What you practice
Each set draws new quadrats with plant symbols and a legend. You count plants using an edge rule, read % cover off a 10 × 10 string grid, find local frequency from 25 small squares, turn quadrat counts into a population estimate, read a belt transect’s kite diagram and draw one from a table, and work out Simpson’s index to compare two sites. Wrong answers that match a common slip get a note, such as counting every edge plant or squaring n instead of using n(n − 1).
Pick your course first. AQA, OCR, the IB and AP write Simpson’s index in different forms, and each sheet prints its course’s form and what a higher value means. AQA GCSE sets leave the index out, as the GCSE specification does.
Example: one count, two forms of Simpson’s index
Set SMP5A2 (AQA A-level) counts 8 red clover, 7 white clover, 14 ribwort plantain and 11 daisies, so N = 40. Σn(n − 1) = 8 × 7 + 7 × 6 + 14 × 13 + 11 × 10 = 390, and d = 40 × 39 ÷ 390 = 4.00. On an OCR or AP sheet the same counts give Σ(n/N)² = 430 ÷ 1,600 = 0.269 and D = 1 − 0.269 = 0.73. Different numbers, same meaning: higher is more diverse. Open set SMP5A2
Questions
How do you calculate percentage cover with a quadrat?
Use a quadrat with a 10 × 10 grid, so each small square is 1%. Count the squares more than half covered by the species: that number is its % cover. This sheet uses the more-than-half rule; if your teacher uses another, follow theirs.
Do plants on the edge of a quadrat count?
Pick one rule and keep to it. This sheet counts a plant touching the top or left side and leaves out one touching the bottom or right side, so edge plants are counted about half the time. Plants wholly outside the frame never count.
How do you estimate population size from quadrats?
Find the mean number per quadrat, then multiply by how many quadrats would fit in the whole area. AQA’s GCSE example: 50 daisies in ten 0.25 m × 0.25 m quadrats (0.625 m² sampled) in a 400 m² field gives 400 ÷ 0.625 × 50 = 32,000.
How do you draw a kite diagram?
Draw a centre line for each species along the distance axis. At each distance, make the kite as wide as the abundance, half above the line and half below, then join the points. Where the species is missing, the kite pinches to the line.
Which Simpson’s index formula should I use?
The one your course gives. AQA A-level uses d = N(N − 1) ÷ Σn(n − 1). OCR A-level and the AP Biology equation sheet use 1 − Σ(n/N)². The IB guide names Simpson’s reciprocal index but prints no formula; IB sheets here use its usual form, N(N − 1) ÷ Σn(n − 1). Each sheet prints its course’s form.
Does a higher Simpson’s index mean more diversity?
Yes, in every form here. N(N − 1) ÷ Σn(n − 1) is 1 for a single species and grows with more species spread more evenly; 1 − Σ(n/N)² runs from 0 to nearly 1. Σn(n − 1) ÷ N(N − 1) on its own, without the 1 − in front, works the other way: lower means more diverse.
What is the difference between percentage cover and frequency?
Percentage cover is how much ground a species covers, useful for grass and moss where single plants are hard to count. Frequency is how often it turns up: the % of quadrats, or of small squares in one quadrat, that contain it. AQA A-level asks for both.
Sources, read October 27, 2026: AQA A-level Biology specification (PDF), 3.4.6 index of diversity and 3.7.4 quadrats, belt transects, % cover and frequency; AQA GCSE Biology practicals handbook (PDF), required practical 9 and the population estimate; OCR A Level Biology A specification (PDF), 4.2.1(d) Simpson’s Index of Diversity; AP Biology Course and Exam Description (PDF), Topic 8.5 and the equation sheet; IB Biology guide, first assessment 2025: the Simpson reciprocal index, quadrat sampling and transects.