Theories of Population Growth & Decline - AP Human Geography

The physiological population density of a country refers to the number of people in that country divided by the size of arable land in that country. It essentially refers to how much land is being used to provide sustenance for the population of a country. It is different from arithmetic population density, which simply refers to the number of people in a country divided by the size of the total land area of that country.

Distance decay is the name of the theory that states that as the distance between two places increases, the interaction between those two places decreases. This is not necessarily determined by the actual geographic distance between two places, but might instead be determined by how easy it is to access one place from another place. For example, London and New York are much further away than London and the Shetland Islands (off the Northern coast of Scotland), but it takes less time to get from London to New York in the modern era.

The demographic transition model demonstrates the changing nature of birth and death rates as related to the economic development of various countries. In the first phase, birth and death rates are both high; in the second phase, death rates fall, but birth rates remain high (leading to rapid growth); in the third phase, birth rates fall to a level similar to death rate and population growth begins to slow; in the fourth phase, birth rates and death rates are fairly stable and very low, which usually leads to slow population growth. Many demographers have in recent years argued for a fifth phase that reflects when death rates begin to outstrip birth rates again, leading to a negative “natural increase rate.” It could be argued that Italy and Japan (to take two examples) are currently in this speculated “fifth phase.”

In stage two of the demographic transition model, the death rate is falling due to improved access to education, health care, shelter, sustenance, etc., but the birth rate continues to be high. This causes massive population growth. This is currently happening in Bangladesh and many of the more developed countries in Africa and South Asia.

A population pyramid is used to show the percentage of the population who fit into various age groups. If a population pyramid is rectangular, that means the bottom section (where babies are represented), the middle section (where working adults are represented), and the top section (where older people who are often retired are represented) are roughly similar sizes. This generally means the population of a country is close to steady, but is possibly growing slightly. A country like Norway or the United Kingdom would have a rectangular population pyramid.

Most geographers and demographers agree that over the course of the twenty-first century, the population growth will slow considerably. This is based on the belief that the current rapid population growth is an anomaly in Earth’s history and that the planet cannot sustain such rapid growth for another century.

The demographic transition model primarily equates the rate of population growth with the various levels of economic development around the world. The first phase is “preindustrial” and the fourth phase is “industrial.” This model has been criticized by many demographers and geographers as being too simplistic.

The crude birth rate (CBR) is an important statistic that describes the number of births per thousand people in a given period of time (usually a year). The average number of children born to a woman during the course of her lifetime is called the total fertility rate (TFR).

The Demographic Transition Model can be divided into pre-industrial, transitional, and industrial segments. A pre-industrial country (like Somalia, for example) will have high birth rates and high death rates. This is the starting place for all of humanity—many people being born and many people quickly dying. The next phase is transitional; in this phase, first the death rate starts dropping, and then shortly thereafter, the birth rate begins to drop too. The final phase is called "industrial." When a country is well-developed and women are relatively liberated, then birth rates and death rates remain fairly stable and much, much lower than in a pre-industrial society.

An Essay on the Principle of Population, published at the end of the eighteenth century, was written by Thomas Malthus. In this essay, Malthus argues that the rate of food production grows arithmetically, whereas population grows exponentially. Malthus argued that this would lead to a situation where population outstrips food production, leading to widespread famine and starvation. His writings are extremely influential even to this day; however, many geographers dispute his conclusions and argue that technology augments food production, allowing it to keep up with population growth. It remains to be seen whether Malthus’ predictions will come true in the twenty-first century or beyond.

The Natural Increase Rate and the Demographic Accounting Equation are both calculations used to determine the current or projected population growth of a region. The Demographic Accounting Equation is more comprehensive because it takes into account immigration and emigration, whereas the Natural Increase Rate only takes into account the birth and death rates.

Thomas Malthus was an economist during the Enlightenment in Europe. In the late eighteenth century he wrote Essay on the Principle of Population (1798) which argues that food production increases arithmetically (linearly) and that population grows geometrically (exponentially). According to Malthus this would mean that eventually the population growth would exceed the growth in food production causing widespread starvation and suffering. Malthus’ arguments continue to be widely influential today.

The demographic accounting equation is used to predict population growth and future population of a country or region. It takes into account birth rates, death rates, immigration, and emigration and so is seemingly quite thorough. The problem with the demographic accounting equation is that all of these statistics (birth rates, death rates, immigration rates, emigration rates) are subject to dramatic change over time. As a result, using contemporary statistics to project into the future becomes a dubious proposition.

The “doubling time” refers to the amount of time it takes for the population of a region to double. The number is based on the annual increase in population as a percentage of the original population. An annual increase of one percent of the population will cause the population to have a “doubling time” of seventy years. An annual increase of four percent of the population will lead to a “doubling time” of sixteen years. This might seem incredibly fast, and indeed it is. The reason for this is that population compounds as it grows, much like compound interest.

All of these answer choices affect the “natural increase rate” of a country. The following are general rules, of which there are some exceptions: as access to education becomes more universal the “natural increase rate” will decline; government policies that institute population control programs will cause the “natural increase rate” to decline; the more empowered women are in a society, and the more gender equality there is, the lower the “natural increase rate.” Finally, cultural customs—like emphasizing the importance of motherhood—can have significant impacts on the “natural increase rate.”

Demography is the study of human population. Specifically, demography generally involves the study of statistics concerning things like birth and death rates, fertility and public health, income, ethnicity, and outbreaks of disease.

The “demographic accounting equation” is used to predict future population growth for a region or country. It is determined first by calculating the “natural increase rate,” which is the number of births minus the number of deaths. Then by adding the “natural increase rate” to the “net migration number.” The “net migration number” is the number of immigrants minus the number of emigrants in a given time period.

In highly developed countries like the United States the natural increase rate is generally very low, it might even be negative. But, the natural increase rate only reflects the changes in population caused by births and deaths; it does not consider immigration and emigration. Much of the population growth in the United States is driven by immigration and the fact that newly arrived immigrants are much more likely to have large families.

In stage three of the demographic transition model a country’s death rate has already fallen and it’s birth rate is in the process of falling as well. The death rate in Morocco has fallen dramatically in the half-century since independence, but the birth rate has only recently begun to fall to match this decline. Nigeria and South Sudan are both still in stage two of the demographic transition process (falling death rates, steady birth rates); Sweden and Denmark are both in stage four of the demographic transition process (low death rates, low birth rates).