Quadrats, Transects & Field Sampling

Revise Quadrats, Transects & Field Sampling for Biology 6 (IAL) WBI16 (A2 Level) — revision notes, 8 practice questions and instant AI marking.

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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.

abiotic factors

Easy marks

4. How Many Quadrats?

The method

running mean levels off

5. Mark-Release-Recapture

The Lincoln index

Practice Questions

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