Why ecologists take samples

1. A habitat holds far more organisms than anyone could count one by one, so an ecologist measures a small part of it and scales the result up to the whole area.
2. The small part that is measured is called a sample, and the values recorded in it are used to estimate the values for the whole habitat.
3. Counting every individual would take far too long, would trample and damage the habitat, and would be impossible for organisms that are buried, hidden under stones or moving between areas.
4. A sample is only useful if it is representative, meaning the species and their proportions inside the sample match those across the whole habitat.
5. Choosing where to place a sample by eye introduces bias, because a person is drawn towards patches that look unusual, easy to reach or rich in the species being studied.
6. Random sampling removes that bias by using a random number generator to select the coordinates of each sample position, so every point in the habitat has the same chance of being chosen.
7. A single sample may land on an unusual patch, so many samples are taken and a mean is calculated, which makes the estimate more reliable.
8. Two separate measurements are taken from the samples: abundance, which answers how many, and distribution, which answers whereabouts.

Abundance, distribution and density

1. Abundance is recorded either as a count of individuals, as a percentage cover, or on a descriptive scale such as abundant, common, frequent, occasional and rare.
2. Counting individuals works for animals and for plants that grow as separate units, such as daisies, dandelions and plantains.
3. Percentage cover is used instead for plants that spread sideways into a mat, such as grass, moss or clover, where one plant cannot be separated from the next.
4. Population density allows two habitats of different sizes to be compared fairly, because it is always expressed per unit area.
5. Density is calculated as $density=\frac{number\; of\; individuals}{area}$ and is written with units such as $m^{-2}$.
6. Distribution describes the pattern across the habitat, for example whether a species is spread evenly, clumped into patches, or restricted to one end of the area.
7. A pattern in distribution is usually explained by an abiotic factor that changes across the habitat, such as light intensity, soil moisture, soil pH, temperature or exposure to wind.
8. A pattern may also be explained by a biotic factor, such as competition for light from a taller species or grazing by rabbits along one edge of a field.

Equipment for collecting organisms

1. A quadrat is a rigid square frame of known area, commonly $0.5m\times 0.5m$, which encloses an area of $0.25m^2$.
2. A gridded quadrat is divided by wires into $100$ small squares, so the number of small squares a species occupies can be read straight off as a percentage cover.
3. A pooter is used for small insects: the collector sucks on one tube while the other tube is held over the insect, and the insect is drawn into the jar by the moving air.
4. A fine gauze is fixed across the inner end of the mouth tube, which stops the insect or any dust being drawn into the collector's mouth.
5. A sweep net is swung through long grass in a figure of eight, and the invertebrates knocked off the stems collect in the bag at the end of each sweep.
6. A pond net is swept through open water and through weed, and the catch is emptied into a shallow white tray so that pale, small animals show up clearly against the background.
7. A pitfall trap is a container sunk into the soil with its rim level with the surface, so that ground-dwelling invertebrates walk in and fall to the bottom.
8. A stone raised on pebbles is placed over the pitfall trap, which keeps out rain and stops birds taking the catch.
9. A beating tray is held under a branch while the branch is tapped sharply, so that the invertebrates living on the leaves drop onto the sheet and can be collected.
10. An identification key is then used to name what has been caught, using paired statements about visible features that are followed one step at a time until a single species is left.
11. A key only gives the right answer if each feature is checked properly, so specimens are examined in a tray or under a hand lens rather than identified from memory.

Scaling a sample up to a population estimate

1. The mean number per quadrat is found by adding the counts from every quadrat and dividing by the number of quadrats used.
2. The mean is then converted into a population density using $density=\frac{mean\; number\; per\; quadrat}{area\; of\; one\; quadrat}$.
3. The estimated total population is found with $population=density\times total\; area\; of\; the\; habitat$.
4. The same calculation can be done in one step with $population=\frac{total\; area}{area\; of\; one\; quadrat}\times mean\; number\; per\; quadrat$.
5. Both areas must be in the same unit before dividing, so a quadrat area given in ${cm}^2$ is converted to $m^2$ first, using $1m^2=10000{cm}^2$.
6. Percentage cover is estimated from a gridded quadrat by counting the small squares that are more than half filled by the species, then treating that count as a percentage.
7. Percentage cover is scaled up to an area rather than to a number, so $40\%$ cover across a $200m^2$ field means about $80m^2$ of the field is covered by that species.
8. The answer is always an estimate, because it is calculated from a small fraction of the habitat rather than from a complete count.

Using a transect to study distribution

1. Random sampling spreads quadrats evenly over a whole area, so it estimates abundance well but hides any pattern that runs in one direction.
2. A transect is used instead when the question is how a species changes across a gradient, such as from open grass into the shade under a hedge, or from the water's edge up a sand dune.
3. A transect is systematic sampling, because the sampling points are set at fixed intervals along a line rather than chosen at random.
4. A line transect records only the species that touch the line at each marked point, which is quick and suits a steep, narrow gradient.
5. A belt transect places a quadrat at each interval along the line and records abundance inside it, which gives quantitative data at every point.
6. A continuous belt transect places quadrats end to end with no gaps, while an interrupted belt transect places them at set intervals such as every $2m$, which covers a long gradient in a reasonable time.
7. At every point, the abundance of the species and the value of at least one abiotic factor are recorded, so the two sets of data can be compared.
8. Light intensity is measured with a light meter, air and soil temperature with a thermometer, soil pH with a pH probe, and soil water with a moisture meter.
9. The results are plotted with distance along the transect on the horizontal axis, so that abundance and the abiotic factor can be read against the same scale.
10. A matching rise and fall in the two lines shows a correlation between the species and that factor, but it does not by itself prove the factor is the cause.

Capture-recapture for animals that move

1. Quadrats are useless for animals that run, fly or swim away, because the animal leaves the frame before it can be counted.
2. Capture-recapture is used instead, and it estimates the population from the proportion of marked animals that turn up in a second catch.
3. A first sample is caught, the number is recorded, every animal is marked harmlessly, and all of them are released back into the same place.
4. Enough time is left for the marked animals to mix evenly back into the population, usually one or two days for small mammals.
5. A second sample is then caught in the same way, and the total caught and the number carrying a mark are both recorded.
6. The population is estimated with $N=\frac{n_1\times n_2}{m_2}$, where $n_1$ is the number caught and marked first, $n_2$ is the number caught in the second sample and $m_2$ is the number of marked animals in that second sample.
7. A small proportion of marked animals in the second catch means the marked individuals were diluted by a large population, so the estimate comes out high.
8. The method assumes there are no births, deaths, immigration or emigration between the two samples.
9. It also assumes the mark stays on and is not washed, rubbed or licked off, since a lost mark makes the population look larger than it is.
10. It assumes the mark does not affect survival, so a bright paint that makes an animal easier for a predator to see would push the estimate too high.
11. Finally it assumes every individual has an equal chance of being caught, which fails if an animal trapped once learns to avoid the trap, or returns to it for the bait.

Making a field investigation reliable

1. The same size of quadrat is used throughout, because counts from quadrats of different areas cannot be combined into one mean.
2. The same person estimates percentage cover for the whole investigation, since two people judge the halfway point of a small square differently.
3. All the samples are taken on the same day and at a similar time, because light, temperature and animal activity change through the day and through the season.
4. At least ten samples are taken, and more are added until the running mean stops changing much, which shows that enough of the habitat has been covered.
5. An anomalous result is identified before the mean is calculated and is checked rather than quietly deleted, for example a quadrat that happened to land on a footpath.
6. Repeating a transect along two or three parallel lines shows whether a pattern holds across the habitat or was a feature of one line.
7. The method is described in enough detail for another student to repeat it exactly, which is what makes the results reproducible.