Quadrat Sampling – Estimating Population Size
Integrated ScienceEcologyAges 13–14
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Sign in to playA 20 m × 12 m field holds four plant species (daisies, clumped dandelions, plantain and ferns) plus grass. Place quadrats at random or along a transect from shade into sun or across a trampled path, count the plants and the grass cover in each one, and estimate the population as N = mean per quadrat × field area / quadrat area, then compare it with the true count. Change the quadrat size and number to see how accuracy changes; tables give species frequency and Simpson's diversity index for each zone.
Lesson: Estimating population size and density with quadrats; distribution of organisms along a transect
What it shows
Ecologists cannot count every plant in a habitat, so they sample with square frames called quadrats. The mean number of plants per quadrat, multiplied by the field area divided by the quadrat area, estimates the population size. This simulation builds a field where daisies need sunlight, dandelions grow in clumps, plantain tolerates trampling and ferns prefer shade. Random quadrats test the estimate against the true count, while a transect shows how each species changes across a light gradient or a path. Grass is recorded as percentage cover, and Simpson's index compares the diversity of zones.
How to use
Choose a Habitat, a Sampling method and the Species to estimate. Set Quadrat side and Number of quadrats, then press Sample automatically or Place 1 quadrat, or tap the field to place one yourself. Watch the estimate graph approach the true count. Untick Show all plants to see only what lies inside the quadrats. Use Clear quadrats or New field to start again.
Parameters you can change
- Habitat Even meadow, Woodland shade to open sun, Meadow with a trampled path
- Quadrat placement Random, Along a transect
- Quadrat side 0.5–2 m
- Number of quadrats 5–40 quadrats
- Species to estimate Daisy, Dandelion, Plantain, Fern
Questions to explore
- Why is the estimate for clumped dandelions less reliable than for daisies when only a few quadrats are used?
- Along the transect from shade into sun, how do the numbers of ferns and daisies change, and why?
- Which zone has the highest Simpson's diversity index, and what does that tell you about it?
