Population

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Population Dynamics
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Characteristics of a Population
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Populations are dynamic and change in response
to environmental stress and/or change in
environmental conditions
Populations change due to
a. Size if many individuals, have competition
for resources
b. Density if grouped close together, more
possibility for spread of infection
c. Dispersion even or clumped
d. Age Distribution very young and very old
cant add to population
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What are the Limits to Growth?
Population growth defined as an increase in the
size of a population over time.
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Four variables that limit population growth
a. Measured in number of births in a year
per thousand.
b. Must take into consideration the biotic
potential of a population
(biotic potential -- Maximum reproductive
rate of an organism)
a. Declining mortality is the primary cause
of most population growth in last 300
years.
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b. Measured in number of deaths per
thousand persons in a given year.
3. Immigration
a. Developed regions expect 2 million
immigrants a year for next 50 years.
(Locals complain immigrants take away jobs
and overload social services. )
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4. Emigration
Carrying Capacity of Earth
The number of individuals of a given species
that can be sustained indefinitely in a given
space
Growing populations of orgs do NOT demonstrate
linear growth rather, curve is J-shaped and is
called exponential growth(page 97)

When carrying capacity is approached, curve
becomes S-shaped and is called logistic growth
this may show fluctuations above and below the
carrying capacity(page 99)
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• Logistic Growth (graphically displayed as an
  S-shaped curve) is where the rate of death and
  birth will be equal, so that populations will
  stop growing (number of individuals is
  maintained)
• Sections of curve beginning growth, rapid
  growth, leveling off, carrying capacity,
  fluctuations (see page 98)

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Curve can change depending upon the amount of
resources in the environment
(Regions with more resources can support
a larger carrying capacity than regions with
fewer resources) Carrying capacity is determined
by 2 factors
1. Environmental Resistance
(All of the limiting factors that act
together to limit the growth of a
population) Ex. Predation, competition
for resources, crowding, stress
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Stress and population effects

• Aggression
• Decrease in parental care
• Decreased fertility
• Decreased resistance to disease
• (documented experimentally in populations of rats
  and mice)
• Unfortunately, happens with people as well

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2. Biotic Potential
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Logistics Growth (S-curve)
Carrying capacity
Number of Yeast Cells
Time (hours)
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Environmental limits to population growth

• Limiting factors biotic or abiotic factors that
  determine whether or not an org can live in a
  particular environment
• (these also regulate the size of a
  population)
• 2 types of limiting factors
• 1. density-dependent factors
• 2. density-independent factors

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Density dependent

• Ex. Disease, competition, parasites, food
• Have an increasing effect as the population
  increases
• -Influenza epidemic of 1918
• -plant disease

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Density independent

• Most are abiotic factors temperature, storms,
  floods, drought, habitat disruption
• pollution
• Affect all populations regardless of density
• -flood kills all earthworms in field
• -cold winter wipes out mosquitoes

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Density-Dependent Limiting Factor
Growth of Aphids
Exponential growth
Peak population size
Rapid decline
Steady population size
Steady population size
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How has Our Population Grown and Changed?
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Demography

• Study of human population growth characteristics
• Looks at growth rate, age structure, geographic
  distribution
• Can tell if pop is growing by looking at the
  difference between the birth rate and the death
  rate
• In US, death rate is declining, life expentancy
  is increasing, fertility rate is decreasing

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1954-2004
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What is a Population Pyramid?
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Population Pyramid (aka age structure
diagrams)
1. Some population statistics are best
represented graphically.
2. One of the easiest to interpret are
population pyramids
b. Divide the population up into an equal
number of age classes (cohorts)
i. For human populations, these are often
5-year periods.
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c. Within each cohort, count the number of
females and males.
d. For each cohort, divide the number of
males and females into the total population
size.
e. Graph these numbers, with the vertical axis
representing cohort age, and the horizontal
axis representing the proportion of the
population held within each cohort.
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ii. The larger the population, the greater the
death rate.
b. When a population increases in size, births
decrease and/or deaths increase
ii. Interspecific
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World Population Growth
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Countries Ranked by Population 2004 Rank
Country Population 1.
China
1,298,847,624 2. India 1,065,070,607 3.
United States 293,027,571 4.
Indonesia 238,452,952 5. Brazil 184,101,109
6. Pakistan 159,196,336 7. Russia 143,782,338
8. Bangladesh 141,340,476 9.
Nigeria 137,253,133 10. Japan 127,333,002 No
te Data updated 4-30-2004 (Release notes).
Source U.S. Census Bureau, International Data
Base.
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China
India
United States
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