Nobody can count every plant in a meadow or every beetle in a wood. This lesson defines ecosystem and niche, sets out the three levels of biodiversity, and shows how random quadrats, Simpson's index and mark-release-recapture turn a fair sample into numbers you can compare.
An ecosystem is a community of organisms and its physical environment, interacting; a niche is the role of a species in its ecosystem, including what it feeds on, where it lives and how it interacts with other organisms and the environment. Biodiversity can be assessed at three levels: the number and range of ecosystems and habitats, the number of species and their relative abundance, and the genetic variation within each species. Samples must be random to avoid bias. Frame quadrats placed at random coordinates give density, frequency or percentage cover of plants and slow-moving animals, and Simpson's index of diversity, D = 1 − Σ(n/N)2, combines richness and evenness on a scale of 0 to 1. Mobile animals are estimated by mark-release-recapture with the Lincoln index, N = (n1 × n2) ÷ m2.
Fig. 1: two sites with the same five species and the same total; the even site has the higher Simpson's index
Section 01
Ecosystems, niches and three levels of biodiversity.
Biodiversity is more than a list of species. Before sampling, be clear what you are measuring.
Ecosystem: a relatively self-contained, interacting community of organisms together with the abiotic environment in which they live.
Niche: the role of a species in an ecosystem: how it obtains energy and nutrients, where it lives, and how it interacts with other species and its physical environment. Two species cannot occupy exactly the same niche for long, as one out-competes the other.
Level of biodiversity
What is assessed
ecosystems and habitats
the number and range of different ecosystems and habitats in an area (for example forest, grassland, wetland, coral reef)
species
species richness (the number of species) and the relative abundance of each, that is, how evenly individuals are spread among the species
genetic
the genetic variation within each species: the number of different alleles of each gene in the population
Section 02
Random quadrats and Simpson's index.
Choose the wild meadow and press Place 10 random quadrats. Read the counts of the five species and Simpson's D. Then switch to the lawn, where clover dominates, and sample again.
Why random: if you choose where to place quadrats you tend to pick interesting or easy spots, so the sample is biased. Lay two tapes at right angles to make a grid and use random numbers (a calculator or table) as coordinates.
Frame quadrat: a square frame (often 0.25 m2 or 1 m2). Record density (number of individuals per m2), frequency (the percentage of quadrats in which a species occurs) or percentage cover (the percentage of the quadrat area covered, used when individuals cannot be counted, such as grasses).
Enough quadrats: keep sampling until the running mean, or the number of species found, levels off.
Simpson's index: D = 1 − Σ(n/N)2, where n is the number of individuals of one species and N the total of all species. Work out n/N for each species, square it, add the squares, and take the sum from 1.
Meaning of D: values close to 1 mean high diversity (many species, evenly spread); values close to 0 mean low diversity, with one or a few species dominating. A low-diversity ecosystem is less stable and more easily disrupted by change.
Examiner trap
Square each n/N before adding, then subtract the total from 1. Adding the n/N values first always gives 1, and forgetting the final subtraction gives a number that falls as diversity rises.
Section 03
Mark, release, recapture.
Press 1. Catch and mark, 2. Release and wait, then 3. Second catch. Compare the Lincoln index estimate with the true population shown in brackets, and repeat to see how much the estimate varies.
Method: catch a sample of a mobile animal (pitfall traps, sweep nets, small mammal traps), count it (n1) and mark each individual harmlessly. Release them where they were caught.
Wait: leave enough time for the marked animals to mix randomly back into the population, then take a second sample (n2) and count how many are marked (m2).
Lincoln index: estimated population N = (n1 × n2) ÷ m2.
Assumptions: marks are not lost and do not make animals more visible to predators or less likely to survive; marked animals mix fully; there are no births, deaths, immigration or emigration between samples; every individual has the same chance of being caught.
Examiner trap
Put the recaptured marked number m2 on the bottom. If marked animals are more likely to be caught a second time (trap-happy), m2 is too large and the population is underestimated.
Stage 1 · Learn
Check what the sim just showed you
Four quick checks. Each correct answer earns XP and records this skill on your profile.
Quick check+10 XP
In the quadrat tab, why is the lawn's Simpson's index lower than the wild meadow's?
Quick check+10 XP
How does the simulator choose where to put each quadrat?
Quick check+10 XP
In the mark-release-recapture tab, why must you press Release and wait before the second catch?
Quick check+10 XP
A run of the simulator gives n1 = 30, n2 = 40 and m2 = 6. What is the Lincoln index estimate?
Stage 2 · Exam
Exam-style questions
Unlocks once the four checks above are done. Worth more XP, written to A Level Paper 4 standard.
Finish the four checks above to unlock the exam questions
Exam style+20 XP
Three species are counted in random quadrats: 10, 6 and 4 individuals. What is Simpson's index of diversity, D?
Exam style+20 XP
Which statement best describes the niche of a species?
Exam style+20 XP
Which method is most suitable for estimating the population of woodlice in a garden?
Exam style+20 XP
Two forests have the same number of tree species. Forest P has D = 0.85 and forest Q has D = 0.40. Which conclusion is valid?
Skill unlocked
Sampling and measuring biodiversity, mastered.
This skill is now recorded on your profile. Keep the chain going.