The size of populations of invertebrates or small mammals in an area can be estimated using mark-release-recapture technique. This technique is particularly useful for animals with shells, such as snails and limpets or invertebrates with exoskeletons such as woodlice. However it can also be carried out on small mammals using Longworth traps.

A sample of animals is captured, counted and marked in some way. These are then released and allowed to re-mix with the population. A second sample is taken later. The population can be estimated from the proportion of marked animals in the second sample.

Fieldwork

You will need:

  • Apparatus for collecting invertebrates, such as pooters or pitfall traps
  • Nail varnish or similar for marking captured invertebrates
  1. Take a sample from the population. For snails, pitfall trapping can be used (as long as a non-lethal option is chosen) or you can sort through leaf litter. Count them and mark them in some way. Since snails have a hard shell, nail varnish or correcting fluid is ideal.
  2. Release the animals back into the population. You now know how many marked animals there are in the population.
  3. Leave the marked animals to mix randomly with the rest of the population. Normally the population is left overnight.
  4. Take a second sample in exactly the same way as the first. Record (a) the total number of animals in the second sample, and (b) the number of marked animals in the second sample (recaptures)
  5. You can now calculate a population estimate. If the animals have been mixing randomly, then the proportion of marked to unmarked animals in the population should be the same as the proportion of marked to unmarked animals in the second sample.

Lincoln Index

The Lincoln Index is used to estimating population size.

\(\mathsf{Population = total\; number\; animals\; in\; 1st\; sample\; \times\frac{total \;number\; of\; animals\; in\; 2nd\; sample}{number\; of\; marked\;animals\; in\; 2nd\; sample}}\)

Assumptions

Several assumptions made when estimating the size of population by mark-recapture. Some are more realistic than others:

  • The population is closed. In other words, there is no migration into or out of the population. In addition, it assumes there are no deaths or births.
    (Births and deaths can be either real or virtual. A real death happens when the animal actually dies while a virtual death means the animal is no longer available for sampling. Perhaps it has gone into hibernation or like a caterpillar has turned into a pupa.)
  • All members of the population mix randomly. In other words, each member of the population has an equal chance of capture. However, this may not be the case for everything. For example, male mice and voles are inquisitive and more likely to enter a trap then females. Lactating females are going to stay close to their nests, so you may effectively only be sampling from half the population.
  • The marks are not lost between samples.
  • The mark does not harm the animal. Does painting nail varnish onto a snail’s shell make it more obvious to a predator like a bird?
  • The marks do not affect the chances of recapture.

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