12 800, then 41 000. Repeated multiplication is the signature of the exponential (log) phase.
From 8 to 10 hours the count hardly moves, 41 000 to 42 000. That is the stationary phase — the population has levelled off at the maximum the flask can support.
From 10 to 12 hours it falls from 42 000 to 30 000. That is the death phase, because more bacteria are dying than are being produced.
Now the explanation. Name the factor that has changed: the nutrients in the broth are being used up, so food supply becomes the limiting factor.
Add the second and third factors that bite in a dense population: competition for the remaining nutrients and for space increases, and disease spreads more easily when the bacteria are crowded.
Finish with the balance statement, which is the mark most people leave out: the birth rate has fallen and the death rate has risen until they are equal, so the population stays constant.

Answer: 0–2 h lag phase, 2–8 h exponential (log) phase, 8–10 h stationary phase, 10–12 h death phase. The population stops rising because the nutrients in the broth run short, so food supply becomes a limiting factor, competition for food and space increases and disease spreads more easily in the dense population, until the death rate equals the birth rate and the numbers stay constant.

Example 2. Using the same data, calculate the percentage increase in the bacterial population between 4 hours and 6 hours, and the mean rate of increase over that period.

Show the working

Pull out the two values first: at 4 hours the count is 1600 per cm³, and at 6 hours it is 12 800 per cm³.
Find the change by subtracting: $12800 - 1600 = 11200$ per cm³.
For the percentage, divide by the value you started from — 1600, not 12 800 — and multiply by 100. Nothing is given to you in the paper, so recall the relation.
$percentage change = \frac{11200}{1600} \times 100$
Work it out: $11200 \div 1600 = 7$ , and $7 \times 100 = 700$ , so a 700% increase. Say increase — the direction is part of the answer.
Do not be alarmed by a figure over 100%. The population went up more than eightfold, so a large percentage is exactly what you should expect.
For the rate, divide the same change by the time taken, which is 2 hours.
$rate = \frac{11200}{2}$
That gives $5600$ , and the unit comes from the two axes: bacteria per cm³ per hour. A rate without its unit will not score the final mark.

Answer: A 700% increase, and a mean rate of increase of 5600 bacteria per cm³ per hour.

Example 3. A pond contains water lilies, water fleas, sticklebacks and herons, along with water, mud and dissolved oxygen. Identify the population, the community and the ecosystem in this description, and state two factors that could affect the rate of growth of the stickleback population.

Show the working

Take the three terms in order of size, and match each to the definition rather than guessing from the example.
A population is one species in one area at one time, so the sticklebacks living in the pond now are a population. So are the herons, separately — one population per species.
A community is all of the populations of different species, so it is the water lilies, water fleas, sticklebacks and herons together — every living thing in the pond.
An ecosystem adds the non-living surroundings and the interactions, so it is the whole pond: the community together with the water, mud, dissolved oxygen and light, interacting together.
Now the factors, and here you should pick from the four the syllabus names rather than inventing others. Food supply: the number of water fleas available for the sticklebacks to eat.
Predation: the herons eat the sticklebacks, so more herons means the stickleback population grows more slowly or falls. Competition and disease would both be acceptable alternatives if you explained them.

Answer: The sticklebacks in the pond are a population. The water lilies, water fleas, sticklebacks and herons together are the community. The whole pond — that community plus the water, mud, dissolved oxygen and light, interacting together — is the ecosystem. Two factors affecting stickleback growth: food supply (the number of water fleas available) and predation (the number of herons feeding on them).

Where people go wrong

Mistake: Writing "the population goes up" or "the numbers fall" without naming a factor.

Instead: Name the factor and the direction it acted: "the population increases because food is plentiful and there are few predators".

The question is asking why the change happened, and the mark is attached to the factor. A description of the curve is not an explanation of it, and this is the fastest way to lose marks in the whole topic.

Mistake: Thinking that nothing is happening during the lag phase.

Instead: The organisms are reproducing, but there are very few of them, and they are still adjusting to the conditions and producing the enzymes they need.

A flat line at the start looks like inactivity, but the mark is for explaining why the increase is small — few individuals reproducing — not for saying that growth has not started.

Mistake: Saying the population has "stopped reproducing" in the stationary phase.

Instead: Reproduction continues, but the birth rate and the death rate are equal, so the total stays constant.

A horizontal line means no net change, not no activity. Stating that the two rates have become equal is usually a mark in its own right.

Mistake: Confusing population, community and ecosystem, especially calling all the organisms in a habitat a population.

Instead: One species is a population; all the populations together are a community; the community plus its physical environment, interacting, is an ecosystem.

These are three separate definitions and each is examined as a one- or two-mark recall. Using the wrong one is treated as a wrong answer, not a near miss.

Mistake: Calculating a percentage change by dividing by the final value.

Instead: Divide the change by the original value, then multiply by 100.

Percentage change measures the change relative to where the population started. Dividing by the new value gives a different number, and there is no formula sheet in Biology, so the relation has to be recalled correctly.

Check yourself

Give the definitions of a population and a community.
Answer
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.
What does an ecosystem contain that a community does not?
Answer
The environment — the non-living surroundings such as soil, water, air and light — with the community and the environment interacting together as a unit.
State the four factors affecting the rate of population growth that this syllabus requires.
Answer
Food supply, competition, predation and disease.
Name the four phases of the sigmoid curve of population growth in order, and describe the shape of the curve in each.
Answer
Lag phase — almost flat and low. Exponential (log) phase — rising steeply and getting steeper. Stationary phase — flat and high. Death phase — falling.
Why does the population rise so steeply during the exponential phase?
Answer
There are now enough individuals to produce many offspring, and no factor is yet limiting growth — food and space are plentiful, competition is low, and disease and predation are not yet significant — so the population doubles repeatedly.
Explain what causes the stationary phase.
Answer
Food supply becomes limiting as the population becomes large, competition for food and space increases, disease spreads more easily in a dense population, and predation may increase, so the birth rate falls and the death rate rises until they are equal and the population stays constant.
A population of beetles falls from 850 to 340 over one summer. Calculate the percentage decrease.
Answer
The change is $850 - 340 = 510$ ; $510 \div 850 \times 100 = 60$ , so a 60% decrease. Divide by the original value, 850, and state that it is a decrease.
On a graph of population against time, how would you tell the stationary phase from the death phase?
Answer
The stationary phase is a horizontal line at the highest population — the numbers are not changing. The death phase is a line sloping downwards — the numbers are falling because the death rate is greater than the birth rate.

Exam questions

Try each part first. Open the mark scheme after you have written your answer.

Question 1. A student grows yeast in a flask of sugar solution at a constant temperature and counts the cells every two hours. The counts, in cells per cm³, are: 0 h, 500; 2 h, 560; 4 h, 4000; 6 h, 32 000; 8 h, 95 000; 10 h, 96 000; 12 h, 72 000.

(a) Identify the phase of population growth occurring between 0 and 2 hours, and the phase occurring between 8 and 10 hours. 2 marks No calculator

Mark scheme

B1 0–2 h: lag phase
B1 8–10 h: stationary phase

(b) Calculate the percentage increase in the number of yeast cells between 4 hours and 6 hours. 2 marks

Mark scheme

C1 change = 32 000 − 4000 = 28 000, divided by 4000 and multiplied by 100
A1 700% increase

(c) Explain why the number of yeast cells increases so rapidly between 4 and 8 hours. 3 marks No calculator

Mark scheme

B1 there are now many cells present, and each can reproduce, so numbers double repeatedly
B1 sugar / food is plentiful, and there is plenty of space
B1 no factor is yet limiting growth / competition, disease and predation are not yet significant

(d) Explain why the number of yeast cells does not increase after 8 hours. 3 marks No calculator

Mark scheme

B1 the sugar / food supply is running out, so food supply becomes a limiting factor
B1 competition for food and space increases as the population becomes dense (accept disease spreads more easily)
B1 the birth rate falls and the death rate rises until they are equal, so the population stays constant

Question 2. Rabbits are released onto a large island that has plenty of grass and no predators of rabbits. The rabbit population is counted each year. It rises slowly at first, then very rapidly, and after several years it levels off.

(a) Describe what is meant by the term population. 2 marks No calculator

Mark scheme

B1 a group of organisms of one species
B1 living in the same area, at the same time

(b) Describe what is meant by the term community. 1 mark No calculator

Mark scheme

B1 all of the populations of different species in an ecosystem

(c) Identify two factors that will affect the rate of growth of the rabbit population on this island. 2 marks No calculator

Mark scheme

B1 food supply (the amount of grass available)
B1 competition (for food, space or shelter) or disease; predation is not credited here because the island has no predators of rabbits

(d) Explain why the rabbit population eventually stops increasing, even though the island is large. 3 marks No calculator

Mark scheme

B1 the grass is eaten faster than it can regrow, so food supply becomes a limiting factor
B1 competition for food and space increases, so fewer rabbits survive or reproduce
B1 disease spreads more easily in a dense population, so the death rate rises until it equals the birth rate

How this is examined

Explaining what causes each phase of the sigmoid curve: A four- to six-mark Explain following a population growth graph, sometimes split into "Explain the shape of the curve between A and B" for two named regions. Each mark needs a named factor plus its effect on births or deaths. Lag: "few individuals present to reproduce, and organisms are adjusting to the conditions". Exponential: "no factor is yet limiting — food and space are plentiful and competition is low — so numbers double repeatedly". Stationary: "food supply becomes limiting, competition and disease increase, so birth rate equals death rate". Death: "food is exhausted, so the death rate exceeds the birth rate". A description of the curve's shape with no factor named scores nothing.
Identifying the four phases on a population graph: A two- to four-mark Identify, usually with letters marked on a curve or time intervals given in a table: "Identify the phase shown between 6 and 8 hours." The exact names: lag phase, exponential phase (log phase is equally acceptable), stationary phase, death phase. Match them by shape — flat and low, steeply rising, flat and high, falling. "Growth phase" or "plateau" will not be credited in place of the syllabus names.
The factors that change the rate of population growth: A two- or three-mark State or Suggest, often applied to a described situation: "Suggest two factors that would slow the growth of this population." Name factors from the syllabus list — food supply, competition, predation, disease — and, if the command is Suggest or Explain, add how each acts in the situation given: "less food, so fewer survive and reproduce", "more predators, so more are eaten". Do not offer predation as an answer when the stem says there are no predators.
Percentage change and rate from population data: A two- or three-mark Calculate from a table or graph, often just before the Explain part of the same question. Show the change first, then divide by the original value and multiply by 100; state increase or decrease. For a rate, divide the change by the time and give the unit from the axes, such as individuals per hour. No formula sheet is provided in Biology 0610, so both relations must be recalled.
Population, community and ecosystem as three separate definitions: A one- or two-mark Describe or Identify at the start of an ecology question, sometimes as "Identify the community in this pond". Quote the defining phrases: population — "one species, in the same area, at the same time" (the time clause is often the second mark); community — "all the populations of different species"; ecosystem — "the community and their environment, interacting together". Examples alone do not score if the definition is not given.

Words to know

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.
Limiting factor: The factor in shortest supply, which is holding back the rate of population growth — commonly food supply, or space to live in.
Lag phase: The first phase of the sigmoid curve, in which the population rises slowly because few individuals are present and the organisms are adjusting to the conditions.
Exponential (log) phase: The phase in which the population rises rapidly and repeatedly doubles, because no factor is yet limiting growth.
Stationary phase: The phase in which the population stays constant at the maximum the environment can support, because the birth rate equals the death rate.
Death phase: The final phase, in which the population falls because the death rate is greater than the birth rate, food having been exhausted.

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