Chapter 4: The Human
Population and the Environment
Basic Concepts of Population
Dynamics
• A population is a group of individuals of the
same species living in the same area.
• A species is all individuals that are capable
of interbreeding.
• A species is made up of populations.
Basic Concepts of Population
Dynamics
• Five key properties of any population
–
–
–
–
–
Abundance
Birth rates
Death rates
Growth rates
Age structure
Basic Concepts of Population
Dynamics
• Demography is the statistical study of
human populations.
• The general study of population changes is
called population dynamics.
• How rapidly a pop changes depends on GR
– Growth rate = birth rate – death rate
Age Structure
• The proportion of the population at each
age.
– Implication for current and future social and
economic conditions
– Impact on the environment
Age Structure
• Four general types
– Pyramid- population w/ many young and high
death rate (short average lifetime)
– Inverted pyramid- top heavy
– Column- birth rate and death rate are low and a
high % of pop is elderly
– Column w/ a bulge- event in the past caused a
high birth or death rate for some age group
Age Structure
Kinds of Population Growth
• Exponential Growth
– A pop increasing by a constant percentage per
unit time.
• Human pop growth peaked at 2.1%
– 1965-1970
• Now at 1.2%
History of Human Population
Growth
• 1. Early period of hunter and gathers- total pop < a
few million
• 2. Rise of agriculture- allowed for increase in pop
density and inc in human pop
• 3. Industrial revolution- improvements in health
and food supply led to rapid inc in pop
• 4. Today- rate of growth slow in industrialized
nations but high in less developed nations
Human Population Growth
Human Population Growth
Present Human Population Rates
of Growth
• Current world population >6.6 billion
– With annual growth rate of 1.2%
• At this rate 84 million people added to
Earth in 1 year
• Correlation between poverty and population
growth
– Positive feedback
Present Human Population Rates
of Growth
Current US growth
rate 0.6%
Projecting Future Population
Growth
• Doubling time (time required for a pop to
double in size) is very sensitive to growth rate
–
–
–
–
It changes quickly as g.r. changes
US w/ a g.r. of 0.6% has a doubling time of 117 yrs
Nicaragua w/ a g.r. of 2.7%, d.t. = 26 yrs
Northern Europe w/ a g.r. of 0.2%, d.t. = 350 yrs
Logistic Growth Curve
• S shaped curve
– Increase exponentially only temporarily
– Then growth rate would decline
– Reach an upper pop limit @ logistic carrying
capacity (g.r. = 0)
Logistic Growth Curve
• Little evidence that animal populations
actually follow this growth curve
• Involves assumptions
– Constant environment
– Constant carrying capacity
– Homogeneous population
• Unlikely if death rate continue to decrease
Demographic Transition
• Three stage pattern of change in birth rates
and death rates
– Occurred during the process of industrial and
economic development of Western nations
– Leads to decline in pop growth rate
Demographic Transition
• Stage 1
– Nonindustrial country
– Birth rate and death rate high, growth rate low
• Stage 2
– Period of high growth rate
– W/ industrialization death rate declines but
birth rate stays high
Demographic Transition
• Stage 3
– Birth rate drops toward death rate
– Growth rate decreases
– Will take place if parents come to believe that
having a small family is to their benefit.
Population and Technology
• Impact that all humans pose on the
environment is a result of two factors
– Number of people
– Impact of each person on the environment
• Total impact of the human pop on enviro =
average impact of an ind x total # of ind
–T=PxI
Population and Technology
• Modern technology increases the use of
resources and enables us to effect the enviro in
new ways.
– E.g. CFCs, automobiles
• Population x technology reveals irony
(T=P x I)
– Improving standard of living increases P
– Countering the benefits of declining I
Human Carrying Capacity
• How many people can live on Earth at the
same time?
• Answer depends on the quality of life
people desire and are willing to accept.
– Estimates vary based on assumptions made.
– “packing-problem”
– Deep ecology
Quality of life
• If people of the world were to live at the
same level as those in US
– High resource use
– Carrying capacity would be low
• If people of the world were to live at the
same level as those in Bangladesh
– Poverty and heavy drain on biodiversity
– Carrying capacity would be much higher
Potential Effects of Medical Advances
on Demographic Transition
• Second decline in death rate
– Leads to Stage IV
• A second stable state would arise if birth rate then
falls
– Stage V
• Decision needs to be made
– Stop research on diseases of old age
– Reduce birth rate
– Or do neither and wait for Malthus’ projections
Human Death Rates
• Acute or epidemic disease- appears rapidly
in pop, affects a large % and then declines.
• Chronic disease- always present in a pop,
typically occurring in small %
• Emerging diseases could effect both
industrial and less developed nations
– SARS
– West Nile Virus
– Epidemic flu
Longevity and Its Effect on
Population Growtrh
• Maximum lifetime- maximum possible age
to which an ind of a species can live.
• Life expectancy- the average # of years an
ind can expect to live.
– Higher in developed nation
– Japan highest, 82 years
– Botswana, Lesotho and Swaziland lowest, 35
years
Each age class within a population has its own life expectancy.
Limiting Factors
• Human populations will eventually be
limited by some factor or combo of factors
– Short-term - affect pop during the year in which
they become limiting
– Intermediate-term - effects are apparent after 1
yr but before 10yrs.
– Long-term – effects are not apparent for 10yrs
How Can We Achieve Zero
Population Growth?
• Simplest and
one of the most
effective is to
delay the age of
1st childbearing
by women.
How Can We Achieve Zero
Population Growth?
• Birth Control
– Breast-feeding can delay resumption of
ovulation
– Family planning methods from abstinence to
induction of sterility w/ natural agents
National Programs to Reduce
Birth Rates
• The choice of population control methods is
an issue that involves social, moral and
religious beliefs
– Vary from country to country
– Wide range of approaches
• Information
• Accesses to birth control
• Rewards and penalties