In other words, it is a ratio: population over resources. If a given environment has a population of 10, but there is food and drinking water enough for only 9 people, then that environment is overpopulated, while if the population is 100 individuals but there are food and water enough for 200, then it is not overpopulated.
Over population can result from increases in births, a decline in mortality rates, which is linked to increases in life expectancy, or from an unsustainable use and depletion of resources. Advances in technology can reduce the threat of overpopulation by making new resources available, or by increasing the productivity of existing resources.
Resources to be taken into account when estimating if an ecological niche is overpopulated include clean water and air, food, shelter, warmth, or other issues related to survival. In the case of human beings, there are others such as arable land and, for all but tribes with primitive lifestyles, lesser resources such as unemployment, money or other economic resources, education, fuel, electricity, healthcare, proper sewage treatment and waste management, and transportation.
In the context of human societies, overpopulation occurs when the population density is so great as to actually cause an impaired quality ...