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Bar graph shows the number of years it has taken to add each billion people to the world population. By 1800, there were about a billion people on Earth. It took 123 years, until 1930, for the number to reach two million. Thirty-three years later, in 1960, the number reached three billion, and 15 years after that, in 1975, the number reached four billion. The population reached five billion in 1987, and six billion in 1999, each twelve years apart. Currently, the world population is nearly seven billion. The population is projected to reach 8 billion in 2028, and 9 billion in 2054.
The time between the addition of each billion human beings to Earth decreases over time. (credit: modification of work by Ryan T. Cragun)

Click through this interactive view of how human populations have changed over time.

Overcoming density-dependent regulation

Humans are unique in their ability to alter their environment with the conscious purpose of increasing its carrying capacity. This ability is a major factor responsible for human population growth and a way of overcoming density-dependent growth regulation. Much of this ability is related to human intelligence, society, and communication. Humans can construct shelter to protect them from the elements and have developed agriculture and domesticated animals to increase their food supplies. In addition, humans use language to communicate this technology to new generations, allowing them to improve upon previous accomplishments.

Other factors in human population growth are migration and public health. Humans originated in Africa, but have since migrated to nearly all inhabitable land on the Earth. Public health, sanitation, and the use of antibiotics and vaccines have decreased the ability of infectious disease to limit human population growth. In the past, diseases such as the bubonic plaque of the fourteenth century killed between 30 and 60 percent of Europe’s population and reduced the overall world population by as many as 100 million people. Today, the threat of infectious disease, while not gone, is certainly less severe. According to the World Health Organization, global death from infectious disease declined from 16.4 million in 1993 to 14.7 million in 1992. To compare to some of the epidemics of the past, the percentage of the world's population killed between 1993 and 2002 decreased from 0.30 percent of the world's population to 0.24 percent. Thus, it appears that the influence of infectious disease on human population growth is becoming less significant.

Age structure, population growth, and economic development

The age structure of a population is an important factor in population dynamics. Age structure is the proportion of a population at different age ranges. Age structure allows better prediction of population growth, plus the ability to associate this growth with the level of economic development in the region. Countries with rapid growth have a pyramidal shape in their age structure diagrams, showing a preponderance of younger individuals, many of whom are of reproductive age or will be soon ( [link] ). This pattern is most often observed in underdeveloped countries where individuals do not live to old age because of less-than-optimal living conditions. Age structures of areas with slow growth, including developed countries such as the United States, still have a pyramidal structure, but with many fewer young and reproductive-aged individuals and a greater proportion of older individuals. Other developed countries, such as Italy, have zero population growth. The age structure of these populations is more conical, with an even greater percentage of middle-aged and older individuals. The actual growth rates in different countries are shown in [link] , with the highest rates tending to be in the less economically developed countries of Africa and Asia.

Questions & Answers

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Brian Reply
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Monique
Part of compound microscope
Bakish Reply
a. body b. stage clip c. adjacent knob d. arm e. eye piece
Kpodo
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Akpo Reply
how do you know when you want to urinate
Akpo
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Coded
As the bladder fills up .. the signals are sent to the brain specifying that its filling up and should be emptied and the fuller it gets, the more signals/ alerts are sent to brain ...leading to the urge to urinate .... to go pee
Khalida
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Coded
hello
Issiya
hi
Abigail
I want the this ecologycal terms
Catherine Reply
what are enzymes
Alvin Reply
enzymes are organic catalysts which speed the rate of chemical reaction but it's self is not used up in the process
Azeezah
what is fertilization
Ofosu Reply
fusion of male and famele gametes
Biology
fusion of female and male gametes.
Biology
form zygote to eggs
Ahmad
also known as syngamy
RESHMA
why is water called a polar molecule
Jabari Reply
enzyme that not found in mitochondria
Mukesh Reply
Internal carotid arteries
Anjola Reply
A closed circulatory system is a closed-loop system, in which blood is not free in a cavity. Blood is separate from the bodily interstitial fluid and contained within blood vessels. In this type of system, blood circulates unidirectionally from the heart around the systemic circulatory route, and th
Peter Reply
food
Ramatulai
Is DNA a proteins or genetic materials ?
Bisallah
what is meiosis
Ram Reply
meiosis is a reduction Division in which four hapliod off springs are produced
Nandanee
what is electrochemical gradient?
Joneth Reply
what is mitosis?
Aliness Reply
(cytology) the division of cell nucleus in which the genome is copied and separated into two identical halves.it is normally follow by cell division
Adekunle
it Is the division of the mother cell into two daughter cells,genetically identical to each other and to their parent cell
Joneth
it is a study of living organisms
Prudence Reply
What are other types of cell
ATAMA Reply
plant and animal cell
Jessy
prokaryotic and eukaryotic cell
Joneth
meaning inside and outside cells
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Source:  OpenStax, Biology. OpenStax CNX. Feb 29, 2016 Download for free at http://cnx.org/content/col11448/1.10
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