Fertility rates refer to the rates of birth per 1,000 women of reproductive age in a given population. When the fertility rate is at the replacement level, a population will remain stable, neither growing nor shrinking. However, when the fertility rate deviates from the replacement level, the size of the population will change. Fertility rates above the replacement level will cause the population to grow; fertility rates below the replacement level will cause the population to shrink.
The population reached 6 billion people around 1999, and increased to around 7 billion by 2012. However, in some countries the birth rate is falling while the death rate is not, leading to a decline in the population growth rate. The population growth rate has been decreasing in higher income countries; however the number of people added to the global population each year continues to increase due to increasing growth rates in lower income countries.
High fertility rates lead to population growth, which, under certain circumstances, can cause a condition known as "overpopulation. " Overpopulation is not a function of the number or density of individuals, but rather the number of individuals compared to the resources they need to survive. In other words, it is a ratio: population to resources. Humans are not unique in their capacity for overpopulation; in general terms, overpopulation indicates a scenario in which the population of a living species exceeds the carrying capacity of its ecological niche.
When estimating whether an area is overpopulated, resources to be taken into account include clean water, food, shelter, arable land, and various social services (such as jobs, money, education, fuel, electricity, medicine, proper sewage and garbage management, and transportation).
Overpopulation can have deleterious effects. When population outstrips available resources, calamity can result, including famine, shortages of energy sources and other natural resources, rapid and uncontrolled spread of communicable diseases in dense populations, and war over scarce resources, such as land. Dense populations may also settle available land and crowd out other land uses, such as agriculture.
Different rates of growth
Presently, every year the world's human population grows by approximately 80 million. However, that population growth is not distributed evenly across all countries. Most population growth comes from developing countries, where birthrates remain high. Meanwhile, about half the world lives in nations with sub-replacement fertility. In some of these countries, the population has actually begun to shrink (e.g., Russia). All of the nations of East Asia - with the exceptions of Mongolia, the Philippines, and Laos - have fertility rates below replacement level. Russia and Eastern Europe are dramatically below replacement fertility. Western Europe also is below replacement. In the Middle East Iran, Tunisia, Algeria, Turkey, and Lebanon are below replacement. Some countries still have growing populations due to high rates of immigration, but have native fertility rates below replacement: Canada, Australia, and New Zealand are similar to Western Europe, while the United States is just barely below replacement with about 2.0 births per woman.
A new fear for many governments, particularly those in countries with very low fertility rates, is that a declining population will lead to underpopulation and will reduce the gross domestic product (GDP) and economic growth of the country, as population growth is often a driving force of economic expansion. To combat extremely low fertility rates, some of these governments have introduced pro-family policies that include incentives, such as payments to parents for having children and extensive parental leave for parents.