Population size is effected by the number of births, deaths, emigrations and immigrations.

Population size can be measured in a number of ways

1/ Total counts, for example counting all the birds nesting on a sea cliff.

2/ Taking samples and extrapolating, for example to work out the barnacle population on a rocky shore a random number of quadrats could be taken. The average number of barnacles per metre square worked out and then the total population for the area estimated from that.

3/ However, it often difficult to use these methods, especially on animals that move around a lot. The technique often used to get round these problems is called mark - release - recapture. The principle is that animals are caught and marked in some way. Some time later a fresh sample of animals is collected. From the proportion of marked animals in the second sample the population size can be estimated.

For these methods to work the following conditions must be met.

(a) The population must be isolated from other populations.

(b) There must be no emigration or immigration.

1. Chose an animal population to investigate. Suitable animals and habitats are:

(a) Banded snails (Cepaea nemoralis and Cepaea hortensis), especially on sand dunes.

(b) Ladybirds in a garden.

(c) Ants, especially woodland ants near a nest.

2. Capture (by any suitable method!) as many animals as you can. Mark them and release them where you found them. Make sure that you record the number of animals you mark.

Marking is best done using something like Humbol model paint. Place a small spot of paint on the animal using the end of a match stick on to the shell or wing case. In dry conditions it is possible to use Tippex. Do not place the paint on any soft tissues, or so that it will prevent the wing cases from opening.

 

 

 

 

 

 

3. Leave for at least 24 hours, after that time go out and capture as many animals as possible. Count the number that are unmarked and the number that are marked from being captured before. Immediately release the animals unharmed where you found them.

4. Estimate the population size using the following equation.

Population size = Number of animals marked X total caught second time

number of marked animals caught second time

5. When you write up the experiment include an evaluation. Does your population of animals meet the conditions for this method?

For an example of project follow this link.

A more accurate method of mark-release-recapture is called the Bailey's triple catch. It involves a longer sequence of marking and recapturing.