Why quadrats have limitations for measuring animal abundance

Quadrats are commonly used to estimate plant abundance and distribution.

However, they have limitations when measuring animal populations:

• Most animals are mobile and do not remain in a fixed area making them difficult to sample accurately.
• Slow moving animals could be counted but fast moving animals may be missed.
• Sampling with a quadrat may not give a representative sample as individuals move in and out.

Exceptions where quadrats can be used to sample animals:

• With very slow-moving animals like barnacles and mussels that are attached to one spot.
• Where animals are dense and remain within a defined territory.

But for most animal populations, different methods are needed.

The mark-release-recapture technique for motile species

The mark-release-recapture technique is commonly used to estimate the size of animal populations.

Steps in using the mark-release-recapture technique:

1. Capture a sample of individuals from the target population in a defined area.
2. Mark or tag each captured individual in some way (e.g. paint dots on them).
3. Release the marked individuals back into the original habitat.
4. Allow time for marked individuals to mix evenly throughout the population.
5. Recapture another sample in the same area
6. Record numbers of marked and unmarked individuals.
7. Estimate total population size using the following formula:

$\text{estimated population size }=\frac{\text{total individuals in first sample }\times \text{ total individuals in second sample}}{\text{number of recaptured marked individuals}}$

Worked example - Estimating population size

In a habitat, 10 deer were originally captured, marked, and released. Later, 12 deer were captured including 2 marked ones.

What is the estimated total population size?

Step 1: Equation

$\text{estimated population size }=\frac{\text{total individuals in first sample }\times \text{ total individuals in second sample}}{\text{number of recaptured marked individuals}}$

Step 2: Substitution and correct evaluation

$\text{estimated population size }=\frac{10\times 12}{2}$

$\text{estimated population size }=60\text{ deer}$

Assumptions of the mark-release-recapture technique

There are some key assumptions that we make when we use the mark-release-recapture technique. If these assumptions are not met, the resulting population estimate may not be accurate.

Assumptions of the mark-release-recapture technique:

• The marked individuals distribute evenly among the rest of the population.
• The population remains constant with no significant immigration, emigration, births, or deaths.
• The population has a definite boundary.
• The marks are non-toxic and don't increase predation rates or change behaviour.
• The marks are permanent and cannot be lost.