Biology  by Bradford cramly support
Biology by Bradford · IB Biology SL/HL · C4.1.3

Quadrat Field Notebook

Random coordinates, honest counts, one population estimate, and a standard deviation that tells you how it's spread.

01 · Before you go

Plan your survey

This method only works for sessile organisms (plants or animals that stay put) where you can count individuals. Dandelions, daisies, plantains, limpets, barnacles or moss cushions work; grass and anything that runs away don't.

Site & quadrat

Question

Choosing quadrat size. Big enough that most quadrats contain some individuals, small enough to count accurately. Rough guide: 0.25 m² for small plants on a field, 0.01 m² (10 cm × 10 cm) for barnacles.
02 · Pack the bag

Kit list

03 · In the field

How to sample randomly

  1. Mark out the site.Lay two tape measures at right angles along two edges of the area: one is the x-axis, the other the y-axis. Record the dimensions in your plan.
  2. Generate random coordinates.Use the generator in section 4. Don't throw quadrats over your shoulder or pick "typical" spots; both are biased.
  3. Find each coordinate.Walk x metres along the first tape, then y metres at right angles into the site. Place the quadrat's corner exactly there, always the same corner, facing the same way.
  4. Count every individual.Count those inside the frame. For ones on the edge, agree a rule (e.g. count if more than half is inside) and use it every time.
  5. Record straight away.Type the count next to its coordinate. A zero is real data: record it.
  6. Keep going.Sample all your planned coordinates. At least 10 quadrats; 20+ gives a more reliable mean.
  7. Leave no trace.Don't pick or pull anything up. Step carefully between quadrats.
04 · Data

Coordinates & counts

Generate your random coordinates, then fill in each count as you reach it. Everything saves on this device; export in section 6.

05 · Back at base

Mean, SD, estimate

Counts per quadrat. Shaded band = mean ± 1 SD.

What the SD tells you. The standard deviation measures how much the counts vary between quadrats. If it is small compared with the mean, most quadrats held similar numbers: the population is evenly spread. If it is large (perhaps even bigger than the mean) some quadrats were packed and others empty: the population is patchy or clumped.
Check it on your calculator

You don't need to memorise the formula. Enter your counts in your calculator's statistics mode (1-variable) and read off x̄ (mean) and s or sₓ (the sample standard deviation, sometimes labelled σₙ₋₁). In a spreadsheet use =AVERAGE() and =STDEV.S().

s = √( Σ(x − x̄)² ⁄ (n − 1) )
Going further: SD vs √mean (beyond the syllabus)

Record at least five quadrats to see this.

06 · Keep it

Save your data