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In fact, about 75 percent of the population of the world live on 5 percent of the Earth’s surface. It refers to the part of the world that people have set up permanent residence in and use for agricultural and economic purposes. Ecumene comes from an ancient Greek term for the inhabited world. To understand why people live in the concentrations they do, you need to understand that only part of our planet is fit for human habitation. The largest concentration of people in the Western Hemisphere live on the northeast coast of the United States and upward along the southeastern coastline of Canada. The rest of the world’s population is unevenly dispersed over the remainder of the planet. Following close behind, the next most densely-populated areas are in South Asia, Southeast Asia, and Europe. The largest cluster of people on Earth live in East Asia, with about 20 percent of the world’s population residing in that region. On any map showing population density, you can see clearly see which regions have many people per unit of land area, and which regions have very few, if any, inhabitants. The simple definition of population density is the number of persons per unit of land area. You may also confuse population distribution (the pattern in which people live) with its density. You can fall into that trap when you look at population density. When we talk about population and its characteristics, it is easy to take an overly simplistic view. For the AP® Human Geography Exam, it is also important to know how to calculate population density and the implications it has on society and the environment. We will discuss population density as a way to explain the relationship between people and their environment. It is important to know and be able to apply the concepts of population density, at every scale from local to global. Population is an important topic in AP® Human Geography and is heavily tested on the exam. What makes up a population? What makes it grow or decline? Why do populations move to a new location? In this AP® Human Geography study guide, we will explore the concept of population density as a sub-set of demography, which is the study of the characteristics of a human population. That movement or migration raises several questions. Human populations have moved and settled in a variety of locations over a long period of time. We look at population through double lenses of space and place.