Populations, Ecosystems and Sampling with Quadrats

What this lesson covers in the specification

Incandio is aligned to this specification. It is not published by, endorsed by or affiliated with Pearson, and it reproduces none of Pearson's wording — the statement numbers are given so you can check every lesson against your own copy.

• 4.1 — The terms population, community, habitat and ecosystem
• 4.2 — Practical: investigate population size in two areas using quadrats (required practical)

Specification section 4(a) The organism in the environment — also taught in Biodiversity, Distribution and Limiting Factors.

1 · Understand it

No exam language yet. The only question this section answers is: do I actually understand what is happening?

The four words, from smallest to largest

1. POPULATION — all the organisms of ONE SPECIES living in a habitat at the same time. All the dandelions in a field.
2. COMMUNITY — all the populations of ALL the species living in a habitat. Every dandelion, grass plant, worm, beetle and bird in that field, together.
3. HABITAT — the PLACE where an organism lives. The field itself, or a rock pool, or the bark of one oak tree.
4. ECOSYSTEM — the community of organisms TOGETHER WITH the non-living conditions of the habitat — the soil, the water, the light, the temperature — and the interactions between them.
5. The step that matters is the last one: an ecosystem includes the non-living parts. A community does not.

The populations in a community are not independent of one another. Species rely on each other for food, shelter, pollination and seed dispersal, which is called INTERDEPENDENCE, and they also COMPETE: plants for light, space, water and mineral ions, animals for food, mates and territory. One consequence shows up in counts made year after year. When prey are plentiful, their predators have more food and increase; more predators eat more prey, so the prey fall; with less food, the predators then fall too. The predator numbers rise and fall a little LATER than the prey numbers.

Now the practical problem. Suppose you want to know how many dandelions are in a field. You cannot count them — there might be forty thousand, and you would trample half of them trying. So you count a small part and scale up, which is SAMPLING, and everything difficult about this practical is a consequence of that decision.

Think of it like tasting soup rather than drinking the pot

You do not need to drink a whole pot to know whether it needs salt — one spoonful will do, provided the pot has been stirred. If it has not been stirred, and you take your spoonful from the top, you will learn about the top and nothing else. That is the entire theory of sampling in one image: a small sample tells you about the whole ONLY if it is representative, and the way you take it is what decides whether it is. A quadrat placed where the dandelions look thickest is a spoonful taken from the top.

A QUADRAT is a square frame, usually half a metre or one metre on each side, placed on the ground so that whatever falls inside it can be counted. The counting is the easy part. The difficulty is deciding WHERE to put it, and the answer is that the placement must be RANDOM — because any human decision about where to place it will be biased, even when you are trying to be fair.

How to place a quadrat randomly, and why it is done this way

1. Lay two tape measures at right angles along two edges of the area, forming the axes of a grid.
2. Generate two random numbers, using a random number generator or a calculator.
3. Use them as coordinates — the first as the distance along one tape, the second as the distance along the other — and place the quadrat where they meet.
4. This removes the experimenter's judgement entirely. Choosing a spot that 'looks typical' introduces bias without any intention to cheat, because people are drawn to areas that are easy to reach, interesting to look at, or convenient to walk to.
5. Repeat with a new pair of random numbers for each quadrat, and take at least ten samples so that the mean is more reliable.

Worked example — scaling a sample up to a population

A field measures 50 m × 40 m. Ten quadrats, each 0.5 m × 0.5 m, are placed randomly, and the numbers of dandelions counted are 3, 5, 2, 6, 4, 0, 7, 3, 5 and 5. Estimate the total number of dandelions in the field.

1. Mean number per quadrat = (3 + 5 + 2 + 6 + 4 + 0 + 7 + 3 + 5 + 5) ÷ 10 = 40 ÷ 10 = 4.0.
2. Area of one quadrat = 0.5 × 0.5 = 0.25 m².
3. Mean number per square metre = 4.0 ÷ 0.25 = 16 per m².
4. Area of the field = 50 × 40 = 2000 m².
5. Estimated population = 16 × 2000.

Answer: 32 000 dandelions — an ESTIMATE, based on a total sampled area of 2.5 m² out of 2000 m².

Notice the word ESTIMATE, and notice how small the sample was: two and a half square metres out of two thousand, which is roughly one part in eight hundred. That is why the number of quadrats matters, why they must be randomly placed, and why the answer should never be written as though it were a count. The commonest way to lose marks here is to present an estimate as a fact.

There is one important situation where random placement is the WRONG choice. If you want to know how a species changes across a gradient — from the water's edge up a beach, or from open field into the shade under a hedge — then random sampling scatters your data across the gradient and tells you nothing about the pattern. There you use a TRANSECT: a line laid across the gradient, with quadrats placed at regular intervals along it. Random for 'how many'; systematic along a transect for 'how does it change'.

Finally, comparing two areas — which is what statement 4.2 actually asks for. If you are comparing a mown lawn with an unmown meadow, everything except the factor you are studying should be as similar as possible: same day, same weather, same size of quadrat, same number of quadrats, and the same person deciding what counts as one plant. Otherwise a difference between the areas may be a difference between the two sets of measurements instead.

2 · Grade 9 Notes

A different job from the section above. You have already understood it; this is the precise set of things to LEARN — definitions to reproduce word for word, processes in order, equations with units, and the answers that score full marks.

Statement 4.1 — the four terms, exactly as they must be defined

Population

All the organisms of ONE species living in a habitat at a given time.

Community

All the populations of ALL the species living in a habitat.

Habitat

The place where an organism lives.

Ecosystem

A community of organisms together with the non-living components of their environment, and the interactions between them.

Interdependence

Species in a community depend on each other for food, shelter, pollination and seed dispersal, so a change in one population can affect the others.

The distinction that is tested

A community is living organisms only; an ecosystem also includes the non-living conditions — soil, water, light, temperature.

Required practical 4.2 — estimating population size in two areas using quadrats

1. Lay two tape measures at right angles along adjacent edges of the first area, to act as the axes of a grid.
2. Generate two random numbers and use them as coordinates to place a quadrat.
3. Count the number of individuals of the chosen species inside the quadrat, using a fixed rule for plants lying on a boundary — for example, count those overlapping the top and left edges only.
4. Repeat with new random coordinates for at least ten quadrats.
5. Calculate the mean number per quadrat, then the mean per square metre by dividing by the quadrat's area.
6. Multiply by the total area of the habitat to estimate the population size.
7. Repeat the whole procedure in the second area, and compare.

Variables

Independent (changed) — the area being sampled — for example mown lawn against unmown meadow
Dependent (measured) — number of individuals of the chosen species per quadrat

Sources of error

Estimating a population from quadrat data — the calculation routine

1. Add the counts and divide by the number of quadrats, to give the mean number per quadrat.
2. Work out the area of one quadrat: a 0.5 m × 0.5 m quadrat has an area of 0.25 m².
3. Divide the mean per quadrat by the quadrat area, to give the mean number per square metre.
4. Work out the total area of the habitat, in square metres.
5. Multiply the mean per square metre by the total area, to give the estimated population size.
6. Call it an ESTIMATE, and state that it assumes the species is evenly distributed and that the quadrats were representative.

Reliability and validity in this practical — the phrases examiners look for

• MORE QUADRATS give a mean less affected by any single unusual reading, so the estimate is more reliable
• RANDOM PLACEMENT removes the experimenter's bias, so the sample is representative and the estimate is valid
• A quadrat containing NO individuals is a real result and must be included in the mean — leaving it out biases the estimate upwards
• The estimate assumes the species is EVENLY DISTRIBUTED across the habitat, which is rarely exactly true
• When comparing two areas, use the same size and number of quadrats and sample on the same day

Random placement against a transect — choose by the question being asked

A. RANDOM placement, using coordinates from random numbers: use when estimating HOW MANY there are in an area, because it removes bias and gives a representative sample.

B. A TRANSECT with quadrats at regular intervals along a line: use when investigating HOW A SPECIES CHANGES across a gradient, such as up a beach or into shade — random placement would scatter the data and hide the pattern.

Not this: Choosing a spot that looks typical of the whole area gives a fair sample.

This: Human judgement is biased even when it is trying not to be — towards accessible, interesting or convenient places. Random coordinates remove the judgement entirely, which is the point.

Model answer [3 marks]

Explain why quadrats must be placed randomly when estimating population size. [3]

If the experimenter chooses where to place the quadrats they may be biased, for example towards areas that are easy to reach or where the species appears abundant. This would make the sample unrepresentative of the whole area. Placing quadrats using random coordinates removes the experimenter's judgement, so the sample is representative and the estimate of the population is more valid.

Model answer [3 marks]

A student uses five quadrats to estimate the number of daisies in a large field and obtains a much higher figure than another student who used thirty. Suggest why, and how the method could be improved. [3]

Daisies are unlikely to be evenly distributed, so with only five quadrats a small number that happened to fall on dense patches would raise the mean considerably. A larger number of quadrats gives a mean that is less affected by any single unusual reading and is therefore more reliable. The student should take many more quadrats, placed randomly, and calculate the mean from all of them.

Model answer [2 marks]

Explain the difference between a community and an ecosystem. [2]

A community consists of all the populations of all the species living in a habitat — the living organisms only. An ecosystem consists of that community together with the non-living components of the environment, such as the soil, water, light and temperature, and the interactions between them.

Mark-losing trap. An ecosystem includes the NON-LIVING conditions. A community does not — that is the whole distinction.

Mark-losing trap. A population is ONE species. All the species together is a community.

Mark-losing trap. Divide by the quadrat AREA, not by the number of quadrats twice. A 0.5 m quadrat has an area of 0.25 m², not 0.5 m².

Mark-losing trap. Say ESTIMATE. Presenting a scaled-up sample as an exact count loses the mark.

3 · Prove it — the five questions

The five questions climb Grade 6 → Grade 7 → Grade 8 → Grade 9 → Grade 9 challenge, and are marked inside Incandio on your own device, by rule, with an authored diagnosis of the mistake you actually made. The mark schemes stay in the app so that the practice is worth doing; the questions themselves are here.

1. Grade 6 · Define [1 mark] — What is a population?
2. Grade 7 · Explain [2 marks] — A student places quadrats in a field wherever they can see plenty of daisies, saying this saves time because empty quadrats give no useful information. Explain why this is a poor method.
3. Grade 8 · Describe [5 marks] — Describe how you would use quadrats to estimate the number of buttercups in a field. Select every statement that belongs in a full-mark answer.
4. Grade 9 · Calculate [4 marks] — A field measures 60 m × 25 m. Eight quadrats, each 0.5 m × 0.5 m, are placed at random, and the numbers of plantains counted are 2, 6, 3, 0, 5, 4, 4 and 8. Calculate the estimated number of plantains in the field.
5. 9+ · Analyse [5 marks] — A student compares the number of daisies on a mown lawn and in an unmown meadow. They use ten 0.5 m quadrats on the lawn on a sunny morning in May, and four 1 m quadrats in the meadow on a wet afternoon in July. They find more daisies per quadrat in the meadow and conclude that mowing reduces daisy numbers. Select every statement that belongs in a full-mark evaluation.

The people behind this science

Two ways into the same idea — the one who counted what nobody thought worth counting, and the one who showed why counting populations matters. Inside Incandio each of them answers knowing exactly which lesson you have just finished.

Charles Darwin — the one who counted what nobody thought worth counting

Darwin's last book was about earthworms, and it is the closest thing in the nineteenth century to the work this page describes. He estimated their population density in a field by counting the worm casts on measured areas, weighed the soil they brought to the surface, and calculated that worms turn over the entire topsoil of England within a few years. It is sampling and scaling up, done carefully over decades, on an organism everyone else regarded as beneath attention — and it turned out to be one of the most important processes in the whole ecosystem.

• “How did you estimate how many worms were in a field?”
• “Why measure something everybody else ignored?”
• “How can you tell how much soil a population has moved?”
• “What surprised you most about what worms actually do?”

Rachel Carson — the one who showed why counting populations matters

This page teaches how to measure a population, and Carson is the person who demonstrated what those measurements are for. Silent Spring argued in 1962 that pesticides were accumulating through food chains and destroying bird populations, and the evidence was largely counts — of eggs, of nests, of birds heard in spring. She was attacked ferociously by the chemical industry, gave testimony to a Senate committee while dying of cancer, and DDT was banned in the United States a decade later.

• “What made you notice that bird populations were falling?”
• “How do you tell an ordinary fluctuation from a real decline?”
• “Why is measuring a population before a change so important?”
• “What happened to you after the book was published?”

Carry on through the course

• Previous lesson: Natural Selection and Antibiotic Resistance
• Next lesson: Biodiversity, Distribution and Limiting Factors
• Biodiversity, Distribution and Limiting Factors — Why organisms are where they are and not somewhere else — and how to measure the variety of life in a place rather than just its quantity.
• Food Chains, Webs and Pyramids — Who eats whom, drawn three different ways — and why only one of the three can come out upside down.
• Energy Transfer Along a Food Chain — About nine tenths of the energy disappears at every step. Here is exactly where it goes — and what that means for how the world is fed.
• The Carbon and Nitrogen Cycles — The same atoms, used again and again — and the four bacteria that do the work nothing else can.
• Air Pollution and the Greenhouse Gases — Two entirely different problems that both begin with burning — one that poisons what it touches, and one that changes the energy balance of the planet.
• All of Biology · Biology key terms · required practicals · Incandio Science · The 4BI1 specification