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Population
Dynamics
Unit 6
Principles of Population
Ecology
• Human population growth is central to
many environmental problems
• We can learn about human population
growth by studying other species
• Population ecology = study of the number of
individuals of a particular species that are
found in an area and how and why those
numbers increase or decrease over time
Aspects Population Ecologists
Consider…
• Competition for food or resources
• Predation
• Disease
• Natural disasters (drought, flood,
earthquake, etc.)
• Reproductive success or failure
• How the population interacts with the
community in an ecosystem
Population Density
• Population size is only meaningful when the
boundaries of the population are defined
• 100 people/sq.mile vs. 100 people/100 sq.
miles
• Population Density = number of individuals
of a species per unit of area or volume at a
given time
• Different habitats can support different
densities
Changing Population Sizes
• Formula to know: r = b – d
• Growth rate (r) = birth rate (b) – death rate
(d)
• Birth and death rate usually expressed per
1000 people
• Growth rate (aka natural increase) usually
expressed as a percentage
• If r is positive, there is population growth. If
r is negative, the population is shrinking.
Changing Population Sizes
• Example: If there are 200 births per
10,000 people and 100 deaths per 10,000
people each year, what is the growth rate of
the population?
• 200/10,000 = 20/1,000;
10/1,000
100/10,000 =
• r = (20/1,000) – (10/1,000) = 0.02 –
0.01 = 0.01
• r = 0.01 or 1% growth rate per year
Words to know…
• Dispersal = the movement of individuals
among populations
• Immigration = the dispersal of individuals
into a population from another area or
country
• Emigration = the dispersal of individuals
from a population, bound for another area
or country in which to live
Global vs. Local Population
Local Population Formula
• r = (b – d) + (i – e)
• i = immigration, e = emigration
• Same idea as global population formula
• Per 1000 people
• Positive = growth, negative = decline
Maximum Population Growth
• Biotic potential (intrinsic rate of increase) =
maximum rate at which a population could
increase under ideal circumstances
• Different species have different biotic potentials
• Age at which reproduction begins and ends
• Fecundity rate = how many offspring are produced
for each period of reproduction and how many
reproductive periods per lifetime
• These factors are life history characteristics and
determine if a species has a high or low biotic
potential
Maximum Population Growth
•
In general, larger species like whales or elephants have smaller
biotic potentials while microorganisms have large biotic
potentials
•
1 bacteria can divide in half every 30 minutes  1 to 1 million
in 10 hours
•
Exponential population growth = graph has a “J curve,”
accelerating population growth
•
Logistic population growth = graph shows a line or “S curve,”
stable population growth
What keeps populations in
check?
• Environmental resistance = limits set by the
environment that limits biotic potentials,
such as limited resources, competition,
disease, predation, natural disasters, etc.
• Environmental resistance increases with
population size  negative feedback loop
• Carrying capacity (K) = largest population
that can be maintained for an indefinite
period by a particular environment
RECALL… Reproductive
strategies
• R-strategy = mature rapidly, short life
spans, high fecundity (rate of reproduction),
low biodiversity, niche generalists
• K-strategy = mature slowly, longer life
spans, low fecundity, high biodiversity, niche
specialists
RECALL… Survivorship Curves
•
Type I = high survival rates for early and middle life,
rapid decline in later life (ex. humans)
•
Type II = roughly constant mortality rate at all stages
of life (ex. some species of birds)
•
Type III = high death rate for early and middle life, high
survival rate in later life (ex. octopus)
•
Survivorship = probability that a given individual in a
population will survive to a particular age
More factors that affect
population size…
• Density-dependent factor = environmental
factor that is affected by population density,
such as predation, disease, competition, etc.
• Density-independent factor = environmental
factor that is NOT affected by population
density, such as random weather events
• Both types of factors can be inter-related
Predator-Prey Dynamics
• Predators eat prey  usually have the lower
population number
• More prey available  more predators can
survive  more prey are consumed  less
prey available  less predators can survive
 less prey are consumed  more prey
available …
• Boom or bust cycle = dramatic increases
followed by crashes
Human Population Growth
• Currently exponential  1.3% per year
• RECALL: Thomas Malthus (1766-1834) was
a British economist who predicted that
human population growth would eventually
be stopped by disease, famine and war
• Proven wrong due to Industrial
Revolution…or was he?
Human Population Growth
• Current Population
• Population experts have predicted that we
will reach zero population growth by the end
of the 21st century
Words to know…
•
Zero population growth = the condition when a
population is no longer increasing (or decreasing)
because the birth rate equals the death rate
•
Infant mortality rate = the number of infant deaths
per 1,000 live births
•
Doubling time = the number of years it will take a
population to double in size, given its current growth
rate
•
Replacement-level fertility = the number of children a
couple must have to “replace” themselves
•
Total fertility rate = the average number of children
born to each woman during her lifetime
Demographics
• Applied branch of sociology that deals with
population statistics of different countries
• Highly developed countries (aka developed
countries) = low rates of population growth,
highly industrialized, low infant mortality,
high gross national income in purchasing
power party (GNI PPP)  the amount of
goods and services an average citizen of that
country could buy in the United States
Developing Countries
• Can be divided into two groups:
• Moderately developed countries = birth rates
and infant mortality rates are high, but
declining, medium level of industrialization,
low GNI PPP
• Less developed countries = highest birth rates
and infant mortality rates, shortest life
spans, lowest GNI PPP’s in the world
Demographic Stages
• Demographers recognize 4 demographic
stages based on their observations of Europe
and America
• Because all highly developed and moderately
developed countries with more advanced
economies have gone through this
demographic transition, demographers
generally assume that the same progression
will occur in less developed countries as they
become industrialized.
Demographic Stages
• Preindustrial stage = birth and death rates are high,
infant mortality rate is high, population grows at a
modest rate
• Plague, war, etc. can increase death rate and cause
population growth to slow or decline
Demographic Stages
• Transitional stage = improved health care,
more reliable food and water, beginning of
industrialization
• Lower death rate, birth rate still high  rapid
population growth
Demographic Stages
• Industrial stage = birth rate declines, death
rate still low, slow population growth
Demographic Stages
• Postindustrial stage = low birth and death
rates, population is better educated and
wealthy, tend to desire smaller families and
take steps to limit family size, population
grows very slowly, not at all, or declines
Age Structure of Countries
• Number and proportion of people at each
age in a population  represented in an age
structure diagram
Population Growth Momentum
• When a large population of children mature,
they become the parents of the next
generation
• This set of parents is larger than the
previous population and therefore have more
offspring, even if they have smaller families
Demographics of the United
States
• Largest population of all highly developed
countries
• High population AND overconsumption 
are we overpopulated?
• Should we control family size?
Immigration?
• 1 million legal immigrants each year
U.S. Immigration
• 150 million people currently live outside of
their native country  easier to do in
modern times
• Before 1875, there were no immigration
laws in the United States
• 1875 = Congress passes a law preventing
convicts and prostitutes from coming in
• 1882 = Chinese Exclusion Act
• 1891 = Bureau of Immigration established
U.S. Immigration
•
Early 20th century = Congress sets more limits,
including quotas allowing only a certain number of
people from each foreign country to immigrate  #
of illegal immigrants increased
•
World War II = labor shortages resulted in more lax
immigration laws
•
1952 = Immigration and Nationality Act  now the
Immigration Reform and Control Act  current
immigration law
•
1965 = abolished all national quotas and gave
priority to those with family members already in
USA, those who can fill vacant jobs, and refugees
seeking asylum
U.S. Immigration
• Current waves of immigrants do not look
the same as those who came in the
beginning of the 20th century
• 4 out of 5 immigrants to the US today
come from Asia or Latin America
• From an environmental standpoint: Can the
US sustain such a large population? Large
birth rates vs. simpler lifestyles
Human Population Explosion
• 81% of the world lives in less developed
countries
• 126 of these countries will double their
populations by 2050 at the current rate
• Quality of life vs. quantity of people
Population and World Hunger
• 800 million people do not get enough food
• 86 countries are considered low-income and
food-deficient  especially in South Asia and
sub-Saharan Africa
• Children are especially prone to starvation
• How to solve the problem  population
control or promote economic development?
Distribution of resources?
Economic Effects of Continued
Population Growth
• Economic growth and population growth are
intertwined  both affect the other
• Does population growth help or hurt
economic growth? It depends…
• Standard of living  to raise it, economic
growth must be happening faster than
population growth
Population, Resources, and the
Environment
• Developed vs. Developing Nations
Types of Resources
• Nonrenewable resources = limited supplies,
depleted by use (minerals, fossil fuels, etc.)
• Renewable resources = supplies replaced at a
rate that matches or beats consumption
(trees, fish, soil, water, etc.)
• Renewable can become nonrenewable if used
too much
Poverty vs. Resources
• Poverty is tied to the effects of population
pressures on natural resources and the
environment
• Impoverished people use environmental
resources unwisely for short term gain 
that gain can be survival
• In the long term, resources are degraded
and economic development is hindered
Words to Know…
• Consumption = the human use of materials and
energy
• People overpopulation = a situation in which
there are too many people in a given area,
resulting in pollution, environmental
degradation, and resource depletion, even
though each individual consumes few resources
• Consumption overpopulation = a situation in
which each individual in a population consumes
too large a share of resources, resulting in
pollution, environmental degradation, and
resource depletion
Words to Know…
• Ecological footprint = the average amount of
land and ocean needed to supply an
individual with food, energy, water, housing,
transportation, and waste disposal
• Sustainable consumption = the use of goods
and services that satisfy basic human needs
and improve the quality of life but that also
minimize the use of nonrenewable and
renewable resources so they are available for
future generations
How much do we use?
• Highly developed nations use:
• 86% of aluminum
• 76% of timber
• 68% of energy
• 61% of meat
• 42% of fresh water
• Produce 75% of waste
• We would need 5 Earths to support everyone if
we all consumed as much as Americans
Urbanization
• Geographical distribution of people influences
social, environmental, and economic aspects
of population growth
• Recently, more and more people have been
migrating to cities
• How many people does it take to make a
city? It depends…
• Could be 100 homes clustered together or
50,000 residents
Characteristics of Urban
Populations
• Every city is unique (size, climate, culture,
economy, etc.)
• In general, city populations are more
heterogeneous as far as race, ethnicity, religion,
and socioeconomic status are concerned
• Urban populations tend to be younger  influx
of young adults from rural areas
• In developing nations, cities tend to have more
males (seeking jobs to support families in rural
areas)  Developed nations actually have more
females than males in cities
City as an Ecosystem
•
Urban ecologists study for variables = POET
•
Population = number of people, factors influencing
population, and composition of city by age, sex, and
ethnicity
•
Organization = social structure of city, such as
economic policies, government, and social hierarchy
•
Environment = natural environment and physical
infrastructure, how human activity affects
environment
•
Technology = human inventions that directly affect
urban environment (aqueducts, AC, transportation,
etc.)
Environmental Problems
Associated with Urban Areas
• Destroy or fragment habitats
• Brownfields = large areas of abandoned, vacant
factories, warehouses, residential areas, etc.
• Suburbs continue to expand
• Commuters = more air pollution
• Affect water flow by covering soil with
pavement, generating water pollution, and/or
using more water than can be replaced
• Cities tend to be warmer than surrounding areas
= urban heat island  affects precipitation
Environmental Benefits of
Urbanization
• A well-planned city can reduce the pollution
and preserve rural areas
• Compact development = cities are designed
so that tall, multiple-unit residential
buildings are close to shopping and jobs, use
public transportation, less parking lots, etc.
• Good examples = Portland, Oregon; Curitiba,
Brazil
Urbanization Trends
• Over half of the world’s population lives in cities
• Most urban growth is happening in developing
countries
• The fast population growth is outstripping the
ability of these cities to provide basic services like
medical care, waste disposal, education, law
enforcement, etc.  slums and squatter
settlements
• Urbanization decreases total fertility rates 
more access to birth control
Culture and Fertility
•
Culture = values and norms of a society
•
Gender is an important part of culture  different
societies have different gender expectations
•
High TFRs are traditional in many cultures, especially
because infant mortality rates are high
•
Many cultures value male children over female
children
•
Social and religious impacts on TFR
•
Catholics have higher TFRs than Protestants or
Jewish women; women with no religion have the
lowest rates of all
Social and Economic Status of
Women
• Women do not have the same rights,
opportunities, or privileges as men in most
societies
• A woman’s value is usually tied to her
father’s status and then her husband’s
• Males are more likely to receive healthcare,
education, etc.
• Status quo enforced by laws, customs, etc.
Factors that affect fertility
• Marriage age = the earlier a woman marries, the
more children she is likely to have
• Education = the more education a woman has,
the less children she is likely to have
• Knowledge of how to control fertility
• More job opportunities = value besides procreation
• More focus on improving living standards
• Availability of family planning services = prenatal
care and improved birth spacing improves infant
mortality rate, contraceptives reduce fertility
rates
Government Policies and
Fertility
• Government already controls aspects of
fertility  minimum age for marriage,
government subsidized health care and
family planning services
• Government can provide incentives that
reward or discourage large families
• China’s 1 child policy = enacted in 1979,
relaxed in 1984 for rural families  may
now have a second child if 1st one is a girl
Government Policies and
Fertility
• India is the second most populous nation in the
world  in 1950’s, it became the 1st nation to
establish government-sponsored family planning
• No immediate results  India is a very diverse
country with 14 main languages and 700 other
languages  difficult to maintain broad
government programs
• Indian culture is biased towards male children
• 1976 = men with 3 or more children forced to
have a vasectomy  drop in number of female
children, even after the practice was abandoned
Government Policies and
Fertility
• Mexico is the second most populous nation in
Latin America (after Brazil)
• 33% of population is less than 15 years of age
• Traditionally, the Mexican government
supported population growth, but in 1974, the
government implemented programs to decrease
population growth (educational reform, family
planning, better healthcare, etc.)
• Birth rate went from 6.7 to 2.9
Government Policies and
Fertility
• Nigeria has the largest population of any African
country
• 44% of the population is less than 15 years of
age
• Only 9% of married women use a method of
contraception, less than half of all women have
any knowledge of contraception
• 1986 = government developed a population
policy that improves health care and encourages
later marriages and longer birth spacing
Government Policies and
Fertility
•
In Europe, populations are either stable or shrinking
•
Pronatalists = those who favor population growth 
protecting culture, economic vitality, nation’s
influence, etc.
•
Pensions and old-age security programs  how will
they be supported by a shrinking population?
•
Policies like paid maternity and paternity leave, tax
bonuses, etc. encourage larger families
•
Unemployment problems
•
Immigration vs. native birth
Global Summit of Population
and Development
• 20 year action plan:
• Ensure reproductive rights of individuals  allow
them to make informed decisions
• Empower women  allow economic and
educational opportunities
• Improve reproductive health services
• Some resistance  abortion policies
• Solution = local laws, cultural values, and
religious beliefs will be respected as plan is
implemented
Achieving Population
Stabilization
• Eliminating problems like hunger, poverty,
economic underdevelopment, environmental
issues, etc.
• Increase funding for public health programs
and public education
• Reduce consumption