Population Ecology and
Human Populations
Chapters 5, 6, and 12
Key Questions
1. What are the characteristics of populations, and how
do scientists track population growth?
2. How are k-selected and r-selected species different?
3. What factors affect human population growth?
4. How do we sustain the world population?
5. What are the major food sources in the world, and
what processes exist to increase food production?
What are the characteristics of populations, and
how do scientists track population growth?
Key Question #1
What is population ecology?
• population - individuals that belong to same species and
live in given area at particular time
• community - all populations of organisms within a given
area
• population ecology - study of factors that cause
populations to increase or decrease
Population Terms
• population size (N): total number of individuals within a
defined area at a given time
• population density: number of individuals per unit area
at a given time
• population distribution: description of how individuals
are distributed with respect to one another
• sex ratio: ratio of males to females in a population
• age structure: description of how many individuals fit
into particular age categories in a population
What factors affect population size?
Two factors influence population size
• Density-dependent factors influence an individual’s
probability of survival and reproduction in a manner that
depends on the size of the population
• availability of food, predation, disease, and migration
• Density-independent factors have the same effect on
an individual’s probability of survival and reproduction at
any population size
• weather/climate, pollution, temperature
Population Growth Models
• population growth models: mathematical equations
that can be used to predict population size at any
moment in time
• population growth rate = percent growth
N
(# of years)
• example: In 1980, the population in Lane County was 250,000.
This grew to 280,000 in 1990. What is the annual growth rate
for Lane County?
• Calculate % growth (final population – initial population / initial
population x 100)
• Divide percent growth by # of years elapsed
Model #1
• exponential growth model: estimates a
population’s future size (P) over a given
time period (t) based on the intrinsic
growth rate (r) and the number of
reproducing individuals currently in the
population (P0)
• P= P0ert
• makes a J-shaped curve when graphed
The Exponential Growth Model Graph
When populations are
not limited by
resources, their growth
can be very rapid. More
births occur with each
step in time, creating a
J-shaped growth curve.
Let’s try it
• In 1950, the world's population was 2,555,982,611. With a growth
rate of approximately 1.68%, what was the population in 1955?
You try it now
• At 5pm, you count 26,300 bacteria in your petri dish. If the
growth rate is 2.7%, how many bacteria will there be at
midnight?
But…..?
Populations do not typically experience exponential
growth indefinitely - are limited by two things:
• limiting resource (factor): resource that a population
cannot live without and that occurs in quantities lower
than population would require to increase in size
(space, light, nutrients)
• carrying capacity (K): limit of how many individuals a
population environment can sustain
2. Logistic Growth Model
• describes a population whose growth is initially
exponential, but slows as the population approaches the
carrying capacity of the environment
• makes an S-shaped curve when graphed
• populations can overshoot (temporarily exceed) the
carrying capacity, which may result in a die-off event
The Logistic Growth Model
1. A small population initially
experiences exponential
growth.
2. As the population becomes
larger, however, resources
become scarcer, and the
growth rate slows.
3. When the population size
reaches the carrying capacity
of the environment, growth
stops. As a result, the pattern
of population growth follows
an S-shaped curve.
Doubling Time – The Rule of 70
• Rule of 70: divide 70 by the growth rate to obtain the
approximate number of years required for a population to double
EX: At a 10% annual growth rate, doubling time is 70 / 10 = 7 years
• Important facts:
• do not change % into a decimal – keep as a whole number
• the larger the rate of growth (r), the faster the doubling time
• rate of growth varies considerably among organisms (difference in growth
rate between bacteria and elephants)
• most populations cannot double forever (resource constraints and disease
contribute to leveling off, reaching K)
How are K-selected and r-selected species
different?
Key Question #2
K-Selected Species
• low intrinsic growth rate that causes the population
to increase slowly until it reaches carrying capacity
• long gestation periods lasting several months
• slow maturation/extended parental care
• long life spans
• inhabit relatively stable biological communities
• birds,larger mammals (elephants, horses, primates),
and larger plants
r-selected species
• high intrinsic growth rate, which often leads to
population overshoots and die-offs
• brief gestation periods
• many offspring at once, each of which has low
probability of surviving to adulthood
• exploit less-crowded ecological niches
• rodents, insects, weeds
Survivorship Curves
• survivorship curve: distinct
patterns of species survival as a
function of age
• 3 types:
• Type I - high survival throughout most of
life span, but then individuals start to die
in large numbers as they approach old age
• Type II - relatively constant decline in
survivorship throughout most of life span
• Type III - low survivorship early in life with
few individuals reaching adulthood
What Factors Affect Human Population
Growth?
Key Question #3
Population growth is important
• Every second, 4-5 children are born on earth
• In that same second, 2 other people die
• world population is about 7.6 billion people and
growing at 1.13% per year
• current US population is about 325 million
WORLD POPULATION CLOCK
Words you need to know
1. Demography - vital statistics about people, such as births, deaths,
where they live, total population size
2. Fecundity - physical ability to reproduce
3. Fertility - actual production of offspring
4. Mortality - death
5. Total fertility rate - number of children born to an average woman
during her entire reproductive life
6. Life expectancy - statistical measure of average time an organism is
expected to live, based on year of birth, current age, and other
demographic factors including sex
9. life span - oldest age to which a species is known to survive
10. immigration - movement of people into a country or region
11. emigration - movement of people out of a country or region
12. zero population growth (ZPG) - occurs when births + immigration just equal
deaths + emigration
13. crude birth rate (CBR) - number of births/1,000 individuals per year
14. crude death rate (CDR) - number of deaths/1,000 individuals per year
15. subsistence economy - non-monetary economy which relies on natural
resources to provide for basic needs, through hunting, gathering,
and subsistence agriculture
16. subsistence agriculture - self-sufficiency farming in which farmers focus on
growing enough food to feed their families
Four factors that affect population growth
1. Fertility varies among cultures and at
different times
2. Mortality offsets births
3. Life expectancy is rising worldwide
4. Living longer has profound social
implications
Fertility varies among cultures
Fertility rates have declined dramatically in every region of the
world except Africa over the past 50 years.
Worldwide fertility rates
What can be done to Extend Life Expectancy?
• improve urban sanitation and waste removal
• improve quality of and expand access to clean water
• form public health boards to detect illnesses/quarantine sick
• research causes/means of transmission of infectious diseases
• develop vaccines and antibiotics
• adopt workplace safety laws and limits on child labor
• promote nutrition through steps such as fortifying foods with
vitamins
Why do people have children?
• because there are pronatalist pressures, aspects in the
environment or culture that increase a person’s desire for
children
• children may be only source of support for elderly
parents in countries without social security
• children are valuable not only for future income but as
source of current income/help with chores
• society has a need to replace members who die or become
incapacitated
Education and income affect the desire for
children
• Highly developed countries
• higher education/personal freedom influence women to not have
children
• desire to spend time/money on other priorities limits number of
children
• Less-developed countries
• food/clothing is minimally expensive, so adding one more child is
negligible
Demographic Transition Can Lead to Stable
Population Size
• theory of demographic transition: as a country moves
from a subsistence economy to industrialization and
increased affluence it undergoes a predictable shift in
population growth
• developed country - relatively high levels of
industrialization and income
• developing country - relatively low levels of
industrialization and income
Phases of Demographic Transition
Phase 1 - preindustrial
period characterized by
high birth/death rates
Phase 2 - society begins to
industrialize, death rates
drop rapidly, but birth rates
do not change. Population
growth is greatest at this
point.
Phase 3 - birth rates
decline
Phase 4 - population stops
growing and sometimes
begins to decline as birth
rates drop below death
rates
Factors that help stabilize populations
• social reforms that promote equality among all
groups - reduce the need and desire for large families
• technology
• brings advances to developing world much more rapidly
• spurs change/development
• equal access to high quality health care
• improving women’s lives through better
education/job opportunities
Total fertility declines as women’s
education increases
Future Expectations
• world population is expected to stabilize
sometime during 21st century
• at that point, total number of humans is likely to
be somewhere around 10-11 billion
Age structure diagrams
• visual representations of number of individuals within
specific age groups for a country
• typically expressed for males and females
• population pyramid - age structure diagram that is
widest at bottom and smallest at top, typical of
developing countries
Resource Use
• to estimate the impact of human lifestyles
on Earth, the IPAT equation was developed
• Impact = population × affluence × technology
Per capita ecological footprints. Many countries exceed the global
average footprint of 2.7 ha per capita.
Resource Use
• A country's affluence often corresponds with its
resource use and level of impact
• Gross domestic product (GDP) - measure of
value of all products and services produced in
one year in one country
Local versus Global Impacts,
• scale of an impact depends on nature of economy
and degree to which society has developed
• highly localized impacts are typical of rural,
agriculturally-based societies
• global impacts are more common in affluent or urban
societies
• urban area - contains >385 people/km2 (1,000 people/m2)
Urban Impacts
Urban growth. More than one-half of the world’s population will live in urban settings by 2030.
Urban Impacts
Sustainable development is a common,
yet elusive, goal
• According to The Millennium Ecosystem Assessment:
1. Ecosystem sustainability will be threatened if human
population continues along its current path of resource
consumption
2. Continued alterations to ecosystems that have improved
human will also exacerbate poverty for some populations
3. If we establish sustainable practices, we may be able to
improve the standard of living for a large number of people
How do we sustain the
world population?
Key Question #4
Global Trends in Food Production
and Security
• Food production has been transformed from smallscale, diversified, family operations to expansive
farms of thousands of hectares
• Food security: having access to sufficient, safe, and
nutritious food that meets dietary needs for an active
and healthy life
Important Words Regarding Food
• Undernutrition - not enough calories are ingested to maintain health
• Malnourished - diet that lacks correct balance of proteins, carbohydrates,
vitamins, minerals
• Overnutrition - ingestion of too many calories and a lack of balance of foods
and nutrients
• Famine - condition in which food insecurity is so extreme that large numbers of
deaths occur in a given area over a relatively short period
• Obese - having too much body fat
• Overweight - weighing too much
What factors contribute to malnutrition?
• poverty
• political unrest
• poor governance
• high food prices
• other economic conditions
Examples of Malnourishment
• Iodine - essential mineral that body cannot
make but requires to produce thyroid
hormones
• found in iodized salt, dairy products,
seafood, meat, and eggs and is essential
for producing thyroid hormones
• lack of iodine leads to stunted growth
and reduced mental ability
• goiter: swelling of thyroid gland due to
lack of iodine
• ~40% of world population at risk
Examples of malnourishment
• Iron – helps body make red blood cells and transport oxygen to
tissues
• found in dark leafy greens, red meat, egg yolks
• low iron leads to anemia – blood disorder that causes fatigue,
weakness
• 30% of world population
• Vitamin A - crucial for eye, reproductive, and immune system
health
• found in eggs, green/orange/reddish yellow fruits and veggies
• leading cause of preventable blindness
Examples of Malnourishment
• Vitamin D: required for proper
absorption of phosphorous (K) and
calcium (Ca) from gastrointestinal
tract
• skin produces it when exposed to
sunlight; also found in fish oils, fatty
fish, egg yolks, fortified foods
• softening of bones due to lack of
vitamin D or imbalance of K and Ca is
called rickets
Overeating is a growing world problem
• causes of obese/overweight populations
• increasing world food supplies
• low prices on unhealthy items
• ease of access (fast food/convenience food)
• highly processed foods rich in
sugars/fats
Obesity in America
• 70% of adults are overweight
• 35.9% of adults are obese
• 20% of teens are obese
• 18% of kids ages 6-11 are obese
• 12% of kids ages 2-5 are obese
Grains (corn, rice, wheat)
• make up largest portion of human diet
• constitute 60% of human energy intake
• two kinds
• whole grain: contains entire grain kernel (whole-wheat flour, bulgur,
oatmeal, whole cornmeal, brown rice)
• refined grain: milled to remove outer kernel to give finer texture
and improve shelf life, but removes dietary fiber, iron, B vitamins
(white flour, white rice)
• most are enriched after processing by adding B vitamins and iron
back in, but not fiber
Global Grain Production
(a) Global grain production
grew rapidly from 1950 to
mid-1980s. Growth has
continued since then, but per
capita growth has slowed.
(b) World grain supply (days
of supply for everyone in the
world) has been declining
Meat/Poultry
• 2nd largest component of human diet
• consumption has grown worldwide
• concentrated, high-value source of protein, iron, fats, and
other nutrients
• in US livestock are raised in large scale, concentrated facilities
with up to a million animals per facility
• requires heavy use of antibiotics which cause severe air/water
pollution
• more than ½ of all antibiotics used in US are administered here
cun83198_0714.jpg
Seafood
• rich in nutrients and omega-3 or -6 fatty acids
• over-harvesting and habitat destruction threaten most
of the world’s wild fisheries
• many types of seafood contain large amounts of
mercury which is harmful in large quantities, especially
to a fetus
What processes exist to increase food
production?
Key Question #5
The Green Revolution
• began in 60s/70s
• period when the productivity of global agriculture increased
drastically due to new advances in hybrid plants, machines,
fertilizers, pesticides, herbicides
• high-yield crops were developed and introduced
• multiple cropping began to be used - field used to grow 2+ crops
through year, so constantly has something growing on it
• In 1940s, Norman Borlaug developed strain of wheat that could
resist diseases, was short (reduced wind damage), produced large
seed heads and high yields
Advantages to green revolution
• grow more crops on same amount of land with similar
amount of effort - reduces production and market costs
• improved resistance of crops to diseases
• expanded export of cash crops
• created massive job opportunities (industry/agriculture)
• less forest/natural land needed to be converted to
farmland to produce more food
• Fun fact: from 1961 to 2008, as the human population increased by 100%
and the production of food rose by 150%, the amount of forests and natural
land converted to farm only increased by 10%
Disadvantages to green revolution
• poor farmers could not afford expensive machinery and
fertilizer
• widespread unemployment among manual workers as
machines took over jobs
• increase in pollution and erosion due to widespread use
of chemical fertilizers and synthetic herbicides and
pesticides
• pest resistance to pesticides
• weed resistance to herbicides
Genetic engineering could have benefits and
costs
• Genetic engineering: splicing a gene from one
organism into the chromosome of another.
• Genetically modified organisms (GMOs):
organisms with entirely new genes, or new
organisms, called “transgenic” organisms
Genetic engineering is revolutionizing agriculture
Benefits of genetic engineering:
• Greater yield
• Greater food quality
• Reductions in pesticide use
• Reduction of world hunger by increased food
production
• Increased profits
Genetic Engineering
Concerns about genetic engineering:
• Safety for human consumption
• Effects on biodiversity
• Regulation of GMOs
Is genetic engineering safe?
• The greatest danger is the ecological effects if these
organisms spread into the native populations.
• There are social and economic implications of GMOs
• Will they help feed the world, or will they lead to a greater
consolidation of corporate power and economic disparity?
• Are GMO’s required if we hope to reduce malnutrition and
feed the world?
More Sustainable Fishing
• many countries around the world have developed fishery
management plans with international agreements
• Sustainable Fisheries Act (1996) - shifted fisheries
management from a focus on economic sustainability to a
species-sustainability approach
• Individual transferable quota (ITQ) - fishery management
program in which individual fishers are given a total allowable
catch of fish in a season that they can either catch or sell
Aquaculture
• farming aquatic organisms such as fish, shellfish, and seaweeds
• involves constructing an aquatic ecosystem
• usually requires keeping the organisms in enclosures
Pros of aquaculture
• creates jobs in community
• increases revenue on city, state and national level
• reduces seafood trade deficit
• helps feed a growing U.S. and world population
• encourages local investment
• places more emphasis on protecting coastal waters from pollution
• reduces fishing pressure on certain wild stocks if that species can be
produced through aquaculture rather than fished
Cons of Aquaculture
• conflicts with other water users (lobstermen, fishermen or migrating fish)
• puts excess pressure on wild stocks used to create high protein feed pellets
• amplifies and transfers disease and parasites to wild fish populations
• pollutes water systems with excess nutrients (fish feed & wastes), chemicals and
antibiotics
• compromises native gene pools if farmed fish and native species interbreed
• threatens livelihood of fishermen
• compromises the aesthetic beauty of coastline
• more susceptible to severe weather, predators, disease, global competition