5-1 how populations grow. three important characteristics of a population are its: geographic...
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5-1 How Populations Grow
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Three important characteristics of a population are its:
Geographic Distribution Density Growth Rate
• Geographic distribution, or range, describes the area inhabited by a population.
• Population density is the number of individuals per unit area.
• Growth rate is the increase or decrease of the number of individuals in a population over time.
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What factors affect population size?o the number of births (natality)o the number of deaths (mortality)o the number of individuals that enter or leave
the population (immigration and emmigration)
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Example• A population of turtles was 10 in a certain
habitat. What would be the growth rate if every year 2 turtles emigrated out of the population, 5 turtles immigrated into the population, 10 were born each year and 5 died each year? Is the population increasing or decreasing? What would this population be in 10 years?
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What are exponential growth and logistic growth?
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Exponential Growth• Exponential growth occurs
when the individuals in a population reproduce at a constant rate. (J-curve)
• Under ideal conditions with unlimited resources, a population will grow exponentially.
• The population becomes larger and larger until it approaches an infinitely large size.
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Logistic Growth• In nature, exponential
growth does not continue in a population for very long.
• As resources become less available, the growth of a population slows or stops.
• Logistic growth occurs when a population's growth slows or stops following a period of exponential growth. (S – curve)
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Carrying Capacity• The largest number of
individuals of a population that a given environment can support is called its carrying capacity.
• When a population reaches the carrying capacity of its environment, its growth levels off. The average growth rate is zero.
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5-2 Limits to Growth
• A limiting factor is a factor that causes population growth to decrease:o Density-Dependent Factorso Density-Independent Factors
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Density-Dependent Factors• A limiting factor that
depends on population size.
• Affect large and dense populations most strongly (small, scattered populations not very affected).
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Competition Predation Parasitism and Disease
• When populations become crowded organisms compete for food, water, space, sunlight and other essentials.
• The regulation of a population by predation takes place within a predator-prey relationship, one of the best-known mechanisms of population control.
• A parasite lives in or on another organism (the host) and consequently harms it and possibly spreads disease.
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Density-Independent Factors• Affect all populations in
similar ways, regardless of the population size.
• Examples:o unusual weathero natural disasterso seasonal cycleso certain human activities
—such as damming rivers and clear-cutting forests
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5-3 Human Population GrowthHow has the size of the human population
changed over time?
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Historical Overview
• Like the populations of many other living organisms, the size of the human population tends to increase with time.
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• For most of human existence, the population grew slowly due to limiting factors (such as disease and food scarcity) which kept population sizes low.
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• About 500 years ago, the human population began growing more rapidly.
• Life was made easier and safer by advances in agriculture and industry.
• Death rates were dramatically reduced due to improved sanitation, medicine, and healthcare, while birthrates remained high.
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• With these advances, the human population experienced exponential growth.
Human Population Growth
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Patterns of Population Growth
• The scientific study of human populations is called demography.
• Demography examines the characteristics of human populations and attempts to explain how those populations will change over time.
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Why do population growth rates differ in countries throughout the world?
• Birthrates, death rates, and the age structure of a population help predict why some countries have high growth rates while other countries grow more slowly.
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The Demographic Transition• Over the past century, population growth in
the US, Japan, and much of Europe has slowed dramatically.
• These countries have completed the demographic transition: a dramatic change in birth and death rates.
Population SizeNatality (+) Mortality (-)
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The demographic transition has three stages.
• Stage 1: high death rates and high birthrates.
• Stage 2: death rate drops, while birthrate remains high. The population increases rapidly.
• Stage 3: birthrate decreases, causing population growth to slow.
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• The demographic transition is complete when the birthrate falls to meet the death rate, and population growth stops.
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Age Structure• Population growth
depends, in part, on how many people of different ages make up a given population.
• Demographers can predict future growth using models called age-structure diagrams, which show the population of a country broken down by gender and age group.
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• In the US, there are nearly equal numbers of people in each age group.
• This age structure diagram predicts a slow but steady growth rate for the near future.
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• In Rwanda, there are many more young children than teenagers, and many more teenagers than adults.
• This age structure diagram predicts a population that will double in about 30 years.
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Future Population Growth
• To predict human population growth, demographers must consider the age structure of each country, and the prevalence of life-threatening diseases.
• If growing countries move toward the demographic transition, growth rate may level off or decrease.
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• Ecologists suggest that if growth does not slow down, there could be serious damage to the environment and global economy.
• Economists assert that science, technology, and changes in society may control the negative impact of population growth.