Population & Community Lab | Estimate and model populations

Meadow ready.

Meadow #

How many of one species are in the meadow? Counting every plant takes too long, so sample. The meadow is divided into 40 equal squares — count a few and scale up.

Let me choose the squares myself (click the meadow) Show the 40-square grid

Square	Flowers	Mean	Estimate

No squares sampled yet.

Talking point. Add squares one at a time and watch the estimate settle. Why does it swing so much at first? Each species grows only where the ground suits it — so where you sample matters, and one quadrat is never enough.

You cannot count rabbits by eye — they move and hide. Catch a sample, mark them, let them mix back in, then catch again and see how many carry a mark.

1 Catch and mark

First sample size (n₁)50

2 Release and let them mix

3 Catch again

Second sample size (n₂)50

! Break an assumption

Marks can fall off between samples Rabbits are born, die and wander in or out Marked rabbits are easier to catch second time

Turn one on, run a few trials, and see which way the estimate is pushed. That is the difference between quoting the Lincoln index and understanding it.

Do buttercups and red clover grow together, keep apart, or take no notice of each other? Collect the data, then work through the whole test yourself.

H₀ — there is no association between the two species: each is distributed independently of the other.

1 Record squares

Let me choose the squares myself (click the meadow)

2 Observed frequencies

Tally your field notes into the table below — every box, totals included.

	Red clover present	Red clover absent	Row total
Buttercup present
Buttercup absent
Column total

3 Expected frequencies

If H₀ were true, how many squares would land in each box? For every box, E = row total × column total ÷ grand total. Give each to 2 decimal places.

	Red clover present	Red clover absent
Buttercup present
Buttercup absent

Demo mode — fill the working in for me

Meadow set-up

Leave the set-up hidden for students. Forcing a case is useful for a worked example on the board, or for giving two groups contrasting meadows to compare.