The proportions of persons of different ages have a major impact on the growth rate of a population. A country's population, for example, has a triangle-shaped age structure, with a higher proportion of younger individuals who are at or near reproductive age. This is because young people are more likely to be alive and living in the country being studied.
When the ratio of older to younger members of a population increases, it tends to reduce the overall population growth rate. For example, if the ratio of 15-to-24-year-olds in Japan was reduced from about 40 percent to 10 percent, the population would experience less rapid growth. The reason is that fewer people would be entering their most fertile years, reducing the total number of births plus deaths each year.
A high proportion of very old people within a population can also slow down growth due to lower birth rates and higher death rates among the elderly. For example, if China's percentage of people over 65 rose from 6 to 12%, its population would grow by only 1.5 million per year, rather than 2.5 million as now.
Finally, a small population size can lead to increased mortality because there aren't enough healthy people to carry out essential tasks such as fighting off infections or providing protection from natural disasters. For example, if a community lacks sufficient health care professionals, they will likely die before they can reproduce themselves into another generation.
In the absence of a concerted attempt to regulate family size, the bigger the proportion of the population in the fertile age range, the faster population growth will be, shifting the average age of the population structure towards the younger end of the spectrum. Fertility levels have a strong effect on the age structure of populations. High levels of fertility mean that a large proportion of people are still reaching adulthood and so there is a higher proportion of young people in the population.
Low levels of fertility mean that most people are either already past reproductive age or will reach it soon. So there are fewer adults to support the growing number of children and youth, which can lead to an increase in the proportion of elderly people in the population.
Between these two extremes lies a wide range of fertility levels. Studies have shown that populations with high levels of fertility (more than 3 babies per woman) tend to have younger average ages because they're still including many children who would have been grown by now, while populations with low levels of fertility (less than 2 babies per woman) tend to have older average ages because they no longer include any children who might still be growing up.
Populations at intermediate levels of fertility (between 2 and 3 babies per woman) show both young and old averages together. These are called "young adult" populations because they include many adults but also many teenagers and children.
Population increase can be projected more precisely if this age structure is known. The use of age structure data allows the pace of growth (or decrease) to be linked to the level of economic development of a population. The relatively small number of young individuals may not be able to compensate for the mortality among the older age groups. This leads to an increased risk of death for the whole population.
There are two main types of age structure: the age distribution and the age density. The age distribution describes the proportion of individuals in each of several age groups. The age density is the number of individuals per unit area in each age group. For example, if one were to count the number of people over 60 years old living in a given region, this would be a way of estimating its aging rate - the percentage of people over 60 years old to the total population.
A population is said to be aging rapidly when the proportion of elderly people (aged 65 and over) increases faster than that of children under 5 years old. Aging populations are becoming a major challenge for many countries around the world. It has been estimated that by 2050 there will be 8 billion people on the planet, but only 1.5 billion children under the age of 15. If fertility rates remain low and life expectancy continues to rise, then aging will play a big role in determining how many people there will be. There aren't enough children to keep up with the number of elderly people.
Birth rates, mortality rates, immigration, and emigration all have an impact on population growth rates. If a population is given an infinite supply of food, water, oxygen, and other natural conditions, it will increase exponentially. However many populations are limited by resources such as food, energy, clean water, and air quality-defined limits to growth.
Population size can also be reduced through death and destruction. A disease can kill off a large percentage of a population, causing a rapid drop in size. An earthquake or flood can also cause mass deaths and remove individuals from the gene pool, reducing the size of a population quickly.
Immigration and emigration can add or take away size from a population, depending on whether the amount is greater than or less than the rate of birth plus death. If a population is growing due to high fertility rates but also high rates of migration, its size can still grow indefinitely.
Finally, a population can decrease in size due to low fertility rates or high rates of death without increasing first. Death and loss through war, disease, and disasters can reduce population sizes drastically over time. The more often this happens, the faster a population will go extinct. There are some species that have been able to survive due to their ability to adapt to new environments, but this will not save them from extinction if we don't do anything about it.
The birthrate, mortality rate, and rate at which individuals enter or depart the population are all factors that might influence population size. When the birth rate exceeds the mortality rate, a population might increase. If not enough babies are born or too many people die, then the population might decrease.
Another factor is immigration and emigration. If more people move into an area than leave it, then the population will grow. If not, then it won't. Immigration can be natural like births or marriages, but also artificial such as refugees from war-torn countries or workers moving to larger cities for jobs. Emigration is the opposite of immigration: people move out of an area. Sometimes this is because of wars, but sometimes it's just because they find better opportunities elsewhere.
Finally, the total number of children ever born, including those that die before their first birthday, plays a role in determining population size. If you have more children than you need, then your population will grow; if you have fewer, then it won't. This factor is important in countries where most children are still born alive or die very young: if many people in these countries have many children, then their populations will increase; if not, then they won't.
These are the main factors that influence population size.