How have human populations changed over time?
Before the 1700’s, or the time of the industrial revolution, the human population grew at a relatively slow pace. From the beginning of our species, human societies developed with small groups that were based mostly on kinship, and this eventually lead to the development of bands of hunters and gatherer family groups (10-50 people) that resided together. As the human population grew, societies became more complex. Eventually, bands came together to form larger groups, or tribes, of up to 500 people. Tribes generally developed an agricultural base to the society to supplement the hunting and gathering lifestyle. As larger groups congregated, chiefdoms of several thousand people were formed. Chiefdoms were organized around agriculture and trade, and had leaders determined by heritage. As human populations continued to increase, states developed to manage large and diverse populations. With states, the development of governments, laws, militaries, and markets also came about. Cooperation between governing states, eventually lead to the development of empires with millions of people. Empires fostered technological advancement and communication, which lead to even greater population explosions.
After the industrial revolution, the human population exploded exponentially (figure 6.1). Exponential population growth, or the J-shaped population distribution, means that populations grow geometrically by doubling: for example, 10, 100, 10,000, 100,000,000, etc. This results in populations growing at a very rapid pace. The growth rate of a population can be defined as the annual number of people added to a population from births or migration minus those lost from death or migration. For example, a crude birth rate of 27 per 1,000 people, and a crude death rate of 7 per 1,000 people with no net migration, gives a net gain of 20 people per 1,000. Twenty divided by 1,000 is equal to 0.02 or 2%. Therefore the annual growth rate, expressed as a percentage of the population at a given point in time is 2%. During the 1960’s, for example, the global annual growth rate was approximately 2%. However, today the global crude birth rate is approximately 22 per 1,000 people and the global crude death rate is 9 per 1,000. Therefore, the global population growth rate is 13 / 1000 = 1.3% and this is expected to decrease further in the future. Yet, because we are multiplying this percentage by a huge global population that increases annually, the absolute number of people added to the global population each year (~90 million) has never been higher.
Population growth over time
It is axiomatic that we will not
see such a major change as this in just one part of the earth’s system. Figure 6.2 shows two other aspects that have
been closely associated with this rapid population growth. It shows a major
transformation in sources of energy. In the 15th century the
Spaniards and Portuguese developed new types of sailing vessels,
circumnavigated the globe and produced what can be called our modern global
ecosystem in which all the continents are closely tied together. That produced
a slight increase in population growth rates in Europe and Asia as new crops
came from the
World Population Growth, Urbanization and Energy Transitions
Another key indicator for population growth is a population’s doubling time, which estimates the number or years it takes for a population to double in size, given the current rate of population growth. Thomas Malthus, an English demographer and political analyst, developed this concept, and predicted that population growth would outpace food supply and lead to mass starvation. Approximate doubling time can be calculated by dividing 70 by the growth rate of the population.
Population doubling time = 70 / growth rate
Thus, for a population with a
constant 1% annual growth rate, the population will double every 70 years, and
for a population with a 2% annual growth rate the population will double every
35 years and so on. Less developed countries such as those in Africa, much of
Asia, and Latin America and the
Currently, demographers estimate that the global population may peak somewhere between 8-12 billion people by 2100, with the majority of the population increase occurring in less developed countries (Figure 6.1). Eventually, the human population will reach its carrying capacity, which is another concept developed by Thomas Malthus, defined as the maximum number of people that can be supported by a given area of land. The carrying capacity of the human population is dependant on resource availability, sustainable management of resources, and technological advancement, which may increase or decrease carrying capacity.
How are population growth and age distribution related, and what are the implications?
The composition of a population can be examined using population pyramids, which categorize the population of a country into sex and age groups (Figure 6.3). The structure of the pyramid results from historical trends in birth and death rates, as well as from population migration. Population pyramids are useful for understanding the future age and sex distribution in societies, as well as the societies’ potential for growth or decline. Pyramids are generally read with males on the left, and females on the right side of the pyramid. Each horizontal bar in the pyramid represents people in the population grouped by their age, with young people at the bottom of the pyramid and old people at the top. The length of the horizontal bar in each age and sex category represents the absolute number of people, or the proportion of people in the population, that are in that age and sex category (Figure 6.3).
Population pyramid distributions
A Population’s growth rate and its age distribution are
closely related. Reproductive age is generally defined as 15 to 45 years old.
Rapidly growing, young societies have a pyramid with a wide base and narrow
In addition to high birth rates,
As social and economic systems improve, life expectancy
increases and birth and death rates often decline. Under these conditions, the
shape of the pyramid changes, shrinking its base and becoming a slow growth
pyramid (Figure 6.3). Slow growth pyramids are typical of countries such as the
United States that have low fertility and mortality rates due to the wide-scale
use of birth control and the availability of adequate health care. In the
Some developed countries including
How do population demographics transition with time?
Demographers have developed a useful concept: the demographic transition. This is when a society moves from high to low birth and death rates. This simple definition implies a widespread and radical social transformation from rural-agrarian to urban-industrial society.
Historically there have been two demographic transitions: the past demographic transition, which occurred in the more developed countries and the present demographic transition, which is now occurring in the less developed countries (Figure 6.4). Demographic transitions are considered historical shifts in the population demographics within a country such that the crude birth and death rates (number of births or deaths per 1,000 people) transition from high rates to low rates in the population. Generally, the death rate in a population decreases before a subsequent change in the birth rate and this produces a spike in population growth until birthrates come into line with death rates. However, what causes the shift from high to low birth and death rates, and the pace of the change, differ between the past and the present demographic transitions.
The present and past demographic transition
Countries currently considered to be developed, such as
In the third stage of this demographic transition, social
and economic changes pushed and pulled people toward more urban areas (see Unit
9) and into new industrial occupations. This change from a rural-agrarian
economy to an urban-industrial society put pressure on people to have smaller
families. Therefore, the birth rate began to decline to come into line with the
lower death rate. This change slowed population growth that was occurring from
high birth rates and low death rates. This closed the demographic transition.
All of the more developed countries of the world have passed through this past
demographic transition. Usually in this
final stage, fertility falls below replacement level and these societies may
begin to decline in absolute numbers.
Some developed countries, like
Currently, many countries in the developing world
Countries in the developing world have benefited from some of the technologies that were invented in developed countries. These new technologies allow the developing countries to reduce mortality and fertility much more rapidly than in the past. The introduction of immunizations and vaccinations, the use of insecticides to control mosquito borne vectors, and other modern medical technology have reduced mortality rapidly worldwide. Because the death rate declined rapidly in developing countries while the birth rate remains high, populations in most developing countries surged. In fact, during the introduction period of these modern technologies in developing nations, birth rates even increased in some areas due to better health care in these populations. Although populations in the developing world are still growing at a rapid pace due to the rapid decrease in death rates, birth rates are now also declining in many countries. Globally, population is expected to increase for another 100 years or so, until birth rates in the developing world come into line the with death rate. Global population is expected to level off at somewhere between 8-12 billion people.
How does human population growth impact the Earth?
For the majority of human history, societies were forced to adapt to Earth’s cycles and systems to survive. This is still true to a large degree; however, times have changed in the sense that humans are now the dominant force altering the planet’s surface and atmosphere. Humans are now such a large driving force on Earth that we have the potential to irreparably alter the functioning of the planet. The growth of the human population is one of the principal factors now having large-scale impacts on Earth’s systems and cycles. Since all of the resources that humans use to make societies function (food, water, fuel, etc.) originate from the Earth, as the number of human beings increase exponentially, so does the demand for natural resources like arable land, energy, and freshwater.
Land conversion for agricultural use and habitat destruction are driving more than 27,000 plant and animal species to extinction annually. In addition, forests are being lost at an alarming rate around the world to make room for greater agricultural production. In example, more then half of the original forests on Earth have now been cut down. Agricultural intensification to support more consumers produces many detrimental ecological consequences such as land degradation, desertification, erosion, as well as increased pesticide, herbicide, and chemical fertilizer pollution. Population growth is also putting pressure on marine and freshwater ecosystems. Currently, more than 2 billion people live in areas with chronic water shortages, and in these regions the population is expected to double within the next 25 years, which will put even more pressure on finite and scarce water resources. In addition to impacts on Earth’s systems from direct consumption of resources, as more natural resources are extracted and consumed to supply larger numbers of people, greater quantities of waste are generated, which the Earth must also absorb.
However, the absolute number of
people on Earth is not the only factor that determines the level of
environmental impact on Earth’s systems and cycles. The productive and
consumptive patterns, lifestyles, and affluence of the consuming population
also have massive impacts on the Earth. People in the
Poverty, corruption, inequitable wealth distribution, over-consumption, and inefficient technologies greatly contribute to the current precarious state of the Earth’s environment. However, technological advancement, strong governmental policies, international agreements, and education, will be key to altering our current unsustainable consumption patterns in the developed and developing countries alike. The impact of population growth on Earth’s systems is a significant problem with social, economic, and political facets, and they all need to be considered to develop a working solution for a sustainable future.
How does wealth affect society?
wealth is increased by the process of economic
development. This is defined as the long term increase of output per
capita, or the increase of human productivity. It is measured by something
called the National Income Account,
which gives us measures of GDP and GNP. Gross domestic product (GDP) is one of
the most common measures of wealth and can be defined as the sum of the values
of all final goods and services (things that are directly consumed and not used
to make other products) produced within a country during a given time period.
Gross national product (GNP) is also a common measure of wealth and can be
thought of as the sum of goods and services produced by the people of a nation
in a year. For example, if a citizen of
distribution of wealth around the globe is extremely uneven with developed
countries accounting for only 14% of the population, yet controlling more than
80% of the planet’s wealth. In addition, the distribution of wealth within most
countries is also exceptionally inequitable with wealth often being controlled
by a small fraction of the population, which can lead to corruption and weak
governmental institutions. The distribution of wealth in a society is measured
by the Gini coefficient wherein a score of 0
indicates that there is perfectly equal distribution of income in a society,
and a score of 1 indicates that income distribution is perfectly unequal;
meaning that 1 person has everything and everyone else gets nothing. On
average, most developed nations have a Gini
coefficient between 0.2-0.4, with the
than 1.1 billion people on Earth live on less than $1 a day and more than 2
billion others fare only slightly better with an income of less than $2 a day.
More than 316 million people in Africa, 380 million people in
development also has a strong relationship with public health wherein those in
poverty are often malnourished, have a lower life expectancy, and are more
susceptible to hazards and disease than those in wealthy nations. For example,
in Botswana HIV/AIDS infection rates are so striking, due to poor economic
development and public health infrastructure, that the disease has completely
altered the population pyramid of the country (Figure 6.5). By 2020, deaths due
to HIV/AIDS will reduce the population in
Population pyramid of Botswana
Is the human population expected to continue growing exponentially?
Demographers note that human population growth is now changing from a J shaped exponential pattern to an s-shaped logistical pattern. World population growth peaked in 1960-65 at just above 2 percent and is now (2005) at XX percent. That implies adding XXXX per year in 1965 and XXX per year in 2005. (Figure 6.1). All biological populations have a carrying capacity, which determines the number of organisms that can live in a certain area of land. Since Earth’s area and resources are finite, many ecologists believe that humans are currently overshooting the Earth’s carrying capacity by using technology and energy stores (think fossil fuels), which makes it possible for a population to temporarily overshoot their carrying capacity before the population either: crashes due to overexploitation of their resources, or changes to an s-shaped distribution by coming into line with the population’s carrying capacity (Figure 6.6). It is important to note, however, that we do not know what the earth’s carrying capacity is. Some people believe we are now above the carrying capacity, others believe that the carrying capacity can be greatly increased through better technology, especially with more sustainable sources of energy.
Population overshoot and collapse
The change from an exponential population growth distribution to a logistic growth distribution will depend on how our technological capacity interacts with the resources of the world’s environment. It will also depend on our decisions about the quality of life that human populations are prepared to accept. We now enter into a period of great uncertainty. Population growth rates peaked in1960-65 and the rate of growth continues to decline. It was 1.21 percent in 2000, and is expected to decline to 0.38 percent by 2050. Despite the decline in the growth rate, however, the absolute number of people added to the world’s population will grow from 6.1 billion in 2000 to 9.1 billion in 2050 (medium variant projection). But these are all projections. We do not know what the future will hold. If current trends persist the expected world population in 2050 could be between 7.7 (low variant) and 10.6 (high variant). This could signal a collapse of a population that grew beyond its carrying capacity; or it could be the beginning of a new age in which a world population of 10 billion and more could enjoy the standard of living that is common to the more developed countries today.
But we do have a good idea
where growth will occur in the next generation. Demographers estimate that most
of the population growth that will occur in the next century will occur in developing
countries. By the year 2100 almost 90% of the world’s population is expected to
be living in developing countries (Figure 6.1). Many African countries for
example, have extremely high rates of population growth.
At the same time the massive
Since mortality is declining globally, (albeit, with an uneven distribution) one of the most important ways to decrease population growth rates is to decrease birth rates among populations, and especially for populations in developing countries since birth rates are currently stable in most developed countries. For example, if global birth rates decline to 1.6 children per woman, the world’s population would stabilize at approximately 7.3 billion people by the year 2050 (Figure 6.7). However, if fertility rates remain high, for example at 2.5 children per woman, then the global population would reach 10.7 billion by the year 2050. Thus, controlling birth rates is a highly effective way to control growing population rates.
Global population as a function of fertility rate
The rate of global population growth and the level of fertility will depend on many political, social, and economic factors, such as primary education, primary healthcare, family planning and the empowerment of women and girls. A proven and effective way to control birth rates is to increase education for women and girls. For example, it has been proven that women and girls with greater levels of education choose to have smaller families. It has also been shown that the education of women is positively associated with health, life expectancy, and economic development in a population. By identifying and curbing social practices that produce power differentials between males and females, women and girls will have a greater opportunity to control their own livelihoods and rates of fertility, which will in turn, decrease population rates over time.
One highly effective way of decreasing population is by delaying the age of onset of childbirth by preventing marriage in early adolescence. By delaying marriage from 16 to 25 for example, birth rates can decrease by as much as 2.2 children per woman. Another effective way of controlling birth rates is by promoting family planning education, including traditional methods such as abstinence and breastfeeding, as well as more modern forms of birth control, including condoms, hormonal contraception like IUDs, birth-control pills, sub-dermal implants, and surgical techniques for permanent sterilization. Population control is a delicate and highly controversial issue with regard to human rights, religious doctrine, and governmental control; however, governments and non-governmental agencies alike have key roles to play in controlling population growth for the health and wellbeing of Earth, as well as for the organisms that rely upon its health (including ourselves).