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How is Population Size Affected by Birth Rates and Death Rates?

Population change is calculated by subtracting the number of people leaving a population (through death and emigration) from the number entering it (through birth and immigration) durinng a specified period of time (usually a year):

population change = (births + immigration) - (deaths + emigration)

When these factors balance out, population size remains stable, a condition known as zero population growth (ZPG).

Instead of the total numbers of births and deaths per year, demographers use two sttistics: the birth rate, or the crude birth rate - the number of live births per 1,000 people in a given year; and the death rate, or crude death rate - the number of deaths per 1,000 people in a population in a given year.

annual rate of natural population change (%) = birth rate -death rate / 1,000 persons X 100

= birth rate - death rate / 10

The rate of the world's annual growth (natural increase) dropped 23% between 1965 arid 1996, from 2% to 1.55%.

An annual natural increase rate of 1 - 3% may seem small, but such exponential growth lead to enormous increases iri population size over a over 100-year period. The current annual population increase rate of 1.55% adds 90 million people per year (a Los Angeles every two weeks).

Africa, 13% of world's population, has the highest rate of population growth (2.8% in 1996) of any continent, with a population doubling time of only 24 years. But Asia, with 61% of the world's population, has the largest annual increase. In numbers of people, China (with 1.22 billion, one of every five people in the world) and India (950 million) dwarf all other countries. Together make up 38% of the world's population. The United States (265 million) has the third largest population population but only 4.6% of the world's people.

Between 1996 and 2025, more than 95% of this growth is projected to take place in developing countries, where hunger and poverty and hunger have become a way of life for almost a billion people.

How Have Global Fertility rates Changed?

Replacement-level fertility is the number of children a couple must bear to replace themselves (2.1 in developed countries and as high as 2.5 in some developing countries), mostly because more children die before resching their reproductive years.

Lowering fertility rates to replacement level does not mean an immediate halt in population growth.

The most useful measure for protecting future population change is the total fertility rate (TFR). an estimate of the average number of children a woman will have during her childbearing years, under current age-specific birth rates. In 1996, the worldwide average TFR was 3.0 children per woman. It was 1.6 in MDC's (down from 2.5 in 1950) and 3.4 in developing countries (down from 6.5 in1950).

Africa has the highest rate of 5.7 children per woman.

TFRs in developed countries is expected to remain about 1.6 and those in LDCs to drop to 2.3 by 2025. This will still lead to a projected population of around 8 billion by 2025, with more than 90% of this growth taking place in developing countries. A key variable in population projections is the tiome at which the total fertility rate of the world (or of individual countries) will drop to a replacement level of around 2.1 children per woman; even after that time, world population will continue to grow for 50-60 years before stabilizing.

How Have Fertility Rates Changed in the United States? The U.S. population has grown from 76 million in 1900 to 265 million in 1966. The TFR has oscillated wildly: 3.7 children per woman in 1957 (post-WWII baby boom) and declining to replacement level since 1972. The rate of growth has declined, but the population is still growing faster than most developed countries. It grew by 1.17% in 1996 (more than double the mean rate of industrialized nations). This growth added 3.1 million people: 1.8 million more births than deaths; 935,000 legal immigrants and 400,000 illegal immigrants. The U.S. Bureau of Census projects the population to increase from 265 to 383 million between 1966 and 2025 (45% increase), with no stabilization in site. Each addition to the U.S. population has an enormous environmental impact. The four main reasons for this projected growth are: the large number of baby-boom women still in childbearing years. an increase in the number of unmarried mothers (Spo4ight and Guest Essay). a continuation of higher fertility rates in some racial and ethnic groups. high levels of legal and illegal immigration ( 40% of U.S. population growth).

Case Study: Increasing Fertility Rates and Environmental Problems in Calif. California is the most populous U.S. state with 32 million people in 1995 and is projected to reach 49 million by 2020. The national TFR has stayed below replacement level since 1985, but California's rose from 1.9 to 2.5 between 1985 and 1995. Much of this increase is due to the high fertility of its immigrants.

The impacts from this increase include loss of wetlands, higher proportion of endangered species than all other 48 lower states, sewage systems nearing capacity, water shortages and irrigated cropland losing productivity. Air pollution has damage crops and led to the strictest air-pollution standards in the United States - primarily due to an 83% increase in motor vehicles and a 50% increase in total annual distances traveled between 1971 and 1995.

What Factors Affect Birth Rates and Fertility Rates?

  • Average level of education and affluence - lower rates with higher education.
  • importance of children as apart of the labor force - higher in LDC's (rural).
  • Urbanization - People living in urban areas usually have betetr access to family planning services.
  • Cost of raising and educating children - children don't enter labor force until later in developed countries.
  • Educational and employment opportunities for women - fertility rates decline as female literacy rate increases.
  • infant mortality rate - lower infant mortality rate leads to smaller families.
  • Average age at marriage (or age women have their first child) - fewer children if age of marriage is 25 or older.
  • Availability of private and public pension systems - pensions eliminate the need of having children to support aging parents.
  • Availability of legal abortions.
  • Availability of reliable methods of birth control.
  • Religious belief, traditions, and cultural norms.
  • What Factors Affect Death Rates? The rapid growth of the world's population is largely due to a decline in crude death rates. Most people started living longer (and fewer infants dies) due to better nutrition, medical technology, sanitation, personal hygiene and safer water supplies. Indicators of overall health are life expectancy (average number of years a newborn can expect to live) and infant mortality rate (number of babies out of every 1000 that die before their first birthday). Infant mortality is probably the single most important measure of a society's quality of life: high rates indicate insufficient food, malnutrition and infectious disease (usually from contaminated water). Between 1965 and 1996, the world's infant mortality rate dropped from 20 to 9 (per 1000 live births) in developed countries and from 11 8 to 68 in developing countries (still high). U.S. infant mortality rate was 7.5 in 1996, but 28 other countries had lower rates. Three factors that keep the U.S. rate high are inadequate health care for the poor, drug addition among pregnant women, and a high birth rate among teenagers.

    How is Migration Related to Environmental Degradation?

    Migration is the movement of people into (immigration) and out of(emigration) an area. People voluntarily move from less affluent areas of ~ow opportunity to more affluent areas. Some migration is involuntary. Environmental refugees are displaced by problems such as drought, desertification, soil erosion and resource shortages The number of refugees could increase (from 25 million in 1996 to 50 million by 2010 or 200 million in the next century) if the predictions of global warming are correct. Dealing with 25 million environmental refugees is a high task (GueSt Essay). Most countries restrict international immigration. Migration within countries (rural to urban) plays an important role in the population dynamics of cities, towns and rural areas.