Sampling Techniques and Species Abundance

Why ecologists take samples

An ecologist might want to know how many daisies grow in a field, or whether they become less common towards a shaded edge. Counting every plant would take too long, so the ecologist samples smaller areas. Abundance describes how many of the chosen species are present; distribution describes where they are found. A good sample must represent the habitat rather than just its most convenient patches.

Random quadrats: estimating population size

A quadrat is a square frame of known area. It is useful for plants and other organisms that stay in place while they are counted. To sample a field without choosing favoured spots, place two tape measures at right angles along its edges to make a coordinate grid. Generate pairs of random coordinates, place the quadrat at those positions and count the target species in each one. If individual plants are difficult to distinguish, estimate their percentage cover instead. Record every result, including zero counts, and use many quadrats so that an unusually crowded or empty patch has less influence.

For counts, calculate the mean number per quadrat. The area ratio tells you how many quadrat-sized areas would fit in the whole habitat:

$\text{estimated population size}=\text{mean count per quadrat}\times\frac{\text{total habitat area}}{\text{area of one quadrat}}$

For example, suppose five counts from $1\,\text{m}^2$ quadrats are 2, 3, 3, 4 and 8 plants. Their mean is $20\div5=4$ plants per quadrat. If the field is $20\,\text{m}^2$, the estimate is plants. It is an , not an exact count; five quadrats would be a small sample for a real field survey.

Transects: finding a pattern across a habitat

Random positions help estimate a whole population, but they may obscure a change from one part of a habitat to another. A transect follows a line across such a change—for example, from shade into open ground. Lay a tape measure along the line and place the same-sized quadrat at regular distances, such as every 2 m. At each position, record the target species’ count or percentage cover. To investigate the effect of a factor, measure it at the same positions: a light meter could measure light intensity along a shade-to-sun transect. Keep the quadrat size and counting method the same so the samples can be compared.

Plot abundance against distance along the transect, choosing scales that show all the values clearly. If abundance rises as light intensity rises, the data show an association. Other conditions may also change along the line, so the pattern alone does not prove that light caused the increase.

Data for Abundance along an illustrative transect

Describing the abundance data

The mean uses every count: add the counts and divide by the number of quadrats. For the counts 2, 3, 3, 4 and 8, the mean is 4. In order, the middle count is 3, so the median is 3; the most frequent count is 3, so the mode is also 3. If there is an even number of counts, find the mean of the two middle values. If no count occurs more often than the others, there is no mode. These summaries describe the sampled quadrats; the mean is the one used in the population estimate above.