Ecology: Populations

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EcologyPopulations
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Characteristics of Populations

1. Geographic distribution
2. Density
3. Growth Rate
4. Age Structure

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Geographic Distribution

• Also called Range
• Describes the area inhabited by a population

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Population Density

• The number of individuals per unit area can vary
  tremendously depending on the species its
  ecosystem

How many Cacti are there per square kilometer in
this desert?
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Population Growth

• Three factors that effect population growth
• Number of births
• Number of deaths
• Number of individuals that enter or leave the
  population

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Examples

• What happens if the number of births are greater
  than deaths?
• What happens if the number of deaths are greater
  than the number of births?
• Which situation will allow for population growth?

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Movement within populations

• Immigration - The movement of individuals INTO an
  area
• Emigration - The movement of individuals OUT OF
  an area
• Which of these will allow a population to grow?

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Exponential Growth

• Occurs when the individuals in a population
  reproduce at a constant rate
• Under ideal conditions with unlimited resources,
  a population will grow exponentially

J-shaped curve
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Logistic Growth

• Occurs when a populations growth slows or stops
  following a period of exponential growth
• This happens as resources become less available
  causing the growth of a population to slow or
  stop.

S-shaped curve
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X number of individualst timeK carrying
capacity
Carrying Capacity - The largest number of
individuals that a given environment can support
K
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Limiting Factors

• A factor that causes population growth to
  decrease such as predation, competition, climate
  extremes or human disturbances
• Two Types of Limiting Factors
• Density-dependent
• Density-independent

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Density-dependent

• Limiting only when the population density reaches
  a certain level
• Biotic factors include competition, predation,
  parasitism, disease
• Operates most strongly when a population becomes
  very large and dense

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Competion

• When organisms are crowded this causes more
  competition
• Examples space, food, water, sunlight
• This competition can be between different species
  which may lead to evolution to occupy different
  niches

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Predation

• Predator-prey relationships are one of the best
  mechanisms of population control

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Parasitism and disease

• Parasites take nourishment at the expense of the
  host
• Usually weakens but can cause death
• Similar to predation

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Density-Independent

• Affect all populations in similar ways regardless
  of the population size
• Abiotic factors include natural disasters,
  seasonal cycles, unusual weather
• Can lead to a characteristic crash
• Most populations can adapt to some change
• Some changes can effect entire populations with
  major upsets leading to long-term declines

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Human Population Growth

• Size of human population tends to increase with
  time
• Human population growth was very slow due to
  harsh conditions and limiting factors
• High death rates for years

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Why did population growth increase?

• Life got easier with improvements in industry and
  agriculture
• Safer food supply
• Improved sanitation
• Improved health care
• Decrease in death rate while birth rate had no
  change
• Exponential Growth

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Demography

• Scientific study of human populations
• Birthrates, death rates, and the age structure
  help predict countries growth rates

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Demographic Transition

• The hypothesis as to why population growth has
  slowed dramatically in countries such as US,
  Japan, and Europe
• A dramatic change in birth and death rates
• Historically human societies have had both high
  birth and death rates

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Age Structure Diagrams

• Help predict future population growth
• Bar graph of the number of people in each age
  group in the population

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• United States has a slow but steady growth rate
• What is the shape of Rwandas age structure
  diagram?
• Indicates that there more young children than
  teenagers and more teenagers than adults.
• Predicts a population that will double in approx.
  30 years

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Future Population Growth

• Estimated human population by 2050 will be more
  than 9 billion.
• Human population is still growing exponentially.
  
• Will it level out to logistic growth?
• How might that happen?