Overpopulation may indicate any case in which the population of any species of animal may exceed the carrying capacity of its ecological niche. In common parlance, the term specifically refers to the relationship of human population to the planet Earth. Overpopulation is not the number of people or animals, but rather the number of people/animals in comparison to the resources they need to survive. In other words, a ratio -- population number:resource amount.
If a population contains 10 people, but there is food enough for only 9 people, it is overpopulated. If a population is 100 billion, but there is food enough for 200 billion, then it is not overpopulated. Resources include: clean water, food, shelter, warmth, and arable land. Other lesser resources include: jobs, money, education, fuel, electricity, medicine, proper sewage and garbage management, and transportation. These lesser resources are not needed by animals and human tribes that live in primitive lifestyles.
Every year the world’s human population grows by approximately 80 million. About half the world lives in nations with sub-replacement fertility and population growth in those countries is due to immigration.
Early in the 19th century, Thomas Malthus argued in An Essay on the Principle of Population 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 arithmetically, population grows exponentially. At that point, the population would be restrained through mass famine and starvation. Malthus argued for population control, through "moral restraint", to avoid this happening. As the population exceeds the amount of resources the population lowers, since the lack of resources causes mortality to increase. This process keeps the population in check and ensures it doesn't exceed the amount of resources
Over the two hundred years which followed, famine has overtaken numerous individual regions; proponents of this theory state that these famines were examples of Malthusian catastrophes. On a global scale, however, food production has grown faster than population. It has often been argued that future pressures on food production, combined with threats to other aspects of the earth's habitat such as global warming, make overpopulation a still more serious threat in the future. Perhaps the best-known example of such an argument is The Limits to Growth, a report produced for the Club of Rome in the early 1970s.
The optimist's viewpoint on population growth
Other studies have countered with the claim that the current population level of over six billion may be supported by current resources, or that the global population may grow to ten billion and still be within the Earth's carrying capacity. Buckminster Fuller and Barry Commoner were both proponents of the idea that human technology could keep up with population growth indefinitely. The assumptions that underlie these claims, however, have been strongly criticised. One criticism is that poor people can't afford such technologies.
In any case, many proponents of population control have averred that famine is far from being the only problem attendant to overpopulation. These critics point out ultimate shortages of energy sources and other natural resources, as well as the importance of serious communicable diseases in dense populations and war over scarce resources such as land area.
A shortage of arable land (where food crops will grow) is a problem. About 21% of the earth's land is arable. In the past, 160 acres (650,000 m˛) of farm land crops fed one person. Hydroponics in autonomous building gardens and greenhouses grow more food in less space. Most food production experiments have used vegetable farming because it can support an adult from as little as 15 m˛ of land. High yield vegetables like potatoes and lettuce don't waste space with inedible plant parts, like stalks, husks, vines, and inedible leaves. New varieties of selectively bred and hybrid plants have larger edible parts (fruit, vegetable, cereal) and smaller inedible parts. With new technologies, it is now possible to grow crops on some unarable land under certain conditions.
It is hoped that technologies and methods which follow the concepts of sustainability will allow better lives for more people.
Fossil-fuel subsidies in agriculture
One of the strongest criticisms of an optimistic outlook is that of the fossil fuel subsidy of modern agriculture. The extremely high agricultural outputs of modern farms depend entirely on immense fossil fuel (mostly petroleum) subsidies in the forms of fertilizers, equipment fuel, and other chemicals such as pesticides. With proven reserves of petroleum steadily falling from year to year, fossil-fuel subsidies would seem to be ultimately doomed.
Effects of unregulated population growth
The world's current agricultural production, if it were distributed evenly, would be sufficient to feed everyone living on the Earth today. However, many critics hold that, in the absence of other measures, simply feeding the world's population well would only make matters worse, natural growth will cause the population to grow to unsustainable levels, and will directly result in famines and deforestation and indirectly in pandemic disease and war.
Some other characteristics of overpopulation:
- Birth rate is high
- Life expectancy is low
- Low level of literacy
- High rate of unemployment in urban areas (leading to social problems)
- Rural people are not gainfully employed (caught in cycle of poverty)
- Insufficient arable land
- Little surplus food
- Poor diet with ill health and diet-deficiency diseases (e.g. rickets)
- GDP per capita is low (under US$765 per annum)
- Many live in unhygienic conditions
- Government is stretched economically
- High crime from people who steal resources to survive
The demographic transition
However, others contend that within a generation after the standard of living and life expectancy starts increasing, family sizes start dropping in what is termed the demographic transition. In support they point to the contention that every estimate of maximum global population since the 1960s, when the "population explosion" became a worry, has been significantly lower than the previous estimates. Among those holding this view are the ecologist Paul Colinvaux, who writes on the topic in Why Big Fierce Animals are Rare, and The Fates of Nations.
The status of women
Another point of view on population vs. the standard of living is that of Virginia Abernethy in Population Politics, in which she shows evidence that this effect only holds true in nations where women enjoy a relatively high status. In strongly patriarchal nations, where women enjoy few rights, a higher standard of living tends to result in population growth. She argues that foreign aid to poor countries must include significant components designed to improve the education, human rights, political rights, political power, and economic status and power of women.
"Survival of the fittest"
Some approach overpopulation with a "survival of the fittest," "laissez-faire" attitude, arguing that if the Earth's ecosystem becomes overtaxed, it will naturally regulate itself. In this mode of thought, disease or starvation are "natural" means of lessening population. Two particular objections to this argument are that a) in the meantime, a huge number of plant and animal species become extinct, and b) this would result in terrible pollution in some areas that would be difficult to abate. As well, it obviously creates certain moral problems, as this would cause great suffering in the people that die.
Others argue that economic development is the best way to reduce population growth. Many developed countries in the world today, such as Italy, now have declining populations (ignoring the effects of immigration).
In either case, it is often held that the most productive approach is to provide a combination of help targeted towards population control and self-sufficiency. One of the most important measures proposed for this effort is the empowerment of women educationally, economically, politically, and in the family. The value of this philosophy has been substantially borne out in cases where great strides have been taken to this goal: where women's status has dramatically improved, there has generally been a drastic reduction in the birthrate to more sustainable levels. Other measures include effective family planning programs, local renewable energy systems, sustainable agriculture methods and supplies, reforestation, and measures to protect the local environment.
The largest shortage for important resources is of medicine. A shortage of educated people can lead to a shortage of doctors. Fewer doctors in a population causes the price of medical bills and health insurance to rise higher than lower income people can afford. The effect is people dying of diseases, even though the cures exist.
US immigration policy
Some overpopulation activists have taken the position that population growth must be dealt with on a country-by-country basis, and therefore support a reduction in immigration into some countries, including the United States, in order to stabilize their population. Others disagree with this approach and believe that population should be addressed only as a global issue, not through immigration restrictions. Regardless of differing views on immigration policy, most population activists agree on the need for birth control, family planning, and the empowerment of women. Among the groups who take a position in support of immigration reductions is Negative Population Growth; among those who do not take a position on immigration and concentrate instead on birth control and family planning is Population Connection, formerly Zero Population Growth. This debate has also been ongoing in some parts of the broader environmental movement, especially Earth First! during the 1980s, and more recently has become an especially contentious issue within the Sierra Club.
Overpopulation as a social issue
The density of population has an impact on a broad range of social and economic issues, such as land prices and housing costs. For example, relatively densely populated countries such as Japan have higher land prices than less densely populated countries such as Australia, and even in that country, land prices have doubled and redoubled as the population has increased. It is sometimes argued that reducing the populations of some areas, such as large cities, would have positive benefits for these reasons.
A different overpopulation concern pertains to the population growth of domestic cats and dogs, often among strays. Many groups such as the Humane Society urge pet owners to have their pets spayed or neutered in order to prevent pet overpopulation. Pet overpopulation can be an ecological concern as well as a concern over animal welfare, with overpopulation occurring when there are more domestic cats and dogs than there are people wanting them as pets, independent of ecological carrying capacity.
Wild animal overpopulation
In the wilderness, the problem of wild animal overpopulation is solved by predators. Predators eat the old, sick, and weak animals. This controls populations and insures strong stock. Without spiders, flies would take over the earth. When animal populations grow, so do the number of predators that feed on that particular animal. Animals that have birth defects or weak genes (such as the runt of the litter) also die off, unable to compete over food with stronger, healthier animals. An animal that isn't native to an environment can overpopulate and destroy that environment, especially if it has no predators that eat it. When rabbits were first introduced to Australia, they quickly overpopulated and ate the plants that other native animals needed to survive. The native species died of starvation. Farmers hunted the rabbits to reduce their population and prevent the damage the rabbits did to the crops.
The United Nations Population Projection
The United Nations projects that world population will stabilize in 2075 at nine billion. See link below. Even so, many scientists believe the earth cannot sustain nine billion people, and some put the number that the earth can sustain indefinitely as low as 2 billion.  (http://www.utne.com/web_special/web_specials_archives/articles/799-1.html)