7.4.4 Estimating the Size of a Population

Estimating Non-Motile Organisms

For slow-moving or stationary organisms such as plants, quadrats and transects are used:

1. Random Sampling with Quadrats:

• Quadrats are placed randomly within the study area to avoid bias.
• The abundance of organisms is recorded in the quadrat.

2. Belt Transects:

• Quadrats are placed systematically along a line (transect) to measure changes across a gradient (e.g., light, moisture). Measuring Abundance in Quadrats:

• Percentage Cover:

  • The proportion of the quadrat area covered by the organism.
  • Useful for plants or algae that are difficult to count.

• Frequency:

  • The number of quadrats in which the organism appears, expressed as a decimal or percentage.

Estimating Motile Organisms

For fast-moving or hidden animals, the mark-release-recapture method is used:

Steps:

3. Capture a sample of the population and mark them in a way that does not harm or affect behaviour.
4. Release the marked individuals back into their habitat and allow time for them to mix evenly with the rest of the population.
5. Recapture a second sample and count how many are marked. Population Size Formula:

$$\text{Population size} = \frac{\text{(Number in first sample)} \times \text{(Number in second sample)}}{\text{Number of marked individuals in the second sample}}$$

Assumptions of Mark-Release-Recapture

• The proportion of marked to unmarked individuals in the second sample is representative of the entire population.
• Marked individuals have mixed evenly with the population.
• The population is closed (no immigration or emigration).
• There are no significant births or deaths during the study.
• The mark is non-toxic, does not increase predation risk, and does not affect behaviour.
• Marks remain visible and do not wear off before recapture.