How do population size and population density describe different things?

They answer different questions. Population size is the total number of individuals in the population. Population density is the number of individuals per unit area. [S1]

Think of size as the total headcount and density as headcount relative to space. A count of 100 individuals does not tell you whether they occupy a small patch or a very large habitat. The same size can therefore correspond to different densities, and a larger population can be less dense than a smaller one if it occupies much more area. [S1]

The useful question is: are you asking how many individuals there are altogether, or how many there are per defined area?

Sure. Suppose, purely as a hypothetical example, Site A contains 120 birds across 6 square kilometres, while Site B contains 80 birds across 2 square kilometres. These figures are invented for illustration, not field observations.

1. Compare size: Site A has 120 birds; Site B has 80. So Site A has the larger population.
2. Calculate density by dividing the number of individuals by the area:

• Site A: 120 ÷ 6 = 20 birds per square kilometre.
• Site B: 80 ÷ 2 = 40 birds per square kilometre.

Site A has more birds overall, but Site B has more birds per square kilometre. The density calculation follows the definition of population density as individuals per unit area. [S1] Always label the area unit: “40 birds” is a count, while “40 birds per square kilometre” is a density.

Choose the measure that matches your question. If you want to compare total numbers of individuals, compare population sizes. If you want to compare how many individuals occupy a given amount of space, compare densities. Density can be relevant to questions about within-population interactions: it can affect interactions such as competition for food and finding mates. [S1] That does not mean density alone proves why a particular interaction occurs.

For a clear comparison, state what area each count covers and use compatible units. Make clear which population and place your count refers to. The definition gives you a calculation, but it does not establish that a particular count is complete.

Use this copyable format, replacing the brackets with your own information:

Population: [species or group] at [place] Count (population size): [number] individuals Area covered: [number] [area unit] Density: [count] ÷ [area] = [number] individuals per [area unit] Interpretation: [This population is larger / smaller overall; its density is higher / lower per unit area than the comparison population.]

Example using the hypothetical figures above: “Population: birds at Site B. Count: 80 birds. Area covered: 2 square kilometres. Density: 80 ÷ 2 = 40 birds per square kilometre. Site B has fewer birds overall than Site A, but a higher density.” This keeps the total count separate from the space-adjusted measure. [S1]