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Exponential Growth
ie: Doubling
!   1 2 4 8 16 32
!   J-shaped curve
!   B > D, then
Limits to pop growth?
Nesting Sites: A Limiting Factor
Limits to pop growth
!   Resources, living space, nest sites, parasites, predators,
wastes etc.
!   Carrying Capacity (K) – Maximum number of
individuals of a population that a given environment
can sustain
!   Logistic growth
!   S-shaped curve
!   (G) = r (N)*(U)
!   U = proportion of resources not yet used
!   Can be stable
Logistic growth
limits to population growth
!   Rate of population decrease increases as population gets
!   Related to crowding and competition
!   Disease: more individuals = greater density. Proximity
increases, so rate of transmission increases
!   Predation: more individuals = fewer hiding places, so
predation increases
limits to population growth
!   Rate of population decrease is unrelated to population
!   Abiotic influences – fire, avalanche, tsunami, etc.
Limits to population growth
!   Example: A population of reindeer introduced to
St. Matthew island in 1944 peaked, then
collapsed by 1966
!   Food became scarce, then thousands of reindeer
starved to death during an unusually harsh winter
Overshoot and Crash
Survivorship Curves
!   A survivorship curve is an age-specific plot that
shows how many members of a cohort remain
alive over time
!   Three types are common in nature
!   Type I: Survivorship is high until late in life
!   Type II: Death rate varies little with age
!   Type III: Death rate peaks early in life
Survivorship curves
!   Shows how many members of
those born at the same time
remain alive over time
!   Type 1 – Death rate is low until
later in life.
Annual plants, large animals
with few offspring
!   Type 2 – Death rate is constant
through life.
Lizards, small mammals, birds
!   Type 3 – Death rate is high early
in life.
Organisms with lots of
offspring and little parental
Survivorship curves
Life History Strategies
!   A life history pattern shows how an organism
allocates its resources between growth,
maintenance, and reproduction over the course
of its lifetime
!   An r-selected species is adapted to conditions
that change rapidly and unpredictably – favors
traits that maximize number of offspring
!   A K-selected species is adapted to life in a
stable environment – favors traits that improve
offspring quality
r-Selected Species
!   Shorter development times
!   Early reproduction
!   Fewer breeding episodes
!   Many young per episode
!   Less parental investment
!   Higher mortality rate & shorter lifespan
K-Selected Species
!   Longer development
!   Later reproduction
!   More breeding episodes
!   Fewer young per episode
!   More parental investment
!   Lower mortality rate & longer lifespan
Human population growth
No limits to human
population growth?
!   Large brains allowed us to master a variety of skills:
start fires, build shelters, make clothing, manufacture
tools, and cooperate in hunts
!   Language ensured knowledge of such skills did not die
with the individual
!   The invention of agriculture about 11,000 years ago
provided a more dependable food supply
No limits to human
population growth?
!   Energy of fossil fuels led to mechanized agriculture and
to improved food distribution systems
!   Invention of chemical fertilizers and synthetic
pesticides increased agricultural productivity
!   Improvements in food safety, sanitation, and medicine
lowered death rates and shifted the survivorship curve
!   The population is currently 7 billion and the United
Nations estimates that it will reach 9 billion by 2050
Population growth: Is a
decrease in death rates the
whole story?
Demographic Transition Model
What is human K?
!   Birth rates declining, but
currently only in
developed nations
!   People living longer
!   More getting access to
health care, sanitation
Comparing Age Structure
!   Age structure affects a population’s growth rate
!   The world’s three most populous countries – China,
India, and the US – differ in age structure
!   More than one-third of the world population is in a
broad pre-reproductive base
!   Even if fertility rate declines to replacement level
worldwide, population will continue to increase
Figure 44-15 p804
Figure 44-15 p804
Figure 44-15 p804
Different consequences
!   Ecological footprint – amount of resources (square feet
of Earth’s surface) required to support a given
!   Developed world – 5-8 hectares per person
!   Developing world – 0.5-3 hectares per person
Table 44-2 p805
!   If every couple has 2 children, still take 60
years for
population growth to slow
!   Because nearly 2 billion still to reach reproductive age
!   Massive food shortages?
!   Fossil fuels?
!   Environmental issues?