Define arithmetic density and physiological density, and explain why they differ.

• A. Arithmetic density is people per unit area; physiological density is people per unit of arable land, and physiological density often higher, indicating pressure on productive land.
• B. Arithmetic density is people per unit area; physiological density is people per unit of arable land, and physiological density is always lower.
• C. Arithmetic density measures urban density; physiological density measures rural density.
• D. Arithmetic density equals physiological density always.

Answer: (A)

The main idea here is how population density can be measured in different ways to reveal different aspects of land use. Arithmetic density is the number of people divided by total land area, so it treats all land the same, regardless of whether it can actually support crops or towns. Physiological density, on the other hand, divides the population by the amount of arable land, the land suitable for agriculture, highlighting how intensively agricultural space must sustain people.

Physiological density is often higher than arithmetic density because not all land is usable for farming. A country may have large expanses of non-arable terrain (mountains, deserts, forests), which lowers arithmetic density, but the population relies on a relatively small area of arable land, pushing the number of people per unit of that productive land up. This difference helps explain pressures on food production and water resources that arithmetic density alone can obscure.

The best choice captures both definitions and notes why physiological density tends to be higher, signaling pressure on productive land. The other options misstate the relationships or what each density measures, such as implying physiological density is always lower, equating the two densities, or describing them as urban versus rural measures.