The term for the decrease in population density over time is known as population decline or depopulation. This phenomenon can occur due to various reasons, including high mortality rates, low birth rates, or large-scale emigration. It is often observed in areas with diminished resources, economic downturns, or environmental stressors that make living conditions unsustainable. Understanding population decline is important for ecology and demography, as it has profound implications on ecosystem dynamics, resource management, and social structures.