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 of life, environmental degradation, or a long-term shortage of essential goods and services. Overpopulation is not merely an imbalance between the numbers of individuals compared to the resources they need to survive, or a ratio of population over resources. This is because such an imbalance may be caused by any other number of factors such as bad governance, war, injustice and exploitation, etc. When other such factors come into play in a certain locale, and population density cannot be shown to be the major cause, overpopulation cannot be conclusively said to occur.
The world's human population is currently growing by more than 75 million people per year. This is down from a peak numerical growth of about 88 million per year in the late 1980s. About half the world lives in nations with sub-replacement fertility, and population growth in those countries is due to immigration.
Thomas Malthus (1798) argued that if left unrestricted, human populations would continue to grow until they would become too large to be supported by the food grown on available agricultural land. He proposed that, while resources tend to grow linearly, population grows exponentially. At that point, the population would be restrained through mass famine and starvation. Malthus (1798) also argued for population control, through "moral restraint", to avoid this happening. As the population exceeds the amount of available resources, it decreases, since the lack of resources causes mortality to increase. This process keeps the population in check and