UNIT 3: POPULATIONS
3.1 Generalist and Specialist Species
3.2 K-Selected and r-selected Species
3.3 Survivorship Curves
3.4 Carrying Capacity
3.5 Population Growth and Resource Availability
3.6 Age Structure Diagrams
3.7 Total Fertility Rate
3.8 Human Population Dynamics
3.9 Demographic Transition
GENERALIST VS.
SPECIALIST
• Generalists have broad niches: can survive
over a wide range of conditions and changes
in their environment (diet, climate, pH, light
levels, etc.)
• These species experience more
competition, but make use of whatever is
presented.
• Specialists have narrow niches: can only live in
a narrow range of conditions.
• These species thrive when conditions are
just right and are good competitors, but
when conditions change, they don’t adapt
easily.
• Specialist species tend to be
advantaged in habitats that
remain constant, while
generalists tend to be
advantaged in habitats that
are changing.
EXAMPLES OF
SPECIALISTS
• Orchid mantis – a colorful mantis with
appendages like leaves that thrives only on
orchids in the tropics
• Sword-billed hummingbird – has a beak
longer than its body and has co-evolved
with certain types of flowers its beak fits into
and has difficulty getting food elsewhere
• Koala – lives almost entirely on eucalyptus
which is toxic to most other animals
Orchid Mantis
Sword-billed
hummingbird
Koala
EXAMPLES OF
GENERALISTS
• Raccoon – has a wide diet, omnivore,
lives in any area with trees, brush, or
structures
• Rat – found everywhere in the world
except the Arctic; not picky eaters
• Horseshoe crab – wide diet on sea floor
bed, tolerates wide range of
temperatures, can survive in low oxygen
waters, and can ever be out of water for
extended periods of time
Raccoon
Rat
Horseshoe Crab
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POPULATION DYNAMICS
– DEPENDS ON THE
FOLLOWING FACTORS
1. Size (# of individuals)
2. Density (# of individuals in a certain space)
3. Dispersion (spatial pattern of individuals)
4. Age distributions (proportion of individuals
by age in a population)
• These may change in response to
environmental stresses and changes in
environmental conditions.
WHAT GOVERNS
POPULATION SIZE?
• Births
• Deaths
• Immigration - # of individuals moving into a
population
• Emigration - # of individuals leaving a
population
• Population change = (births + immigration) (deaths + emigration)
BIOTIC POTENTIAL
• All populations vary in their ability to grow.
• The intrinsic rate of increase or biotic
potential (r) is the rate at which a
population would grow if there were no
limits on resources.
• Organisms with high biotic potential tend
to:
• Reproduce early in life
• Have short generation times (time between
generations)
• Reproduce many times
• Have large numbers of offspring each time
LIMITS ON BIOTIC
POTENTIAL
• No population can grow indefinitely!
• There are limiting factors in the environment
which put a halt to population growth
(environment resistance).
• The actual population size is determined by
the relationship between the biotic
potential and the environmental resistance.
This is known as the carrying capacity (K) of
an organism (the number of individuals of a
species that can be sustained indefinitely in
a given location)
MINIMUM VIABLE
POPULATION (MVP)
• Typically the intrinsic rate of increase for a
population requires a certain minimum
population size be present.
• If not
• Individuals may not be able to find
mates
• Interbreeding may occur
• Genetic diversity may be too low to
allow for adaptations to environmental
changes.
EXPONENTIAL GROWTH
• Populations with few resource limitations in
the environment will grow exponentially.
• The population size starts to grow slowly at
first ant then grows faster as the
population increases.
• J shaped curve
RESOURCE AVAILABILITY
• Population growth is limited by environmental
factors, especially by the available resources
and space.
• Resource availability and the total resource
base are limited and finite over all scales of
time.
• When the resources needed by a population
for growth are abundant, population growth
usually accelerates.
• When the resource base of a population
shrinks, the increased potential for unequal
distribution of resources will ultimately result
increased mortality, decreased fecundity, or
both, resulting in population growth declining
to, or below, carrying capacity.
LOGISTIC GROWTH
CURVE
• There is exponential growth when the
population is small enough and hasn’t
encountered significant environmental
resistance, but the growth decelerates and
levels off as it reaches the carrying
capacity.
• S shaped curve.
EXCEEDING CARRYING
CAPACITY
• Sometimes a species will overshoot the
carrying capacity. Typically because of a
reproductive lag time.
• This results in a dieback or crash in the
population.
• Carrying capacity is NOT
a fixed value.
POPULATION GROWTH
AND DENSITY
• Density independent population controls control the population regardless of the
density of the population
• Floods, fires, hurricanes, habitat destruction
• Density dependent population controls - have
a greater affect on the population as the
density of the population increases
• Competition, predation, parasitism, disease
• Ex. Bubonic plague
High density
Low density
POPULATION
FLUCTUATIONS
• Stable - population usually remains about
the same
• Irruptive - population size is maintained,
but suddenly goes up and crashes back to
a slightly lower level
• Irregular - population size goes up and
down for no known reason
• Cyclic- population goes through regular
increases and crashes over time.
REPRODUCTIVE
STRATEGIES
• r-selected species or opportunists
• Reproduce rapidly when conditions are
favorable or when a disturbance opens up an
new opportunity
• Tend to be found in early stages of succession
and are more successful when there are
disturbances.
• K-selected species or competitors
• Do well in competitive situations when the
population size is near the carrying capacity
• Typically follow a logistic growth curve
• May be more prone to extinction
CHARACTERISTICS OF R
STRATEGISTS
• Many small offspring
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Small adults
Early age of reproduction
Short generation times
Little or no parental care
Short lived
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Most offspring die before reaching reproductive age
Population fluctuates wildly around K
Generalists
Not competitive
Adapted to changes in environmental conditions
Early successional species
High biotic potential
CHARACTERISTICS OF K
STRATEGISTS
• Fewer, larger offspring
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Larger adults
Later reproductive age
Longer generation times
More parental care
Longer lived
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Most offspring survive to reproductive age
Population fairly stable around K
Specialists
Competitive
Better suited to stable environments
Late succesional species
Low biotic potential
The availability of
suitable habitat for
individuals of a
population in a
particular area is what
determines its ultimate
population size.
SURVIVORSHIP CURVES - SHOW
THE NUMBER OF SURVIVORS OF
EACH AGE GROUP FOR A
PARTICULAR SPECIES
• Late loss curves - most organisms survive
through their reproductive years and die off
later. Examples: most K strategists.
• Early loss curves - most organisms die off
before their reproductive years. Examples:
most r strategists.
• Constant loss curves - organisms die off at a
fairly steady rate regardless of age. Examples:
songbirds, lizards, and small mammals.
LIFE TABLES
• Show the numbers of individuals at each age
from a survivorship curve.
• Show the probable life expectancy at each
age.
• Life insurance companies rely on these to set
prices.
HUMAN POPULATION
DYNAMICS
FACTORS AFFECTING
HUMAN POPULATION
SIZE
• Population change = (births+immigration) - (deaths
+ emigration)
• Birth rate (crude birth rate) - the number of live
births per 1000 people in a population per year.
• Death rate (crude death rate) - the number of
deaths per 1000 people in a population per year.
• Annual rate of natural population change (%) =
birth rate - death rate
10
• Rule of 70 (Doubling time) = 70 / (% increase)
DEFINITIONS OF FERTILITY
• Replacement level fertility - the number of
children a couple must have to replace
themselves.
• 2.1 in developed countries
• Up to 2.5 in developing countries
• Total fertility rate - an estimate of the
average number of children a woman will
have during her reproductive years.
TFR
• This is affected by the age at which
females have their first child, educational
opportunities for females, access to family
planning, and government acts and
policies.
• If the fertility rate is at replacement levels,
then a population is considered to be fairly
stable.
• Factors associated with infant mortality
rates include whether mothers have access
to good healthcare and nutrition and
changes in these factors can lead to
changes in infant mortality rates over time.
FACTORS AFFECTING BIRTH
AND FERTILITY RATES
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Importance of children to the workforce
Urbanization
Cost of raising and educating children
Educational and employment opportunites for
women
Infant mortality rate
Average age of marriage
Availability of pensions
Availability of legal abortions and birth control
methods
Religious beliefs, traditions, and cultures
FACTORS AFFECTING
DEATH RATES
• Availability of food
• Nutrition
• Public health
• Sanitation and personal hygiene
• Safety of water supplies
INDICATORS OF HEALTH
• Life expectancy - average number of years
a newborn can expect to live
• Life expectancy has increased in developed
and many developing countries
• Life expectancy is dropping in the world’s
poorest nations
• Infant mortality rate - number of babies out
of every 1000 born who die before their first
birthday
• Single most important indicator of a society’s
quality of life
• High infant mortality indicates
• Undernutrition
• Malnutrition
• Infectious disease
AGE STRUCTURE DIAGRAMS - SHOW
THE PROPORTION OF A POPULATION BY SEX
AT EACH AGE LEVEL
• Age Categories
• Prereproductive
• Reproductive
• Postreproductive
• Types of Pyramids
• Rapid growth
• Guatemala, Nigeria, Saudi Arabia
• Slow growth
• U.S., Canada, Australia
• Zero growth
• Spain, Austria, Greece
• Negative growth
• Germany, Bulgaria, Sweeden
POPULATION PYRAMIDS
PROS AND CONS OF
REDUCING BIRTHS
• Opponents of slowing growth:
• Life expectancy is longer than at any
time in history
• The world can still support billions more
people
• We are the world’s most valuable
resource
• Proponents of slowing growth:
• If we do nothing, will cause more deaths
• Cause more environmental harm
DEMOGRAPHIC TRANSITION - AS
COUNTRIES BECOME MORE
INDUSTRIALIZED, FIRST THEIR DEATH RATES,
AND THEN BIRTH RATES DECLINE
• Preindustrial stage
• Little population growth because
conditions are harsh and birth rates and
death rates are high
• Transitional stage
• Industrialization begins, food production
and health care improve so death rates
drop, but birth rates remain high
• Industrial stage
• Birth rates begin to drop and population
growth slows
• Postindustrial stage
• Birth rates continue to decline until they
equal or become less than death rates
DEMOGRAPHIC TRAP
• Rapid growth rate in developing countries
could cause them to become stuck in
stage 2 as rising population surpasses the
ability of the country to provide for
adequate healthcare and outweighs any
economic benefits.
• May cause countries to revert to stage 1.
• Also resources available to today’s
developing nations are not the same as
those once available.
WHAT CAN HELP REDUCE BIRTH
RATES AND POPULATION SIZE?
• Family planning programs
• Empowering women
• Economic incentives
• Improved health care
• Access to education
• Eradicating poverty
• Greater involvement of men in childrearing and birth control
• Reduction of unsustainable use of
resources
CASE STUDIES
THAILAND
• Adopted policy in 1971 to reduce population
growth. (Growth rate 3.2% and avg. of 6.4 children
by 2002 growth rate 0.8% and 1.8 kids)
• What they tried to do?
• Creative family planning methods supported by the
government
• High literacy and advances for women
• Better health care for women and children
• Openness of Thai people for new ideas
• Support of family planning by religious leaders
• Still room for improvement
• High levels of air pollution
• Need to improve public health
UNITED STATES
• Population is growing faster than any other developed
nation (1.2%)
• Higher birth rate than death rate accounts for 60% of
increase, but also large immigration rate.
• Birth rates decreased between 1910 to 1930 as
industrialization took hold and women became more
important in the work force. Remained low through
the 30’s.
• Rose sharply between 1946 and 1964 - baby boom.
• Declined from 1965 to 1976 and then has increased
again.
• High infant mortality rates in U.S. compared to other
developed nations
• Inadequate health care for poor women and their
infants
• Drug addiction
UNITED STATES
POPULATION PYRAMIDS
CHINA
• Between 1972 and 2002, TFR has been cut from 5.7
to 1.8 children
• Forced to do something or face mass starvation
• Has the most strict population control program in
the world
• Couples urged to postpone age of marriage and only
have one child
• Married couples have easy access to birth control
methods and abortions
• Married couples who pledge to have no more than
one child receive benefits such as extra food, free
medical care, but breaking the pledge forfeits the
benefits.
• Population should begin declining in 2042 (amend the
proposals then?)
CHINA
POPULATION PYRAMIDS
2100
Projected for 2050
INDIA
• World’s first national family planning program
in 1952, but in 2002 still second most populous
country.
• Big problems with poverty, malnutrition, and
environmental conditions.
• Half the labor force is unemployed or has only
occasional work
• 53% of children suffer from malnutrition
• Has 16% of world’s pop, but only 2.3% of land and
2% of forests
• Half of cropland is degraded
• 70% of water is seriously polluted
• Still have an average of 3.2 children per woman.
Most couples believe that they need many
children to do work and care for them in old age.
COMPARING INDIA’S
GROWTH TO CHINA’S
AFRICA
• Has most of the world’s poorest countries with
an average life expectancy of only 55 years.
• Serious hunger and malnutrition in infants and
children, but losing high number of young
adults due to AIDS
• HIV/AIDS epidemic is ravaging Africa (6000
people a day die).
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Life expectancy may drop to 35 or 40 years
High number of orphans (40 million by 2010)
Loss of labor force
Drop in food production
• Reduce spread of HIV
• Education
• Health care
AFRICAN NATIONS
• Zimbabwe: from 65 to 39 years
• Bostwana: 62 to 40
• Republic of Congo: 57 to 47
• Congo: 54 to 49
• Ethiopia: 51 to 41
• Ivory Coast: 57 to 46
• Kenya: 66 to 48
• Namibia: 65 to 42
• Nigeria: 58 to 54
• Rwanda: 54 to 42
• South Africa: 65 to 56
• Swaziland: 58 to 39
• Tanzania: 55 to 46