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How do we determine population size?

Population size is an important characteristic of a population. Many processes that affect a population differ depending on how large or small it is. There are several ways to determine the size of a population. Small populations can be counted directly by simply going to the habitat area and counting the number of individuals. For small populations, we can often precisely determine population size. However, even for small populations, the ease of directly counting the number of individuals varies.

Take a look at each of these animals. What factors affect our ability to measure their population size directly? Click the image to see if you guessed correctly.

In addition to these factors, many populations are simply too large for us to count individual by individual. Because of the impracticalities of counting individuals, there are several techniques to estimate population size. Read about each in the table below.

population index We can measure an index, which is a factor that is directly related to population size. For example, this could be the number of nests (instead of counting birds), the number of tracks on the ground (instead of counting possums), or the fecal droppings on the ground (instead of counting deer). 
mark-and-recapture We can capture animals in their habitat, mark them, and release them. Later, we can go back to the habitat, capture again, and count how many marked animals there are. The proportion of marked to captured animals can be used to estimate the entire population size. This process is referred to as mark-and-recapture.
sampling We can estimate how large the population is by counting only members of the population in a particular area (a process called sampling) and then use this to estimate the size of the entire population over its entire inhabited area. 

Different methods are appropriate for determining the population size of different individuals. Test your knowledge of these methods. Connect each population on the left to the most appropriate method of determining its population size on the right.

humans


population index

mark-and-recapture

sampling

count entire population

That is correct.
Humans are spread over a large area but are fairly easy to count individually, so it is best to sample a smaller area.
whales


population index

mark-and-recapture

sampling

count entire population

That is correct.
Whales are often inaccessible because they live in such deep water, but they are extremely long-lived; therefore, it is best to mark and recapture them year after year.
sea stars
population index

mark-and-recapture

sampling

count entire population

That is correct.
Sea stars are fairly small, and they stay in the same place most of the time. Thus, it is easy to count every sea star in a given population.

Complete