populations ch.19. (19-1) understanding populations population: group of 1 species living in the...
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Populations
Ch.19
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(19-1) Understanding Populations
• Population: group of 1 species living in the same place at 1 time
• 3 characteristics:– Size– Density– Dispersion
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Population Size
• # of individuals in a given area
• Difficult to measure
• Estimates often taken
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Population Density
• How crowded a pop. is
• # of individuals per unit area– Ex: 30 people / classroom
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Population Dispersion
• Spatial distribution of individuals1. Clumped: resources are clumped
• Food, living space
2. Even: social interactions• As far away as possible
3. Random: independent location
• Seed dispersal
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Population Dynamics
• Pop.’s change over time due to:– Birth rates– Death (mortality) rates– Life expectancy rates– Age structure– Patterns of mortality
• Survivorship curves
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Age Structure
• Distribution of individuals among different ages in a pop.
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Patterns of Mortality
• Survivorship curves– Type I: death at old age
• Humans, elephants
– Type II: death at any age• Certain bird species
– Type III: death at young age• Oysters, salmon, many insects
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(19-2) Population Growth Rate
• # by which a pop.’s size changes over time
• Depends upon:– Birth rate– Death rate– Emigration– Immigration
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Calculations
• Adds to pop.: birth & immigration
• Subtracts from pop.: death & emigration
• Assume immigration = emigration
• Therefore:
birth rate – death rate = growth rate
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2 Models of Growth
1. Exponential Model
2. Logistic Model
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Exponential Model
• The larger the pop. gets, the faster it grows
• Limitation: only exists under rare conditions– Limiting Factor: condition that restricts
growth of a pop.
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Logistic Model
• Exponential model accounting for limiting factors
• Carrying capacity (K): # of individuals the environment can support for a period of time
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Population Regulation
• 2 types of limiting factors:1. Density-independent
• Reduction of pop. by same proportion no matter what size it is
– Ex: weather (floods, fires)
2. Density-dependent • The larger the pop., the larger the
proportion of reduction– Ex: resource limitations (food, nest sites)
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Population Fluctuations
• All pop.’s fluctuate in size
• Some linked to environmental changes
• Lynx & hare cycles
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Perils of Small Populations
• Vulnerable to extinction– Environmental disturbances (fires,
storms,disease)
• Inbreeding: mating with relatives– Fewer offspring– Decreased genetic variation
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(19-3) History Lesson
• Hunter-gatherer lifestyle– Little pop. growth
• Agricultural revolution– Domesticate animals & cultivate plants for
food– Human pop. grew faster
• Industrial revolution– More growth
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Today’s Growth
• Developed countries: 20% of world– USA, Japan, Germany, etc.
• Developing countries: ~80% of world– S. America, Asia, Africa, etc.– Poorer & growing
much faster
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Demographic Transition Model
• Industrial development causes economic & social progress that affects pop. growth rates