Biology • Year 11 • Module 4 • Lesson 10
Ecological Sampling: Quadrats, Transects and Mark-Recapture
Lock in the three core sampling methods, their formulas, when each is appropriate, and the key vocabulary that underpins all of Module 4's fieldwork questions.
1. Term–definition match
Match each term to its definition. Write the correct term in the right-hand column. Choose from: quadrat, transect, line transect, belt transect, mark-recapture, population estimate, observer bias, closed population, mean density, Lincoln-Petersen index. 10 marks
| # | Definition | Matching term |
|---|---|---|
| 1.1 | A defined square or rectangular frame placed in a habitat to count or estimate the organisms inside. | |
| 1.2 | A straight line or strip laid across a habitat to survey the distribution of species along an environmental gradient. | |
| 1.3 | A survey method in which every species touching a tape measure is recorded — records presence and position but not density. | |
| 1.4 | A strip of defined width surveyed on either side of a line — produces quantitative counts and density data along a gradient. | |
| 1.5 | A method for estimating mobile animal populations by tagging individuals, releasing them and checking what fraction of marked animals appears in a second capture. | |
| 1.6 | The formula N = (M × C) / R used to calculate population size from mark-recapture data. | |
| 1.7 | The calculated number of individuals in a habitat derived from sampled data, not a direct count. | |
| 1.8 | The number of individuals per unit area, calculated from all quadrat counts divided by total quadrat area sampled. | |
| 1.9 | The error introduced when a researcher places sampling units in convenient or visually appealing spots rather than randomly. | |
| 1.10 | An assumption of mark-recapture: no births, deaths, immigration or emigration occur between the two sampling events. |
2. Complete the formulas
Fill each blank with the correct term or symbol. 6 marks
2.1 Quadrat sampling — mean density:
Mean density = ______________ / ______________
State what each part of the formula represents.
2.2 Quadrat sampling — population estimate:
Population estimate = ______________ × ______________
State what each part of the formula represents.
2.3 Mark-recapture — Lincoln-Petersen index:
N = ( _____ × _____ ) / _____
Define each variable: N = M = C = R =
3. True or false — with correction
Circle T or F. If false, write the corrected version. 8 marks
3.1 Quadrat sampling is the best method for estimating the population of eastern grey kangaroos in a grassland. T / F
3.2 A belt transect produces quantitative data about how the density of organisms changes along an environmental gradient. T / F
3.3 If marked animals avoid traps after being released, the mark-recapture estimate will be too low. T / F
3.4 Observer bias in quadrat sampling tends to produce an underestimate of population density. T / F
4. Quick calculations — show your working
Show all working for full marks. 8 marks
4.1 A researcher places eight 1 m × 1 m quadrats across a 3 000 m² coastal heath. The plant counts are: 5, 3, 7, 4, 6, 2, 8, 5.
(a) Calculate the mean density of plants per m². 1 mark
(b) Estimate the total plant population in the heath. 1 mark
4.2 Ecologists trap and tag 30 brown trout in a reservoir. One week later they catch 40 trout, of which 6 are tagged.
(a) Using N = (M × C) / R, calculate the estimated trout population. Show your substitution. 2 marks
(b) If five tagged fish died before the second sample, would your estimate be too high or too low? Explain. 2 marks
5. Classify each investigation
For each ecological investigation below, write the most appropriate sampling method (quadrat, line transect, belt transect, or mark-recapture) in the second column, and the main reason why in the third. 8 marks — 1 per method, 1 per reason
| Investigation | Best method | Main reason |
|---|---|---|
| Estimating the density of sea anemones on a rocky platform. | ||
| Mapping how grass species change from a salt marsh edge into a freshwater meadow. | ||
| Counting barnacle density at five heights across an intertidal zone, with precise numbers needed. | ||
| Estimating the population size of common brushtail possums in a National Park. |
Q1 — Term–definition matches
1.1 quadrat • 1.2 transect • 1.3 line transect • 1.4 belt transect • 1.5 mark-recapture • 1.6 Lincoln-Petersen index • 1.7 population estimate • 1.8 mean density • 1.9 observer bias • 1.10 closed population.
Q2 — Formulas
2.1 Mean density = total individuals counted / total quadrat area sampled (in m²). Numerator = sum of all individual counts across all quadrats; denominator = number of quadrats × area per quadrat.
2.2 Population estimate = mean density × total habitat area (in m²). Mean density is from 2.1; total habitat area is the area being estimated.
2.3 N = (M × C) / R. N = estimated total population; M = number marked and released in first sample; C = total individuals in second (recapture) sample; R = number of marked individuals in the second sample.
Q3 — True / false
3.1 False. Kangaroos are highly mobile — they move in and out of a quadrat before counting can be completed. Mark-recapture is the appropriate method for mobile animals.
3.2 True.
3.3 True. Trap shyness reduces R (marked recaptures) below the expected value. A smaller R makes the denominator of N = (M × C) / R smaller, inflating the estimate — so the estimate is too high, not low. Note: if the question asks about the effect on R, the answer is "R is too small", which inflates the estimate. Award the mark for either interpretation as long as reasoning is correct.
3.4 False. Observer bias leads to placing quadrats in areas of highest density, producing an overestimate of population density, not an underestimate.
Q4 — Calculations
4.1(a) Total count = 5 + 3 + 7 + 4 + 6 + 2 + 8 + 5 = 40 individuals. Total quadrat area = 8 × 1 = 8 m². Mean density = 40 / 8 = 5 plants per m².
4.1(b) Population estimate = 5 × 3 000 = 15 000 plants.
4.2(a) N = (M × C) / R = (30 × 40) / 6 = 1 200 / 6 = 200 trout.
4.2(b) The estimate would be too high. Five tagged fish died, so only 25 marked fish were actually in the population at the time of recapture. The effective M should be ~25, not 30. Using M = 30 inflates the numerator (M × C), producing an overestimate. This also means R is smaller than expected for a true M of 25, further inflating N. This is an open-population violation.
Q5 — Classification
Row 1: Quadrat sampling — sea anemones are sessile (permanently attached); they remain inside the quadrat boundary long enough to be counted.
Row 2: Line transect — a quick survey of which species are present along the salt-marsh-to-meadow gradient is sufficient; precise density is not required.
Row 3: Belt transect — precise counts and density data are needed at each height interval, so a strip of defined width is required rather than just recording species that touch a line.
Row 4: Mark-recapture — brushtail possums are mobile and nocturnal; they cannot be counted in a fixed area during a brief survey. Capture, tagging and recapture allows a statistical estimate of the total population.