Quadrats and transects sim

What the sim shows

Part A, random sampling. A model of a 10 m × 10 m lawn holds a fixed set of daisy plants, growing in patches. Press Random numbers, then drag the quadrat (0.5 m × 0.5 m = 0.25 m²) so its top-left corner sits at those coordinates on the two tapes. Placing quadrats at random means every part of the field has the same chance of being sampled, so you cannot bias the result by choosing thick or bare patches. Count the plants in the close-up: a plant counts if it is inside the frame, or touches the top or left side (and not the bottom or right side); plants touching the bottom or right side do not count, so no plant on a shared edge is counted twice. This is one common edge rule; some classes instead count a plant only if more than half of it is inside the frame. Mean number per quadrat / 0.25 m² = density per m²; density × 100 m² = estimated total. As you count more quadrats the running mean usually becomes steadier; steadier is not the same as correct, because by chance (or through bias) a sample can settle some way from the true value. Part B, a transect. A tape runs from a tree trunk (shade) into the open. At each metre a gridded quadrat (25 squares, 4 % each) is laid beside the tape: count the squares more than half covered by grass to get the percentage cover, and read the light meter. A scatter graph of light against cover shows a positive correlation. A correlation does not prove cause on its own: soil moisture, leaf litter and trampling also change from the trunk to the open. Simplified on purpose: the field is a model with a fixed, seeded set of 1446 plants, so the true total can be counted and compared, which a real survey cannot do; plants never overlap and none sits on the line of an edge; rosettes are drawn as flat circles; the quadrat frame is drawn thicker than scale so it can be seen; the light meter is shown on the bench, but in real life it is held level at each point. Light values are illustrative (≈), in the range of bright overcast daylight in the open and shade under a tree; a real meter reading would also change with cloud and the time of day. Grass cover in Part B is modelled from light plus a fixed allowance for other factors at each point.

• The quadrat you are using is outlined; drag it
• White dots: daisy plants in the model field
• Close-up: the ground inside the quadrat, seen from above

Use it in your lessons

• Try it here: anyone can unlock 2 figures free, and a free account unlocks up to 5. Runs on a laptop, a Chromebook or a phone; nothing to install.
• Present it and share student links with the teacher plan, from US$5 a month, billed yearly.
• Put it on your own website or LMS for US$9, paid once, or every sim from US$29 a month.

Free guide: 12 practicals as sims, and how to run one in a lesson →

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