Determining the size of a population is sometimes difficult, especially for a population of highly mobile organisms. In this animation, we show the mark–recapture method, which is useful for estimating the sizes of such populations. In this method, a population sample is captured, and the individuals are counted and marked before they are released back into the population. The number of marked individuals that are later recaptured in a second sample provides a clue to the overall size of the population.
The mark–recapture method is used for estimating animal population sizes for highly mobile species. The method is used because it is hard to estimate the population size of a species if, when you count them, they move into and out of the sampling area. We can use the method to estimate the population of deer ticks in an area of lawn.
One way to capture ticks is to run a cloth over the grass. Some of the ticks in the population will grab on, much like they would grab onto a passing leg.
Let's say that we capture ten ticks. n1 will be the total number of individuals in this first sample (ten). We can be pretty certain that we haven't captured all the ticks in the lawn, but what proportion have we captured? This is where the mark–recapture method is useful. Each of the ticks is now marked with paint so that we can identify them later. The painted ticks are released back into the lawn.
Later, after the marked individuals have had time to mix with unmarked ones in the population, we capture a second sample. It is important to do this soon enough so that births, deaths, and individual movement do not affect the population size significantly. This time, we capture 12 individuals. In this second sample, the number of marked individuals, M, provides a clue to the overall size of the population in the lawn. Here, M equals three. So the number of marked individuals is just 25% of our second sample. By extrapolation, this tells us that the first sample, the ones we marked, was just 25% of the entire population in the lawn.
We can use an equation to estimate this total population size, which is designated N. The total population size equals n1 times n2 divided by M. So 10 times 12 divided by 3 equals an estimate of 40 ticks in the population.
The mark–recapture method is useful for estimating the size of a population of highly mobile organisms. A sample of the population is captured, marked in some way, and then released back into the population. The marked individuals are a certain proportion of the total, so that when a second sample is taken, the number of marked individuals captured again should represent the same proportion of the second sample. For this method to be most accurate, enough time must elapse between the two samplings so that the marked individuals mix throughout the population. However, not so much time should elapse for births, deaths, or immigration/emigration to significantly affect the population size.