How Can Human Population Size Keep Increasing In Spite Of Decreased Fertility Rates?

Population momentum is a consequence of the demographic transition. Population momentum explains why a population will continue to abound even if the fertility charge per unit declines. Population momentum occurs because it is non just the number of children per woman that make up one's mind population growth, only also the number of women in reproductive age. Eventually, when the fertility charge per unit reaches the replacement rate and the population size of women in the reproductive historic period bracket stabilizes, the population achieves equilibrium and population momentum comes to an terminate. Population momentum is defined as the ratio of the size of the population at that new equilibrium level to the size of the initial population.^[1] Population momentum ordinarily occurs in populations that are growing.

Example [edit]

Assume that a population has 3 generations: First (oldest), Second (child bearing), and Third (children). Further assume that this population has a fertility rate equal to 4 (iv). That is, each generation is twice the size of the previous. If the population of the first generation is arbitrarily set at 100, the second is then 200, and the tertiary is 400. The spreadsheet below shows the initial population in the offset row.

Population momentum example
Time	Gen i	Gen ii	Gen 3	Gen 4	Gen 5	Notes
0	100 (sometime)	200 (fertile)	400 (children)			Population: 700 Fertility Charge per unit: four
1	dead	200 (old)	400 (fertile)	400 (children)		Population: 1000 Fertility Rate: 2
2	dead	dead	400 (quondam)	400 (fertile)	400 (children)	Population: 1200 Fertility Rate: ii

Outset note that the second and third generation of the initial population are each twice the size of the previous. The full of the initial population is 700 = 100 + 200 + 400.

Then presume that at the stop of the third generation, fertility falls to replacement (for simplicity assume that to be two). Now take the population forward in fourth dimension to the next generation, line two of the spreadsheet. The get-go generation dies, and the new generation, the fourth, is equal to the 3rd (because now fertility is replacement). Repeat the process once again to reach the fifth generation (line iii in the spreadsheet). The fifth generation is once more equal to the fourth and now the population'south three generations are equal, and the population has reached equilibrium.

The initial population has grown from 700 to 1,200 even though fertility dropped from four to replacement (2) at the stop of the 3rd generation. Population momentum carried the population to higher levels over the side by side two generations.

Further steps to zero population growth [edit]

Population momentum impacts the immediate nativity and death rates in the population that determine the natural rate of growth. Nevertheless, for a population to have an absolute zero amount of natural growth, the US National Library of Medicine National Institutes of Health suggests that three things must occur.

1. Fertility rates must level off to the replacement rate (the net reproduction rate should be 1). If the fertility rate remains college than the replacement rate, the population would go along to grow.

2. Bloodshed rate must terminate declining, that is, it must remain constant.

3. Lastly, the historic period construction must conform to the new rates of fertility and mortality. This last step takes the longest to complete.^[2]

Implications [edit]

Population momentum has implications for population policy for a number of reasons.

ane. With respect to high-fertility countries (for instance in the developing world), a positive population momentum, meaning that the population is increasing, states that these countries volition proceed to grow despite large and rapid declines in fertility.

2. With respect to lowest-low fertility countries (for example in Europe), a negative population momentum implies that these countries may experience population decline even if they try to increase their charge per unit of fertility to the replacement rate of 2.1. For case, some Eastern European countries prove a population shrinkage fifty-fifty if their birth rates recovered to replacement level. Population momentum can become negative if the fertility rate is under replacement level for a long menstruation of time.

3. Population momentum shows that replacement level fertility is a long-term concept rather than an indication of current population growth rates. Depending on the extant age construction, a fertility rate of two children per woman may stand for to short-term growth or turn down.^[3]

Adding [edit]

To summate population momentum for population A, a theoretical population is constructed in which the birth charge per unit for population A immediately becomes replacement level. Nether such conditions, the population volition eventually stabilize into a stationary population, with no yr-to-year changes in age-specific rates or in total population. The population momentum is calculated by dividing this last total population number by the starting population.^[iv] Momentum, Ω, can be expressed every bit:

$\Omega =(be_{o})Q$

In this equation, b is the crude birth rate while e_o is the life expectancy at nativity. Q is the total number of births per initial nascence.

$Q={i \over r\mu }{R_{o}-1 \over R_{o}}$

This equation is used to derive Q (total births per initial birth), r is the growth rate and µ is the unchanging population mean age at childbearing. R_o is the Net Reproduction Charge per unit of the non-changing population.^[5]

Causes [edit]

Population momentum is typically caused by a shift in the country'due south demographic transition.^[1] When mortality rates drib, the immature survive babyhood and the aging population live longer. Fertility rates remain high, causing the overall population size to grow.^[6] According to population momentum, even if high fertility rates were immediately replaced with replacement level fertility rates, the population would continue to grow due to the pre-childbearing population entering childbearing years.^[i]

Run across also [edit]

Human overpopulation
Census
List of population business organisation organizations
Population stabilization

References [edit]

^ ^a ^b ^c Blue, Laura; Espenshade, Thomas J. (2011). "Population Momentum Across the Demographic Transition". Population and Development Review. 37 (four): 721–747. doi:ten.1111/j.1728-4457.2011.00454.x. ISSN 0098-7921. PMC3345894. PMID 22319771.
^ Bongaarts, John (2009). "Human population growth and the demographic transition". Philosophical Transactions of the Regal Society B: Biological Sciences. 364 (1532): 2985–2990. doi:10.1098/rstb.2009.0137. ISSN 0962-8436. PMC2781829. PMID 19770150.
^ "The impact of population momentum on time to come population growth" (PDF). United nations Dept of Economical and Social Affairs, Population Division. October 2017.
^ Preston, Heuveline, and Guillot (2001) Demography: Measuring and Modeling Population Processes, 165
^ Kim, Young J.; Schoen, Robert (1997-08-01). "Population momentum expresses population aging". Demography. 34 (iii): 421–427. doi:10.2307/3038294. ISSN 0070-3370. JSTOR 3038294.
^ Weeks, John Robert (2016). Population : an introduction to concepts and bug. Boston, MA: Cengage Learning. ISBN9781305094505. OCLC 884617656.

External links [edit]

Mason, A., Lee, South. and Russo, G. (2000). Population Momentum and Population Aging in Asia and Near-East Countries. [ebook] Strategic and Economical Analysis Office. Available at: [i] [Accessed 28 Mar. 2018].
PAPP103 - S08: Population dynamics and momentum, United Nations Population Fund - UNFPA, [ii]