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 Population Dynamics
 Populations
 Organisms do not generally live alone. A population is a group of organisms from
the same species occupying in the same geographical area.
 This area may be difficult to define because:
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A population may comprise widely dispersed individuals which come
together only infrequently, e.g. for mating.
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Populations may fluctuate considerably over time.
 Features of Populations
 Populations are dynamic and exhibit attributes that are not shown by the individuals
themselves.
 These attributes can be measured or calculated and include:
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Population size: the total number of organisms in the population.
Population density: the number of organisms per unit area.
Population distribution: the location of individuals within a specific area.
 Features of Populations
 Population composition provides information relevant to the dynamics of the
population, i.e. whether the population is increasing or declining.
 Information on population composition (or structure) includes:
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Sex ratios: the number of organisms of each sex.
Fecundity (fertility): the reproductive capacity of the females.
Age structure: the number of organisms of different ages.
 Population Dynamics
 The study of changes in the size and composition of populations, and the factors
influencing these changes, is population dynamics.
 Key factors for study include:
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Population growth rate: the change in the total population size per unit
time.
Natality (birth rate): the number
of individuals born per unit time.
Mortality (death rate): the number of individuals dying per unit time.
Migration: the number moving into or out of the population.
 Migration
 Migration is the movement of organisms into (immigration) and out of
(emigration) a population. It affects population attributes such as age and sex
structure, as well as the dynamics of a population.
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Populations lose individuals through deaths and emigration.
Populations gain individuals through births and immigration.
 Population Density
 The number of individuals per unit area (for terrestrial organisms) or volume (for
aquatic organisms) is termed the population density.
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At low population densities, individuals are spaced well apart. Examples:
territorial, solitary mammalian species such as tigers and plant species in
marginal environments.
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At high population densities, individuals are crowded together. Examples:
colonial animals, such as rabbits, corals, and termites.
 Population Distribution
 A crude measure of population density tells us nothing about the spatial distribution
of individuals in the habitat.
 The population distribution describes the location of individuals within an area.
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Distribution patterns are determined by the habitat patchiness (distribution of
resources) and features of the organisms themselves, such as territoriality in
animals or autotoxicity in plants.
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Individuals in a population may be distributed randomly, uniformly, or in
clumps.
 Random Distribution
 A population’s distribution is considered random if the position
of each individual is independent
of the others.
 Random distributions are not common; they can occur only where:
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The environment is uniform and resources are equally available throughout
the year.
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There are no interactions between individuals or interactions produce no
patterns of avoidance or attraction.
 Random distributions are seen in some invertebrate populations, e.g. spiders and
clams, and some trees.
 Uniform Distribution
 Uniform or regular distribution patterns occur where individuals are more evenly
spaced than would occur by chance.
 Regular patterns of distribution result from intraspecific competition amongst
members of a population:
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Territoriality in a relatively homogeneous environment.
Competition for root and crown space in forest trees or moisture in desert
and savanna plants.
Autotoxicity: chemical inhibition of plant seedlings of the same species.
 Clumped Distribution
 Clumped distributions are the most common in nature; individuals are clustered
together in groups.
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Population clusters may occur around a a resource such as food
or shelter.
Clumped distributions result from the responses of plants and animals to:
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Habitat differences
Daily and seasonal changes in weather and environment
Reproductive patterns
Social behavior
 Density Dependent Factors
 Density dependent factors exert a greater effect on population growth at higher
population densities.
At high densities, individuals:
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Are more easily located by
predators and parasites.
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Are more vulnerable to infection and disease.
Density dependent factors are biotic factors such as food supply, disease, parasite
infestation, competition, and predation.
Species Interactions
No organism exists in isolation. Each participates in interactions with other
organisms and with the abiotic components of the environment.
Species interactions may involve only occasional or indirect contact (predation or
competition) or they may involve a close association between species. Symbiosis
is a term that encompasses a variety of such close associations, including
parasitism (a form of exploitation), mutualism, and commensalism.
Parasitism
Many animal taxa have representatives that have adopted a parasitic lifestyle.
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Compete more for resources.
Parasites occur more commonly in some taxa than in others. Insects, some
annelids, and flatworms have many parasitic representatives.
Parasites live in or on a host organism. The host is always harmed by the presence
of the parasite, but it is not usually killed. Both parasite and host show adaptations
to the relationship.
Parasites may live externally on a host as ectoparasites, or within the host’s body
as endoparasites.
Ectoparasites
Ectoparasites, such as ticks, mites, lice, bed bugs, and fleas, live attached to the
outside of the host, where they suck body fluids, cause irritation, and may act as
vectors for pathogens or disease-causing agents.
Insect vectors include human lice, rat fleas, mosquitoes and tsetse flies.
Endoparasites
In many endoparasites, a primary host (in which the parasite becomes sexually
mature) and one or more intermediate hosts (which usually house larval stages)
are required to complete the life cycle.
 Endoparasitic species such as the pork tapeworm, Taenia solium, are highly
specialized to exploit the resources of the host.
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Adaptations, such as specialized mouthparts and lack of a digestive tract,
enable them to spend their life inside the intestines of their vertebrate hosts.
Tapeworm Adaptations
Features of the pork tapeworm, Taenia solium, illustrate typical cestode
adaptations.
Humans are the only primary host. Proglottids containing eggs are passed in the
feces to be ingested by a pig. They hatch and are reingested when the pig flesh is
eaten by humans.
The worm is simply a specialized head region (scolex) and a series of proglottids
which are shed into the feces. There is no gut and respiration is anaerobic.
Plant Parasites
A great diversity of plant parasites exist. Some depend only partly on their host
plant for nutrition; they are photosynthetic but utilize the host’s nutrients, e.g.
mistletoe.
Others are entirely parasitic and are unable to photosynthesize, e.g. cancer root
(Conopholis americana) dodder, and wood rose (Dactylanthus taylorii).
Mutualistic Relationships
Mutualistic relationships occur between some birds (such as oxpeckers) and large
herbivores (such as zebra, Cape buffalo, and rhinoceros). The herbivore is cleaned
of parasites and the oxpecker gains access to food.
Lichens are an obligate mutualism between a fungus and either a green alga or a
cynobacterium. The fungus obtains organic carbon from the alga. The alga obtains
water and nutrient salts from the fungus.
Commensal Relationships
In commensal relationships, one party (the commensal) benefits, while the host is
unaffected.
Epiphytes (perching plants) gain access to a better position in the forest canopy,
with more light for photosynthesis, but do no harm to the host tree.
Commensal anemone shrimps (Periclimenes spp.) live within the tentacles of host
sea anemones. The shrimp gains protection from predators, but the anemone is
neither harmed nor benefitted.
Exploitation
Exploitation describes relationships where one species benefits at the expense of
another. It includes several familiar interactions:
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Predation: Predator kills the prey outright, e.g. lions hunting zebra.
Herbivory: The herbivore feeds on, but usually does not kill, the plant, e.g.
zebra grazing on grass.
Parasitism: The parasite does not usually kill its host, e.g. ticks feeding on
the blood of a zebra.
 Types of Interaction
 Density Independent Factors
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The effect of density independent factors on a population’s growth is not
dependent on that population’s density:
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Physical (or abiotic) factors
* temperature
* precipitation
* humidity
* acidity
* salinity etc.
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Catastrophic events
* floods and tsunamis
* fire
* drought
* earthquake and eruption
 Population Growth
 Population growth depends on the number of individuals added to the population
from births and immigration, minus the number lost through deaths and emigration.
 This can be expressed as a formula:
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Net migration is the difference between immigration and emigration.
Calculating Population Change
Rates of Population Change
Ecologists usually measure the rate of population change.
These rates are influenced by environmental factors and by the characteristics of
the organisms themselves.
 Rates are expressed as:
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Numbers per unit time,
e.g. 2000 live births per year
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Per capita rate (number per head of population),
e.g. 122 live births per 1000 individuals (12.2%)
 Exponential Growth
 Populations becoming established in a new area for the first time are often termed
colonizing populations.
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They may undergo a rapid exponential (logarithmic) increase in numbers to
produce a J-shaped growth curve.
 In natural populations, population growth rarely continues to increase at an
exponential rate.
 Factors in the environment, such as available food or space, act to slow population
growth.
 Logistic Growth
 As a population grows, its increase will slow, and it will stabilize at a level that can
supported by the environment.
 This type of sigmoidal growth produces the logistic growth curve.
 Life Tables
 Numerical data collected during a population study can be presented as a table of
figures called a life table.
 Life tables provide a summary of mortality for a population. The basic data are the
number of individuals surviving to each age interval. This gives the ages at which
most mortality occurs in a population.
 Survivorship Curves
 The age structure of a population can represented with a
survivorship curve. Survivorship curves use a semi-log plot of the number of
individuals surviving per 1000 in the population, against age.
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Because they are standardized (as number of survivors per 1000),
species with different life expectancies can be easily compared.
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The shape of the curve reflects where heaviest mortality occurs:
 Type I Survivorship Curves
 Species with Type I or late loss survivorship curves show the heaviest mortality
late in life. Mortality is very low in the juvenile years and throughout most of adult
life.
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Late loss curves are typical of species that produce few young and care for
them until they reach reproductive age.
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Such species are sometimes called K selected species and include
elephants, humans, and other large mammals.
 Type II Survivorship Curves
 Species with Type II or constant loss survivorship curves show a relatively
constant mortality at all life stages.
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Constant loss curves are typical of species with intermediate reproductive
strategies. Populations face loss from predation and starvation throughout
life.
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Examples include some many types of songbirds, some annual plants, some
lizards, and many small mammals.
 Type III Survivorship Curves
 Species with Type III or early loss survivorship curves show the highest mortality
in early life stages, with low mortality for those few individuals reaching a certain
age and size.
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Early loss curves are typical of species that produce large number of offspring
and lack parental care.
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Such species are r selected species (opportunists), and include most annual
plants, most bony fish (although not mouth brooders), and most marine
invertebrates.
 ‘r’ and ‘K’ Selection
 Two parameters govern the logistic growth of populations.
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The intrinsic rate of natural increase or biotic potential. This is the maximum
reproductive potential of an organism, symbolized by the letter r.
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The saturation density or
carrying capacity of the
environment, represented
by the letter, K.
 We can characterize
species by the relative
importance of r and K
in their life cycles.
 r-Selected Species
 Species with a high intrinsic capacity for population increase are called r-selected
or opportunistic species.
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These species show certain life history features and, to survive, must
continually invade new areas to compensate for being displaced by more
competitive species.
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Opportunists include algae, bacteria, rodents, many insects, and most annual
plants.
 K-Selected Species
 Species that are K-selected exist under strong competition and are pushed to use
available resources more efficiently.
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These species have fewer offspring and longer lives.
They put their energy into nurturing their young to reproductive age.
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K-selected species include most large mammals, birds of prey, and large,
long-lived plants.
 Human Population Growth
 The world population, now over 6.8 billion, is growing at the rate of about 80
million per year.
 Projections put the world population at between 8 and 12 billion in 2050, with nearly
all of this growth expected in the developing world.
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Developing countries include: Africa, Asia (excluding Japan), Latin America
and the Caribbean, and regions of Melanesia, Micronesia, and Polynesia.
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The human population has grown rapidly because of the expansion of agriculture
and industrial production and lower death rates from improvements in hygiene
and medicine.
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In 2006, the population of developed countries grew exponentially at 0.1%
per year
* Developing countries grew (15 times faster at 1.5% per year.
 Population Stabilization
 Human population growth is slower than predicted but because of the large and
increasing population size the world population is still expected to increase
substantially before stabilizing.
 Fertility Rates
 Based on current trends, it is assumed that human fertility rates will continue to
decline and life expectancy will continue to increase. Developing countries are
expected to broadly follow these demographic trends.
 Global Population Growth
 Estimates of likely future growth of the world human population are highly uncertain
and projections for 2050 range from a low of 7.7 billion to a high of 11.2 billion.
 Demographic Transition
 Demographic Transition describes the process where countries become
economically developed then their birth and death rates begin to decline. This
process can be described by the different stages:
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Preindustrial stage shows little population growth due to high birth rate with
high death rate.
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Transitional stage is
industrialization
death rates
birth rates
Rapid
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Industrial stage is birth
dropping and
death rate.
where
begins,
drops and
remain high.
population growth
occurs.
rate
approaching
Population growth
slowly declines.
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Postindustrial stage are
birth
rate and death rate
become
similar and population
growth slows
or declines.
 Demographic Transition
 Generalized model of demographic transition shows that the total fertility of a
society decreases as the society progresses through the demographic transition.
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Some developing countries may have difficulty making the demographic
transition.
 Age Structure
 Age structure refers to the number of organisms of different ages.
 Populations can be broadly grouped into those individuals of:
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pre-reproductive age
reproductive age
post reproductive age
 Analysis of the age structure of
populations can assist in their
management because it can
indicate where most population
mortality occurs and whether
or not reproductive individuals
are being replaced.
 Human Age Structure
 Human age structure varies from country to country.
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In developing countries age structure tends to be in favor of younger
individuals with a large proportion being under 15.
In developed nations, age structure is relatively even throughout the age
groups.
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Age Structure Diagrams: Determining Population Growth
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The rate at which a population grows or declines depends on its age structure. Age
structure diagrams are a graphical illustration that shows the distribution of different
age groups. These groups are further broken into:
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Prereproductive age are persons who are not mature enough to reproduce
Reproductive age are those persons that are capable of reproduction.
Postreproductive age are those persons too old to reproduce.
 Age Structure Diagrams
 Age Structure Diagrams
 Since age structure diagrams are broken down by age, demographers can obtain:
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Birth rate
Maturity rate
Death rate of an entire population.
These diagrams are usually broken down by country and divided into genders.
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32% of the people in developing countries were under 15 years old in 2006
versus only 17% in developed countries.
 Age Structure: United States
 Monitoring the baby boom generation in yellow.
 Age Structure Diagrams
 Pyramid shaped histogram have a birth rate that exceeds the death rate thus
the population is getting larger or growing rapidly.
 Examples: Africa, Asia (Excluding Japan), Latin America
 Age Structure Diagrams
 When the histogram shape is straighter and more box-like until age 45-85 this
shows stable growth population or growing very slowly. In this case the birth
rate almost equals or equals the death rate.
 Examples: US, Australia & Canada: Slow; Denmark, Austria & Italy: Stable
 Age Structure Diagrams
 If there is declining (negative growth) then the birth rate is less than the death
rate. These pyramids will bulge near the top or is invert.
 Example: Germany, Bulgaria & Hungary.
 Demography
 Demography is the study of human populations, their characteristics and changes.
Demographics will measure:
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Statistics about people, such as births, deaths, and where they live as well
as total population size.
 Demographics
 Growth Rate includes birth rate, death rate, immigration and emigration. Usually
calculated in percent growth using the formula of births minus deaths, plus
immigration minus emigration
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Doubling Time is the time it takes for the population to double the number of
people (70 / % growth rate)
 If a population of 10,000 experiences 100 births, 40 deaths, 10 immigrants and 30
emigrants in a year, what is the net annual percentage growth rate?
 If a population of a country grows at a rate of 5% a year, the number of years
required for the population to double is what?
 Factors Affecting Birth and Death Rates
 The number of children women have is affected by:
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Urbanization
The cost of feeding and raising children
Education and economic opportunities for women
Infant deaths
Marriage age
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Availability of contraception and abortion
Government policies and economic incentives
Government policies and economic incentives
Unequal distribution of food (Loss of land)
Importance of children in labor
Religious beliefs and cultural traditions
Increasing poverty
 Urbanization
 Urbanization is the movement of people from rural areas into cities
 Urban areas must import most of its food, water, energy, minerals, & other
resources because of large populations
 Large populations produce and consume enormous quantities of resources that can
pollute the air, water & land.
Disease can easily spread in urban areas because of the density of people
 Urbanization
 Environmental pressures of urbanization from population growth are reduced
because birth rates in urban areas usually are 3-4x’s lower than in rural areas
because cities provide education opportunities.
 Some countries, including China, penalize couples who have more than one or two
children by:
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Raising their taxes
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Charging other fees
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Eliminating income tax deductions for a couple’s third child
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Loss of health-care benefits, free education, food allotments and job
options
 Factors Affecting Birth and Death Rate
 Infant mortality rate: The
number of child and /
or
infant deaths.
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If a mother lives in an
infant
tend to have a lot of
children to ensure some
to adulthood
area with a high
mortality rate she will
Infant mortality rate is
higher in developing
countries than in
developed countries.
will make it
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Along with life expectancy, the infant mortality rate is a good indicator of the
quality of life of a country
Replacement-level fertility: the number of children a couple must bear to replace
themselves. try.
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Slightly higher than two children per couple. (2.2 in developed countries and
2.5 in some developing countries.)
 Factors Affecting Birth and Death Rate
 Total fertility rate (TFR): the average number of children a woman has during
reproductive years.
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In 2006, the average global Total Fertility Rate was 2.7 children per woman.
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1.6 in developed countries (down from 2.5 in 1950.
1.6 in developed countries (down from 2.5 in 1950.
3.0 in developing countries (down from 6.5 in 1950).
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If fertility rate drops to replacement level fertility but the population continues
to grow, this is called population momentum
 Historical Factors Affecting Birth and Death Rates
 Death rates have declined and births have increased because of:
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The Green Revolution increased food supplies and productivity offering
better nutrition opportunities.
The Industrial Revolution improved sanitation and allowed personal
hygiene through safer water supplies.
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Advances in medicine reduced mortality rate.
 Demographics in the USA
 The baby bust that followed the baby boom was largely due to delayed marriage,
contraception, and abortion.
 In 2006, the total fertility rate in the United States was slightly > 2.0
 Demographics in the USA
 Nearly 2.9 million people were added to the U.S. in 2006:try.
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59% occurred because of births outnumbering deaths
41% came from illegal and legal immigration. Population increase in recent
years has been because of immigration
Current U.S. population: 309,786,186
Developed Countries
High rates of resource use (Availability of resources, production, waste)
Result in high levels of pollution and environmental degradation per person (Control
methods, clean up education, )
 The measure of a country’s economic growth is the Gross National Product (GNP)
or the Gross Domestic Product (GDP)
 Demographics:
Developing Countries
 China, the largest, has taken drastic population control methods.
 By 2050, India is predicted to pass China. Pakistan is projected to become 3rd (the
US is 3rd now.)
 However, Russia is losing 600,000 people a year, after being the 4th largest country
in 1950.
 India and China
 For more than five decades, India has tried to control its population growth with only
modest success
 Since 1970, China has used a government-enforced program to cut its birth rate in
half and sharply reduce its fertility rate.
 1994 Global (Cairo) Conference: Population & Development
 The summit at Cairo, Egypt, encouraged action to stabilized the world’s population
at 7.8 billion by 2050, instead of the projected 11-12.5 billion.
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Provide universal access to family-planning services
Improve the health care of infants, children & pregnant women and improve
the status of women by expanding education & job opportunities
Encourage development of national population policies
Increase men’s involvement in child-rearing responsibility & family planning
and increase access to education for girls
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Take steps to eradicate poverty
 Solutions: Population Size
 Family planning has been a major factor in reducing the number of births and
abortions throughout most of the world.
 Women tend to have fewer children if they are:
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Educated
Hold a paying job outside the home.
Do not have their human right suppressed.
 Decrease poverty and make food distribution more efficient.