Revise Quadrats, Transects & Field Sampling for Biology 6 (IAL) WBI16 (A2 Level) — revision notes and instant AI marking. Free to start.
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Edexcel IAL Biology • Unit 6: Practical Skills in Biology II
Quadrats, Transects & Field Sampling
🌾 Big idea: You can never count everything, so you count a sample and scale up. Every mark in fieldwork comes from two decisions: making the sample representative (random where you want an average, systematic where you want a gradient) and making it big enough that the answer would not change if you sampled again.
Summary — What This Topic Covers
Random sampling for abundance, systematic for a gradient
Using quadrats: density, frequency and percentage cover
Belt and line transects, and when each is right
Deciding how many quadrats, and where the number comes from
The mark-release-recapture method and the assumptions it rests on
1. Random vs Systematic Sampling
The distinction
Random sampling gives every point an equal chance of being chosen — use it to estimate abundance across a uniform area.
Systematic sampling places samples at set intervals along a line — use it to study how a community changes along an environmental gradient.
RANDOM — "how much marram grass is in this dune?"
lay two tapes at right angles as axes
generate random coordinate pairs with a random number generator
place a quadrat at each pair
SYSTEMATIC — "how does the community change from shore to inland?"
lay a transect line along the gradient
sample at fixed intervals, e.g. every 5 m
Why "somewhere typical" is wrong
Choosing where to put a quadrat by eye is bias. You unconsciously pick places that match what you expect, so the sample is not representative. Random coordinates remove the choice from you entirely.
The one they test
Using random sampling on a gradient destroys the pattern you are trying to find — the very thing being investigated gets averaged away. Match the method to the question.
2. Measuring with a Quadrat
MEASURE DEFINITION BEST FOR
────────────────────────────────────────────────────────────────────
Density number of individuals per m² countable plants
with clear limits
Frequency % of quadrats in which the species that are
species occurs at all hard to count
Percentage cover % of the quadrat area the grasses, mosses,
species covers spreading species
ACFOR scale abundant/common/frequent/ quick surveys;
occasional/rare subjective
Estimate percentage cover using a gridded quadrat — a 10×10 grid means each square is 1 %
Cover can total more than 100 % where plants overlap in layers, and that is not an error
Density needs individuals you can actually distinguish — hopeless for grasses
ACFOR is subjective; two students will disagree, which is why it is the weakest of the four
3. Transects
Key Terms
A line transect records only the organisms touching the line at set points.
A belt transect places quadrats along the line, giving abundance as well as presence. An interrupted belt transect samples at intervals rather than continuously.
Belt transects give far more information and are almost always the better answer in an exam, unless the question specifically asks for speed over detail. Record the abiotic factors alongside — light, soil pH, soil moisture, distance — because the point of a transect is to relate the biology to the gradient.
Easy marks
Say that abiotic factors are measured at each quadrat. A transect that records only species tells you a pattern exists but nothing about what drives it.
4. How Many Quadrats?
The method
Plot a running mean: calculate the cumulative mean number of species (or individuals) as each quadrat is added, and plot it against the number of quadrats. Where the curve levels off, further quadrats stop changing the answer — that is the minimum sample size.
quadrats: 1 2 3 4 5 6 7 8 9 10
running mean: 4.0 5.5 6.3 7.0 7.4 7.6 7.7 7.7 7.8 7.8
↑
levels off here — about 7 quadrats is enough
What mark schemes look for
This is the answer to "how would you decide how many quadrats to use?" — not "as many as possible" or "ten". The running mean gives a reason, and the reason is the mark.
5. Mark-Release-Recapture
The Lincoln index
population = (number in 1st sample × number in 2nd sample) ÷ number marked in the 2nd sample
first sample: 60 woodlice caught, marked, released
second sample: 50 caught, of which 15 were marked
population = (60 × 50) ÷ 15 = 200
The marked individuals must mix fully back into the population before the second sample
The mark must not harm the animal or make it more visible to predators
The mark must not rub off between samples
No significant births, deaths, immigration or emigration between the two samples
Marked individuals must be equally likely to be recaptured — not made "trap-shy" by the experience
Where it goes wrong
If any assumption fails, the estimate is biased in a predictable direction. If marks rub off, fewer marked animals are recaptured, so the calculated population comes out too high.
Practice Questions
Practice Question 1
A student wants to compare plant species diversity in a mown and an unmown field. Describe how they should place their quadrats and justify the method.
Practice Question 2
Explain why random sampling would be inappropriate for investigating how vegetation changes from the edge of a pond to open grassland.
Practice Question 3
In a mark-release-recapture study, 80 snails were marked and released. A second sample of 60 contained 20 marked snails. Calculate the population size and state two assumptions.
Practice Question 4
Describe how a student could decide how many quadrats to use, and explain why "as many as possible" is not an adequate answer.
What to Memorise
Random → abundanceSystematic/transect → gradientChoosing by eye = biasDensity, frequency, % cover, ACFORGridded quadrat: 10×10 → each square = 1 %Cover can exceed 100 %Belt transect + abiotic factorsRunning mean levels off → enough quadratsLincoln index = (n₁ × n₂) ÷ marked in n₂
Concepts Checklist
Exam Tips
What mark schemes look for
Match the method to the question: random for abundance, transect for a gradient. Choosing the wrong one usually costs the whole section.
The trap
"Place quadrats where the vegetation is typical." It is bias, and it is the most common wrong answer in fieldwork questions.
Easy marks
Say abiotic factors are measured at each quadrat, and name two — light intensity and soil moisture will do for most habitats.
Worth remembering
For "how many quadrats", the running mean levelling off is the answer with a reason attached. A bare number is not.