A population is a group of organisms of the same species living in the same area. For all kinds of purposes it is important to know how a population develops. You can find out about this development by setting up a survey: the population survey. In all cases, it is important to first determine the size of a population, since you start at the beginning. But how do you count a herd of elephants? Or how do you determine the size of a group of flying birds?
Determining the population size
Determining the size of a population is, in most cases, a large, but reasonably easy to do job. You can determine the size of a population in several ways:
- Count the population
- Run a sample
- Work according to the catch-catch method
For each population in itself one has to make a trade-off between these different ways. Often we look at what is most convenient and what is most feasible. In all cases an exact size is not provided, but it is possible to make a fairly accurate estimate. This is often enough to continue research into the development of a population. The three ways of determining the size of a population are explained in more detail later in this article.
Count the population
The first method of determining the size of a population is by simply counting the animals that belong to the population (and thus a particular area is alive). However, in many cases this method is not a good option, as it is often difficult to separate the animals. Think of a flock of birds flying over: you cannot count them quickly because the birds fly criss-cross through each other. Therefore, this method is only suitable for determining the population size of populations whose animals can be easily distinguished. You can think of counting the elephants or the number of large trees in a game park in Africa. The count is usually done from an aircraft flying over the particular habitat.
Run a sample
The second method of counting the number of animals in a population is to perform a sample. This method is mainly carried out in populations whose organisms cannot be easily separated and in which the population density (number of organisms per specific living area) is high. In this method, the number of organisms in a very small area is counted or, in the case of, for example, flocks of birds, photographed. To get a reliable result, this is done in a number of different places in the habitat. The average is taken from this count. You then know, for example, that an average of 15 plants grow on 1 square meter. The number of animals in the entire area can then be determined fairly accurately on the basis of the size of the empty area. For example, if the area is 1000 square meters, the average number of organisms per square meter will be multiplied by 1000.
Work according to the catch-catch method
The third and final way to determine the size of a population is to use the catch-catch method. This will be illustrated by the example of a group of rabbits. Suppose you want to find out the number of rabbits in a rabbit population in a certain forest. You then go hunting to catch a bunch of rabbits. The rabbits from this first catch are marked so that they can be recognized as rabbits from the first catch. Then you release these rabbits and a while later you go on a rabbit hunt again. The second catch is likely to be tagged and untagged rabbits. The size of the entire population can then be calculated as follows:
Number of tagged rabbits 2nd catch / Total number of rabbits 2nd catch = Number of tagged rabbits 1st catch / Whole population
Because the first 3 data from the above formula are known, the entire population can be calculated with a simple cross product. Now suppose that the first catch consisted of 50 rabbits, then 50 are marked. The second catch consists of 20 tagged and 20 untagged rabbits. The total population then becomes:
Number of tagged rabbits 2nd catch / Total number of rabbits 2nd catch = Number of tagged rabbits 1st catch / Whole population
20/40 = 50 / Whole population → Whole population = 40 * 50/20 = 100
This method is quite reliable, but it also has a downside. Animals generally show learning behavior and therefore they are not easy to catch a second time. There is therefore a danger that the number of tagged animals in the second catch is very small. As a result, the number of the entire population goes up sharply (namely, it is divided by the number of tagged animals from the second catch and the smaller the denominator in a fraction, the higher the result of that fraction). As a result, the population is estimated to be larger than it actually is.