Estimating population size

Use this equation to calculate the number of grass plants found in the whole area:

Total number of grass plants =

number \: of \: grass \: plants \: found \: in \: sample \times \frac{total \:area (m^2)}{sample \: area (m^2)}


A student wanted to know how many flowers there were in a 50 m2 field and threw enough quadrats to cover 10 m2, finding 40 flowers. How could the student use the equation above to calculate the estimated number of flowers in the whole field?

\text{total number of flowers in the field}= 40 \times \frac{50}{10} = 200

Kite diagram showing grasses and dandelions over 25m. Grasses rise slightly and fall slightly over long period then rise and fall steeply to 25m. Dandelions rise steeply, then fall gradually to 20m.

In the example above, the distribution of dandelion plants gradually changes from five metres to 20 metres along the transect. A quadrat has been placed at regular intervals of a metre (or a few metres) along the transect.

A gradual change in the distribution of species across a habitat is called zonation. It can happen because of a gradual change in an abiotic (non-living) factor.

A transect is usually used to investigate a gradual change in a habitat rather than to simply estimate the number of organisms within it.


Using the data for the distribution of grasses in the kite diagram above, suggest how far along the transect you would find a well-used footpath.

15 m as this is where the number of grass plants is the least.


When using transects to record the amount of moss found, it is impossible to count the number of plants, so it is usual to estimate percentage cover.

If a quadrat contains 25 sections and moss covers 16 of these sections, what percentage of the quadrat is covered in moss?

\frac{16}{25} \times 100 = 64%

The quadrat method is not practical when attempting to estimate the population of animals which can move fast or be scared off by quadrats landing nearby.

A better way to estimate the population size of an animal species is using the capture-mark-recapture method:

  1. Animals are trapped, eg using pitfall traps.
  2. They are marked in a harmless way and then released.
  3. Traps are used again a few days later.
  4. The numbers of marked and unmarked animals caught in the traps are recorded.

Once the data has been collected, the following equation can be used to estimate the population of animals in a particular habitat:

Population of animals =

\frac{\text{Number found in first sample } \times \text{number found in second sample}}{\text{Number found in second sample which were already marked}}


A student was attempting to estimate how many woodlice were living at the bottom of her garden. She sat for one hour collecting every woodlouse she saw into a jar. She counted them, there were 26, and marked their back with a small amount of white paint. She then released them back where she found them. A week later she repeated the exercise at the same time of day for one hour. She counted all of the marked woodlice she collected that day - there were 15. She also found another 10 who were unmarked, totaling 25 woodlice.

Use the equation above to estimate the number of woodlice in the area.

\frac{26 \times 25}{15} = 43


What must the student assume for this result to be deemed accurate (close to the true value)?

  1. There was no immigration or emigration during the time between the samples being taken.
  2. The marking of the organism neither directly harmed the organism nor made it easier for it to be seen by predators.