Low densities may cause an extinction vortex and lead to further reduced fertility. This is called the Allee effect after the scientist who identified it. Examples of the causes in low population densities include:
For humans, population density is the number of people per unit of area, usually quoted per square kilometer or square mile (which may include or exclude, for example, areas of water or glaciers). Commonly this may be calculated for a county, city, country, another territory or the entire world.
The world's population is around 7,500,000,000 and Earth's total area (including land and water) is 510,000,000 square kilometers (197,000,000 sq. mi.). Therefore, the worldwide human population density is around 7,500,000,000 ÷ 510,000,000 = 14.7 per km2 (38 per sq. mi). If only the Earth's land area of 150,000,000 km2 (58,000,000 sq. mi.) is taken into account, then human population density increases to 50 per km2 (129 per sq. mile). This includes all continental and island land area, including Antarctica. If Antarctica is also excluded, then population density rises to over 55 people per km2 (over 142 per sq. mile). However, over half of the Earth's land mass consists of areas inhospitable to human habitation, such as deserts and high mountains, and population tends to cluster around seaports and fresh-water sources. Thus, this number by itself does not give any helpful measurement of human population density.
Several of the most densely populated territories in the world are city-states, microstates and dependencies. These territories have a relatively small area and a high urbanization level, with an economically specialized city population drawing also on rural resources outside the area, illustrating the difference between high population density and overpopulation.