Techniques for Estimating Population Density and Size Imagine you are a wildlife biologist interested in studying the rare and possibly endangered white-footed Guano Island mouse. The mice were once widespread. Now, they are known to live only on Guano Island, a tiny dot off the coast of Peru, with an area of only 50 hectares. Unlike many rodents, the Guano Island mice are not territorial; they hide during the day, but at night they roam freely over the island in search of thornbush seeds, their favorite food. One of the first things you might want to know about the mouse and thornbush populations is population density, the number of individuals per unit of area or volume. There are two methods for studying population density; the mark-recapture method and the sample plot method. The mark-recapture method is a sampling technique used for determining wildlife population density. Animals are trapped, marked, and released. Later, traps are set again, and the proportions of marked and unmarked animals captured during the second trapping gives an estimate of the size of the whole population. The mark-recapture method is a good choice for active, elusive creatures like mice. The sample plot method involves counting all the organisms in selected plots of known size, followed by extrapolating to estimate the size of the entire population. Mice don't stay put, so getting a count of them is really difficult! So, let's use the mark-recapture method. We'll set traps on the island, arrange them randomly and check the traps in the morning to see how many have been caught. The data table at the top of the screen shows how many mice we caught. Now we tag the mice and release them and the traps are reset so we can capture another batch of mice. We'll check the traps again to see how many marked and unmarked mice we've caught the second time. The data table shows how many mice have been caught this time. Also note that we have recaptured some of the mice that were caught and marked previously. Now let's use this data for estimating the total population size of the Guano Island mouse. The equation shown here can be used to estimate total population size. Using the simple formula, we arrive at a population 468 Guano Island mice. Now let's estimate the population density of the Guano Island mice the number of mice per hectare. The area of Guano Island is 50 hectares. To find population density, we divide population size by the area. In this case, we divide 468 mice by an area of 50 hectares. Our estimate of the population density of mice on the island is 9 mice per hectare. Thornbush seeds are the favorite food of the Guano Island mouse. Let's look at the status of the thornbush population. The Sample Plot Method is the best method for studying the population density of the thorn bush. While it might be possible to count all the thornbushes on an island this small, the island is rugged and a total count would be a lot of work. The sample plot method involves counting all the organisms in selected plots of known size, then extrapolating to estimate the size of the entire population. It is a relatively easy way to make a quick estimate of thornbush population density. Our sample plots are 1 hectare in area. Let's scatter a number of sample plots randomly on the island. Now, we know the number of thornbushes in all the sample plots. Now let's estimate the average population density of thornbushes the number of thornbushes per hectare. With the sample plot method, the average population density can be estimated by dividing the number of individuals in all the sample plots by the area of all the of sample plots. This is easy to do in our study, because we used one-hectare plots. Using the simple formula; 47 thornbushes in sample plots divided by 6 hectares, we find that there are 8 thornbushes per hectare. So, what is the total number of thornbushes on Guano Island? Knowing the area of the island (50 hectares) and the population density of thornbushes, we can calculate the estimated total population size of thornbushes on Guano Island To find the population, we multiply thornbushes per hectare times the Area. In this example, we multiply 8 thornbushes per hectare times 50 hectares which equals a thornbush population size of 400.
