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Populations
Objective: A4 - Analyze how populations & communities respond to
abiotic & biotic factors and recognize that long-term survival of a
species is dependent on changing resources that are limited
Review:
Population - a group of organisms, all of the same
species living in a specific area.
Density - # of organisms
3 important characteristics of a population are:
- distribution
- density
- growth rate
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Population density –
the number of individuals
per unit area
3 factors that can affect population:
- number of births
- number of deaths
- number of individuals that enter or
leave the population
Immigration – movement of individuals into an area
Emigration – movement of individuals out of an area
Population Growth
Population Growth of Houseflies
Population size
1 million
500,000
100
One year
● J-shaped growth curve
— illustrates exponential population growth
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Population Growth of Houseflies
Population size
1 million
500,000
100
One year
• Exponential growth – occurs when the individuals
in a population reproduce at a constant rate
(as a population gets larger, it also grows at a faster rate)
– No limits on growth
– Under ideal conditions with unlimited resources
– Could not occur indefinitely in nature
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• Logistic growth – occurs when a
population’s growth slows or stops
- as resources become less available,
following a period of exponential growth
- Limits on growth
- A S-shaped
curve illustrates
logistic growth
Exponential
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Carrying capacity
• Carrying capacity – largest number
of individuals of a population that a
given environment can support
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• Limiting factor – a factor that causes
population growth to decrease
– Any biotic or abiotic factor that restricts the
existence, numbers, reproduction, or
distribution of organisms
– Examples
• Lack of food
• Overcrowding
• Competition for resources
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FOOD
PREDATORS
DISEASE
Exponential growth
SPACE
Characteristics of Population Growth
Population
Carrying capacity
S curve
J curve
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Time
2 kinds of limiting factors:
• Density-dependent limiting factor
– Exp. competition, predation,
parasitism, disease
biotic
• Density-independent limiting factor
– Exp. Unusual weather, natural
disasters, seasonal cycles,
certain human activities
abiotic
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Resources
• Glenco Interactive Chalkboard 2003
• Prentice Hall Biology 2004
• www.mun.ca/biology/scarr/Logistic_growth.htm