Investigating Ecosystems: Sampling, the Lincoln Index and Abiotic Measurement

You cannot count everything, so you sample. An ecosystem investigation estimates the size of populations and measures the abiotic factors that might explain them. The technique depends on whether the organism moves. For plants and slow animals, a quadrat — a square frame of known area — is placed and everything inside is counted, estimated as percentage cover, or recorded as present or absent. Quadrats are placed randomly using coordinates from a random-number table, never by throwing, because a person unconsciously chooses interesting spots. Enough quadrats are needed for the running mean to stop changing. When the question is how a community changes along a gradient — from shore to dune, from shade to sun — quadrats are placed at fixed intervals along a transect line instead, which is systematic sampling and deliberately not random.

Moving animals: capture–mark–release–recapture. A sample is caught, marked harmlessly and released (M individuals). Later a second sample is caught (n) and the marked ones counted (m). If marked animals have mixed back into the population, the proportion marked in the second sample equals the proportion marked in the whole population: m/n = M/N, so the Lincoln index gives N = M × n ÷ m. With 50 marked, 40 recaptured and 8 marked among them, N = 50 × 40 ÷ 8 = 250. The estimate rests on assumptions that must be stated: no births, deaths or migration between samples; marks do not fall off, harm the animal or change its chance of capture; and released animals mix randomly. The weakness is sensitivity when m is small — recapturing 7 instead of 8 moves the estimate by more than 30 animals — so larger samples and repeats are needed.

Abiotic factors and their instruments. Temperature (thermometer or probe, at a fixed time and depth), light (light meter, unreliable under changing cloud), pH (meter, calibrated first), soil moisture (weigh, dry, reweigh), dissolved oxygen (probe, measured on site because a sample changes in the bottle), salinity, flow velocity (float and stopwatch), slope (clinometer), wave action, turbidity. Every measurement is repeated and averaged, taken under the same conditions, and recorded with the time and place. A single reading is an anecdote; reliability comes from repetition.

What the examiner rewards. Naming the method that fits the organism; explaining why random or systematic; stating the assumptions of the Lincoln index and one situation in which each fails; and identifying the limitation of an instrument rather than just naming it. A data table with units, repeats and a mean scores; a single number does not.