ch. 5 populations. ch. 5 outline 5-1: how populations grow –characteristics of populations...
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Ch. 5 Populations
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Ch. 5 Outline• 5-1: How Populations Grow– Characteristics of Populations– Population Growth– Exponential Growth– Logistic Growth
• 5-2: Limits of Population Growth– Density-Dependent Factors– Density-Independent Factors
• 5-3: Human Population Growth– Patterns of Population Growth
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Characteristics of Populations• Geographic Distribution
• Density
• Growth Rate
• Age Structure
• Geographic Distribution (or range) is the area inhabited by a population
• Population density: the number of individuals per unit area
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Population Growth• Three factors can affect population
size:– The number of births– The number of deaths– The number of individuals leaving or
entering the population
• A population is growing when its birthrate is greater than its death rate.
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Population Growth• The movement of Individuals also
affects population size.– Immigration: The movement of
individuals to an area– Emigration: The movement of
individuals out of an area
• What might cause the movement of individuals either to or from and area?– Food Supply– Finding a mate
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Exponential Growth• Exponential growth: occurs when
individuals in a population reproduce at a constant rate– A population will grow exponentially
under ideal conditions:• No predation• Plenty of Food and Space
• The graph of a population growing exponentially:– J-shaped Curve
• Bacteria often grow exponentially
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Exponential Growth Curve
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Logistic Growth• Logistic Growth: occurs when a
population’s growth slows or stops following a period of exponential growth– Takes into account a limited amount of
resources
• Graph of Logistic Growth:– S-shaped Curve
• The largest number of individuals that a given environment can support is called the carrying capacity.
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Limits to Population Growth• What causes population size to
decrease?– Limiting Factors: a factor that causes
population growth to decrease
• Examples of Limiting Factors:– Competition– Predation– Parasitism– Drought (or other climate extremes)– Human Disturbances
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Density-Dependent Factors• A limiting factor that depends on
population size is called a density-dependent factor.– They only become limiting when the
population density reaches a certain level. (They don’t affect small, scattered populations)
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Examples of Density-Dependent Factors• Competition– When populations are crowded, organisms
begin competing for food, space, mates, sunlight, etc…
– Competition between two different species also exists leads to adaptations and natural selection (EVOLUTION!!!)
• Predation– The predator-prey relationship is one of the
best known mechanisms for population control.• Ex. Moose-wolf population near Lake Superior
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Examples of Density-Dependent Factors• Parasitism and Disease– Parasites take nourishment at the
expense of their host, often weakening them (leading to death).
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Density-Independent Factors• Density-Independent Factors: affect
all populations in similar ways, regardless of the population size.
• Examples:–Weather/seasonal changes– Natural disasters– Human Activities
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Historical Overview• For most of history, the human
population has grown very slowly
• About 500 years ago, agriculture and industry made life easier and the population growth rate increased.
• With the Industrial Revolution came improved medical care and increased sanitation, which lead to exponential human population growth.
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Patterns of Population Growth• The Earth cannot sustain exponential
population growth forever. It DOES have a carrying capacity. (Resources are limited!)
• Demography: the study of human populations– Attempts to explain how they will change over
time
• Birthrates, death rates, and age structures of populations help predict why some countries have high growth rates while others have low growth rates.
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Patterns of Population Growth• In the last century, Japan, the United
States, and much of Europe have slowed their population growth.–Why?• With improved nutrition, sanitation, and
medical care, the death rate was slowing and the birthrate was still high (cause an increase in the population). BUT, as societies become more modern, families are tending to have less children (as education level and standards of living increase).
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Patterns of Population Growth• However, the worldwide population
is still growing exponentially.
• Population growth also depends on age structure.– How many people of different ages
make up a population?
• Age-Structure Diagrams: Graph the number of people in different ages in a population– Ex: USA vs. Rwanda.