Population, Human

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Population, Human

Carrying capacity and growth of the human population

Environmental effects of human populations

The number of human beings on Earth has increased greatly during the past few thousand years, but especially during the last two centuries: from 1850 to 1950 the human population doubled, from 1.265 billion to 2.516 billion, and has more than doubled from 1950 to the present. moreover, it is certain that the human population—at over 6.5 billion in 2006— will continue to increase, at least for a time.

The recent growth of the human population has resulted in intense damage to the biosphere, representing a global environmental crisis. The degradation has occurred on a scale and intensity that is comparable to the enormous effects of such geological processes as glaciation. The impact of the human population on any one region, as on the biosphere as a whole, is a function of two interacting factors: (1) the actual number of people and (2) their per-capita environmental impact, which largely depends on the degree of industrialization of the society and on the lifestyles of individuals. In general, more damage is done to Earth to support a person living a highly industrialized lifestyle than to support one living a pretechnical-agricultural or hunter-gatherer lifestyle. However, a direct correlation between industrialized comfort and birth-rate is often observed: the more well-to-do a population is (e.g., that of Europe or the United states), the lower its birthrate tends to be.

Size of the human population

The human species, Homo sapiens, is by far the most abundant large animal on Earth. Our world population is growing at about 1.5% annually, that is, at about 80 million people per year. If the percentage of annual growth remains constant, the number of people added yearly increases: 1.5% of 100 is 1.5, but 1.5% of 200 is 3. Thus, 80 million new people per year is an approximate figure valid only for a short space of years. Also, global birth rates vary from year to year. If the current rate of growth is maintained, the size of the human population will double in less than 50 years, at which time there will be more than 12 billion people on earth—assuming that there is some way to keep them alive.

No other large animal is known to have ever achieved such an enormous abundance. Prior to over-hunting during the nineteenth century, the American bison (Bison bison ) numbered 60–80 million animals and may have been the world’s most populous wild large animal. The most abundant large animals in the wild now are the white-tailed deer (Odocoileus virginia-nus ) of the Americas, with 40–60 million individuals; and the crabeater seal (Lobodon carcinophagus )of Antarctica, with 15–30 million. These species have populations less than 1% of that of human beings at this time.

Large animals domesticated by human beings have also become enormously abundant, and these companion species must be supported by the biosphere in concert with their human patrons. Therefore, they must be considered an important component of the environmental impact of the human population. These animals include about 1.7 billion sheep and goats, 1.3 billion cows, and 0.3 billion horses, camels, and water buffalo. Human beings are also accompanied by enormous populations of smaller domesticated animals, including 10–11 billion chickens and other fowl.

Carrying capacity and growth of the human population

A population of organisms changes in response to the balance of the rates at which new individuals are added by births and immigration and the rate at which they are lost by deaths and emigration. Zero population growth occurs when the growth and loss parameters are balanced. These demographic relationships hold for all species, including ours.

The history of Homo sapiens extends to somewhat more than one million years. For almost all of that time relatively small populations of human beings were engaged in subsistence lifestyles that involved hunting wild animals and gathering wild edible plants. The global population of human beings during those times may have been as large as a million or so individuals. However, colonization of new parts of the world (e.g., Asia, Europe, Australia, Polynesia, the Americas) and occasional discoveries of new tools and weapons allowed prehistoric human beings to grow in numbers and become more effective at gathering food. This in turn allowed the population to increase.

About 10,000 years ago, the first significant developments of primitive agriculture began to occur. These included the domestication of a few plant and animal species to achieve greater yields of food for human beings. The development of these early agricultural technologies and their associated sociocultural systems allowed enormous increases in environmental carrying capacity for human beings and their domesticated species, so that steady population growth could occur. Even primitive agricultural systems could support many more people than could a subsistence lifestyle based on the hunting and gathering of wild animals and plants.

Further enhancements of Earth’s carrying capacity for the human enterprise were achieved through other technological discoveries. For example, the discovery of metals and their alloy—first copper and bronze, later iron and steel—allowed the development of superior tools and weapons. Similarly, the invention of the wheel and of ships made possible the easy transportation of large quantities of valuable commodities from regions of surplus to those of deficit. At the same time, increased yields in agriculture were achieved through a series of advances in breeding of domesticated plants and animals and in farming techniques. The evolution of human sociocultural systems has thus involved a long series of discoveries and innovations that increased the effective carrying capacity of the environment, permitting growth of the population. As a result of this process, there were about 300 million people alive in AD 0, and about 500 million in 1650, at which time the rate of population growth increased significantly. This trend has been maintained to the present. The recent explosive growth of the human population has several causes. Especially important has been the discovery of more effective medical and sanitary technologies, which have greatly decreased death rates (especially infant and child death rates) in most human populations. There have also been enormous advances in the technologies that allow effective extraction of resources, manufacturing, agriculture, transportation, and communications, all of which have allowed further increases in the carrying capacity of the environment.

As a result of these relatively recent developments, the global population of human beings increased from about 500 million in 1650 to over one billion in 1850, two billion in 1930, four billion in 1975, and five billion in 1987. In 2005, the human population was approximately 6.56 billion individuals.

More locally, there have been even greater increases in the rate of growth of some human populations. In recent decades some countries have achieved population growth rates of 4% per year, which if maintained would double the population in only 18 years. One third of all the world’s births occur in India and China, the two most populous countries in the world (about 1.049 billion and 1.28 billion persons, respectively).

These sorts of population growth rates place enormous pressure on ecosystems. For example, the human population of central Sudan was 2.9 million in 1917, but it was 18.4 million in 1977, an increase of 6.4 times. During that same period the population of domestic cattle increased by a factor of 20 (to 16 million), camels by 16 times (to 3.7 million), sheep by 12.5 times (to 16 million), and goats by 8.5 times (to 10.4 million). Substantial degradation of the carrying capacity of dry lands in this region of Africa has been caused by these increases in the populations of human beings and their large-mammal symbionts, and there have been other ecological damages as well (e.g., destruction of trees and shrubs for cooking fuel).

Another example of the phenomenon of rapid population growth is the number of people in the province of Rondonia in Amazonian Brazil. This population increased twelvefold between 1970 and 1988, mostly through immigration, while the population of cattle increased by 30 times. These population increases were accompanied by intense ecological damage, as the natural rainforests were “developed” to sustain human beings and their activities. (The areas in question are not, for the most part, “developed” in the sense of being transferred from their wild state to a sustainable agricultural state, but in the sense of being stripped and degraded for short-term profit.)

Future human population

The growth rate of the global human population achieved a maximum during the late 1960s, when it was 2.1% per year. If sustained, this rate was capable of doubling the population in only 33 years. This rate of increase slowed somewhat to 1.5% per year in the 1999, equivalent to a doubling time of 47 years, and in 2002 had slipped to about 1.3%. Even at today’s comparatively modest growth rates, the human population increases by about 80 million people each year.

Reasonable predictions can be made of future increases of the human population. The predictions are based on assumptions about the factors influencing changes in the size of populations, for example in the rates of fecundity, mortality, and other demographic variables. Of course, it is not possible to accurately predict these dynamics because unanticipated changes, or “surprises,” may occur. For example, a global war could have an enormous influence on human demographics, as could the emergence of new diseases. AIDS is an example of the latter effect, because this lethal viral disease was unknown prior to the early 1980s.

As a result of these uncertainties, it is not possible to accurately forecast the future abundance of human beings. However, reasonable assumptions about demographic parameters can be based on recent trends in birth and death rates. Similarly, changes in the carrying capacity of Earth’s regions for the human economy can be estimated from recent or anticipated advances in technology and on predictions of environmental changes that may be caused by human activities. These types of information can be used to model future populations of human beings.

A typical prediction of recent population models is that the global abundance of human beings could reach about seven billion by 2011 or 2015, depending on birth rate variations, and 8.9 billion by 2050. Models tend to predict that the human population could stabilize at about 10–12 billion. in other words, the global abundance of human beings will probably double again before it stabilizes. This prediction is likely to be fulfilled unless there is an unpredicted, intervening catastrophe such as a collapse of the carrying capacity of the environment, an unprecedented and deadly pandemic, or a large-scale nuclear war.

The structure of human populations

Population structure refers to the relative abundance of males and females, and of individuals in various age classes. The latter type of structure is significantly different for growing versus stable populations and has important implications for future changes in population size.

Populations that have not been increasing or decreasing for some time have similar proportions in various age classes. In other words, there are comparable numbers of people aged five to 15 years old as those 35–45 years old. The distribution of people is even among age classes except for the very young and the very old, for whom there are, in many societies, disproportionately high risks of mortality. (In industrialized societies, the death rate for infants and young children may be very low; for the elderly, it remains high.)

In contrast, populations that are growing rapidly have relatively more young people than older people. Therefore, the age-class structure of growing populations is triangular, that is, much wider at the bottom than at the top. for example, more than one half of the people in a rapidly growing human population might be less than 20 years old. This type of population structure implies inertia for further growth because of the increasing numbers of people that are continually reaching reproductive age.

Human populations that are growing rapidly for intrinsic reasons (i.e., birth rather than immigration) have a much higher birth rate than death rate and a markedly triangular age-class structure. The so-called demographic transition refers to the intermediate stage during which birth rates decrease to match death rates. Once this occurs, the age-class structure eventually becomes more equitable in distribution until zero population growth is achieved.

Environmental effects of human populations

The huge increases in size of the human population have resulted in a substantial degradation of environmental conditions. The changes have largely been characterized by deforestation, unsustainable harvesting of potentially renewable resources (such as wild animals and plants that are of economic importance), rapid mining of non-renewable resources (such as metals and fossil fuels), pollution, and other ecological damages.

At the same time that human populations have been increasing, there has also been a great intensification of per-capita environmental impacts. This has occurred through the direct and indirect consequences of increased resource use to sustain individual human beings and their social and technological infrastructure: meat production, fuel-burning, mining, air and water pollution, destruction of wild habitat, and so forth.

KEY TERMS

Carrying capacity— The maximum population of a species that can be sustained by a given habitat.

Cultural evolution (or sociocultural evolution)— The process by which human societies accumulate knowledge and technological capabilities and develop social systems, allowing increasingly effective exploitation of environmental resources.

Demographic transition— This occurs when a rapidly growing population changes from a condition of high birth rate and low death rate to one of low birth rate in balance with the death rate, so that the population stops increasing in size.

Demography— The science of population statistics.

Doubling time— The time required for an population to double in size.

This trend can be illustrated by differences in the intensity of energy use among human societies, which also reflect the changes occurring during the history of the evolution of sociocultural systems. the average per-capita consumption of energy in a hunting society is about 20 megajoules (millions of joules) per day (MJ/d), while it is 48 MJ/d in a primitive agricultural society, 104 MJ/d in advanced agriculture, 308 MJ/d for an industrializing society, and 1025 MJ/d for an advanced industrial society. the increases of per-capita energy usage, and of per-capita environmental impact, have been especially rapid during the past century of vigorous technological discoveries and economic growth.

In fact, global per-capita economic productivity and energy consumption have both increased more rapidly during the twentieth century than has the human population. This pattern has been most significant in industrialized countries. in 1980, the average citizen of an industrialized country utilized 199 gigajoules (GJ, billions of joules) of energy, compared with only 17 GJ/yr in less-developed countries. Although industrialized countries only had 25%percnt; of the human population, they accounted for 80% of the energy use by human beings in 1980. Another illuminating comparison is that the world’s richest 20% of people consume 86%percnt; of the goods and services delivered by the global economy, while the poorest 20% consumes just 1.3%percnt;. More specifically, the United States—the world’s richest country as measured on a net, though not on a per-capita, basis—consumes approximately 25% of the world’s natural resources and produces some 75% of its hazardous wastes and 22% of its greenhouse gas emissions, while having only about 4.5% of the world’s population.

See also Extinction.

Resources

BOOKS

Birdsall, N., A.C. Kelley, S. Sinding (eds). Population Matters: Demographic Change, Economic Growth, and Poverty in the Developing World. Oxford University Press, 2003.

Weeks, John R. Population: An Introduction to Concepts and Issues. Belmont, CA: Wadsworth Publishing, 2004.