A plague on the pest – rabbit calicivirus disease and biological control
Box 3 | Estimating population size and density
One of the first things to do when studying a population is to find some way of working out the number of individuals in it, for without some estimate of the numbers you cannot begin to record any changes in the population.
Counting organisms that are stationary
With fixed organisms such as trees or barnacles, one can count the individuals in an area if the area is small. However, if you need to estimate the number of trees in a 1000 hectare forest or of barnacles on a rocky shore, it is only necessary to count several sample areas. From the numbers in the sample areas you can estimate the total number or you can work out the average population density of the trees per hectare or barnacles per square metre. You can follow this procedure:
- Define the whole area (A) in which the population is to be estimated.
- Choose a small sampling unit (area a). This is an area in which you expect to be able to see and count all individuals. For example, your sampling unit might be a rectangle with an area of 1 square metre. Ecologists call these sampling units quadrats.
- Choose a sample size (n). This is the number of quadrats that you will select in area A. The selection of quadrats must either be random (if you can select a lot of quadrats) or representative (if you are restricted to only a few).
- Count the number of individuals in each quadrat.
- Find the average number of individuals per quadrat. To do this, divide the total number of individuals by the number of quadrats.
- Calculate the estimated number of individuals (N) in the whole area, as follows:
Total population = Average number per quadrat × Total area/Area of quadrat.
Of course, your estimated number of individuals (N) is not the actual number of animals in the area, but a reasonable approximation. How close it is to the real number will depend on how large a and n are. The more quadrats you select, and the larger each is, the better the estimate will be, but more (and larger) quadrats require more time and effort.
Calculating the population density is simple once you know the total number of individuals in a population and the total area. Divide the total number of individuals by the total area and express the result as number of individuals per unit area.
Figure 1. In this example the study area is 100 × 100 metres. Therefore A = 10,000 square metres. Each cross represents a shrub. Each quadrat measures 10 × 10 metres. Therefore a = 100 square metres. There are 12 quadrats so n = 12. Use the steps given for this method to estimate the population illustrated in the figure. How accurate was your estimation?
Counting organisms that move around
For many organisms that move around, such as small mammals and fish, carrying out a census is not easy. Animals can be surveyed (eg, red kangaroos are large enough to be counted from the air) but the technique must be used very carefully and systematically by counting in several locations and at different times. Another method is called 'mark, release and recapture'. The animals to be counted are captured, marked in some way so that they can be recognised later, and then released. With a little calculation, you can estimate the total population size from the proportion of marked and unmarked animals in the second trapping session, as the following procedure shows.
- Define the area in which the population is to be estimated.
- Decide how to catch and mark the species in question.
- Catch a reasonably large number of animals within the study area, all during a relatively short time (usually minutes to hours).
- Mark all the animals caught and release them. (The method of marking must not harm the animals nor make them conspicuous to predators.) The number caught, marked and released is M animals.
- Leave the area for a while to allow the marked animals to mix completely with the rest of the population.
- Repeat the capture procedure. Sort the animals caught into marked and unmarked. The number of marked animals is m and the total number of animals (marked + unmarked) is n.
- The estimated total number of animals (the population size N) in the area is calculated as follows:
Total number = Number marked × Total number caught/Number of marked ones caught.
Can you see why this is so? It is a prediction of the number of animals that would have to be caught to be sure of catching all the marked animals.
Figure 2. The proportion of marked to unmarked animals captured at the second sampling can be used to give an indication of the total population size. Use the steps given for this method to estimate the population illustrated in the figure. How accurate was your estimation?
Of course, N is only an estimate and how close it is to the real population size will depend on a number of factors: the number of animals you manage to catch and mark as a percentage of the total, whether the marked animals mix properly with the unmarked ones, and whether any lose their marks or are affected by being marked. Migration, births and deaths that take place between the two sampling events can also result in inaccurate estimates.
Boxes
Box 1. Biological control
Box 2. The history of myxoma virus in Australia
Related site
Kangaroo counting (Box 1 of Nova: Science in the news topic, Is Australian wildlife fair game?)
Posted February 1997.