Population size, density, & dispersal (article)
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What is a population?

In everyday life, we often think about population as the number of people who live in a particular place—New York City has a population of 8.6 million.1‍ Or Monowi, Nebraska has a population of one. Just think—you could double the population of Monowi if you felt like moving there!

In ecology, a population consists of all the organisms of a particular species living in a given area. For instance, we could say that a population of humans lives in New York City, and that another population of humans lives in Gross. We can describe these populations by their size—what we often mean by population when we're talking about towns and cities—as well as by their density—how many people per unit area—and distribution—how clumped or spread out the people are.

Measuring population size

To find the size of a population, can’t we just count all the organisms in it? Ideally, yes! But in many real-life cases, this isn’t possible. For instance, would you want to try and count every single grass plant in your lawn? Or every salmon in, say, Lake Ontario, which is 393 cubic miles in volume?1‍ Counting all the organisms in a population may be too expensive in terms of time and money, or it may simply not be possible.

For these reasons, scientists often estimate a population's size by taking one or more samples from the population and using these samples to make inferences about the population as a whole. A variety of methods can be used to sample populations to determine their size and density. Here, we’ll look at two of the most important: the quadrat and mark-recapture methods.

Quadrat method

For immobile organisms such as plants—or for very small and slow-moving organisms—plots called quadrats may be used to determine population size and density. Each quadrat marks off an area of the same size—typically, a square area—within the habitat. A quadrat can be made by staking out an area with sticks and string or by using a wood, plastic, or metal square placed on the ground, as shown in the picture below.

After setting up quadrats, researchers count the number of individuals within the boundaries of each one. Multiple quadrat samples are performed throughout the habitat at several random locations, which ensures that the numbers recorded are representative for the habitat overall. In the end, the data can be used to estimate the population size and population density within the entire habitat.

Mark-recapture method

For organisms that move around, such as mammals, birds, or fish, a technique called the mark-recapture method is often used to determine population size. This method involves capturing a sample of animals and marking them in some way—for instance, using tags, bands, paint, or other body markings, as shown below. Then, the marked animals are released back into the environment and allowed to mix with the rest of the population.

Later, a new sample is collected. This new sample will include some individuals that are marked—recaptures—and some individuals that are unmarked. Using the ratio of marked to unmarked individuals, scientists can estimate how many individuals are in the total population.

Example: using the mark-recapture method

Let’s say we want to find the size of a deer population. Suppose that we capture 80 deer, tag them, and release them back into the forest. After some time has passed—allowing the marked deer to thoroughly mix with the rest of the population—we come back and capture another 100 deer. Out of these deer, we find that 20 are already marked.

If 20 out of 100 deer are marked, this would suggest that marked deer—which we know are 80 in number—make up 20% of the population. Using this information, we can formulate the following relationship:

number marked first catch (M)total population (N)‍=‍number marked second catch (x)total number of second catch (n)‍

MN‍ =‍ xn‍

Next, we rearrange the equation:

N‍ =‍ nMx‍

And finally, we plug in the values from the deer example:

N‍ =‍ (100 total second catch)(80 marked first catch)(20 marked second catch)‍ =‍ 400 deer‍

This approach isn’t always perfect. Some animals from the first catch may learn to avoid capture in the second round, inflating population estimates. Alternatively, the same animals may preferentially be retrapped—especially if a food reward is offered—resulting in an underestimate of population size. Also, some species may be harmed by the marking technique, reducing their survival. The approach also assumes that animals don’t die, get born, leave, or enter the population during the period of the study.

Alternative approaches to determine population size include electronic tracking of animals tagged with radio transmitters and use of data from commercial fishing and trapping operations.