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5-1 How Populations Grow
Photo Credit: © Frans Lanting/Minden Pictures, Inc.
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2. Characteristics of Populations
What characteristics are used to describe a population?
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3. Characteristics of Populations
Three important characteristics of a population are its:
geographic distribution
population density
growth rate
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Population Growth
What factors affect population size?
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Population Growth
Population Growth
Three factors can affect population size:
the number of births
the number of deaths
the number of individuals that enter or leave the population
A population can grow when its birthrate is greater than its death rate.
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Population Growth
Immigration, the movement of individuals into an area, is another factor that can cause a population to grow. Emigration, the movement of individuals out of an area, can cause a population to decrease in size.
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Exponential Growth
What are exponential growth and logistic growth?
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Exponential Growth
Exponential Growth
Under ideal conditions with unlimited resources, a population will grow exponentially.
Exponential growth occurs when the individuals in a population reproduce at a constant rate.
The population becomes larger and larger until it approaches an infinitely large size.
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Exponential Growth
Exponential Growth
In the presence of unlimited resources and in the absence of predation and disease, a population will grow exponentially. Both hypothetical graphs show the characteristic J-shape of exponential population growth.
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Logistic Growth
Logistic Growth
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.
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Logistic growth is characterized by an S-shaped curve.
This graph shows the S-shaped curve of logistic growth. As resources become less available, the population growth rate slows or stops. The growth of this population has leveled off at its carrying capacity.
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Limiting Factors
What factors limit population growth?
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13. Density-Dependent Factors
Wolf and Moose Populations on Isle Royale
The relationship between moose and wolves on Isle Royale illustrates how predation can affect population growth. In this example, the moose population was also affected by changes in food supply, and the wolf population was also affected by disease.
Moose Wolves
Density-Dependent Activity
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5-1
Population density is the number of individuals
that are born each year.
per unit area.
that immigrate.
that emigrate.
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5-1
When the birthrate of a population exceeds its death rate, the population
decreases.
increases.
stays the same.
increases then decreases.
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5-1
An S-shaped curve on a graph of population growth is characteristic of
exponential growth.
logistic growth.
carrying capacity.
delayed growth.
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5-1
Exponential growth in a population slows down or stops as
resources become limited.
rate of immigration increases.
rate of emigration decreases.
birth rate increases.
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5-1
Exponential growth rate means that each new generation of a population
adds the same number of new individuals as the previous generation did.
increases at the same rate as the previous generation.
is the same size as the generation before.
increases by a varying amount.
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5-2 Limits to Growth
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Limiting Factors
What factors limit population growth?
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Limiting Factors
Limiting Factors
The primary productivity of an ecosystem can be reduced when there is an insufficient supply of a particular nutrient.
Ecologists call such substances limiting nutrients.
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Limiting Factors
A limiting nutrient is an example of a more general ecological concept: a limiting factor.
In the context of populations, a limiting factor is a factor that causes population growth to decrease.
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23. Density-Dependent Factors
A limiting factor that depends on population size is called a density-dependent limiting factor.
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24. Density-Dependent Factors
Density-dependent limiting factors include:
competition
predation
parasitism
disease
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25. Density-Dependent Factors
Density-dependent factors operate only when the population density reaches a certain level. These factors operate most strongly when a population is large and dense.
They do not affect small, scattered populations as greatly.
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26. Density-Dependent Factors
Competition
When populations become crowded, organisms compete for food, water space, sunlight and other essentials.
Competition among members of the same species is a density-dependent limiting factor.
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27. Density-Dependent Factors
Competition can also occur between members of different species.
This type of competition can lead to evolutionary change.
Over time, the species may evolve to occupy different niches.
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28. Density-Dependent Factors
Predation
Populations in nature are often controlled by predation.
The regulation of a population by predation takes place within a predator-prey relationship, one of the best-known mechanisms of population control.
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29. Density-Dependent Factors
Wolf and Moose Populations on Isle Royale
The relationship between moose and wolves on Isle Royale illustrates how predation can affect population growth. In this example, the moose population was also affected by changes in food supply, and the wolf population was also affected by disease.
Moose Wolves
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30. Density-Dependent Factors
Parasitism and Disease
Parasites can limit the growth of a population.
A parasite lives in or on another organism (the host) and consequently harms it.
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31. Density-Independent Factors
Density-independent limiting factors affect all populations in similar ways, regardless of the population size.
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32. Density-Independent Factors
Examples of density-independent limiting factors include:
unusual weather
natural disasters
seasonal cycles
certain human activities—such as damming rivers and clear-cutting forests
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5-2
A limiting factor that affects all populations in similar ways regardless of their size might be
drought.
disease.
predation.
crowding.
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5-2
Which of the following would be a limiting factor affecting the panda population of China?
programs that educate people about endangered species
capture of some pandas for placement in zoos
laws protecting habitat destruction
a disease that kills bamboo plants
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5-2
Density-dependent factors operate most strongly when a population is
large and dense.
large but sparse.
small and sparse.
small, but growing.
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5-2
Within a limited area, if the population of a predator increases, the population of its prey is likely to
increase.
decrease.
remain about the same.
become extinct.
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5-2
Which of the following is a density-independent factor affecting populations?
predation
disease
a destructive hurricane
parasites
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38. 5-3 Human Population Growth
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Historical Overview
How has the size of the human population changed over time?
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Historical Overview
Historical Overview
Like the populations of many other living organisms, the size of the human population tends to increase with time.
For most of human existence, the population grew slowly.
Limiting factors kept population sizes low.
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Historical Overview
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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Historical Overview
With these advances, the human population experienced exponential growth.
Human Population Growth
The size of the human population has increased over time. After a long, slow start, the worldwide population grew exponentially following improvements in medicine, sanitation, agriculture, energy use, and technology.
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43. 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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44. Patterns of Population Growth
Why do population growth rates differ in countries throughout the world?
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45. Patterns of Population Growth
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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46. Patterns of Population Growth
The Demographic Transition
Over the past century, population growth in the United States, Japan, and much of Europe has slowed dramatically.
According to demographers, these countries have completed the demographic transition, a dramatic change in birth and death rates.
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47. Patterns of Population Growth
The demographic transition has three stages.
In stage 1, there are high death rates and high birthrates.
In stage 2, the death rate drops, while the birthrate remains high. The population increases rapidly.
In stage 3, the birthrate decreases, causing population growth to slow.
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48. Patterns of Population Growth
The demographic transition is complete when the birthrate falls to meet the death rate, and population growth stops.
Birthrates, death rates, and the age structure of a population help predict the rate of population growth. Birthrates and death rates fall during the demographic transition. In Stage I, both the birthrate and death rate are high. During Stage II, the death rate drops while the birthrate remains high. Finally, in Stage III, the birthrate also decreases.
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49. Patterns of Population Growth
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.
Age-structure diagrams show the population of a country broken down by gender and age group.
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50. Patterns of Population Growth
U.S. Population
In the United States, 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.
80+
Males
Females
60–64
Age (years)
40–44
This graph shows the age structure of the U.S. population.
20–24
0–4
Percentage of Population
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51. Patterns of Population Growth
Rwandan Population
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.
80+
Males
Females
60–64
Age (years)
40–44
This graph shows the age structure of the Rwandan population.
20–24
0–4
Percentage of Population
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52. Future Population Growth
To predict human population growth, demographers must consider the age structure of each country, as well as 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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53. Future Population Growth
This table, based on actual and projected data from the U.S. Census Bureau, International Database, shows data on world population.
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54. Future Population Growth
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.
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5-3
The size of the human population began to increase exponentially after the
bubonic plague.
development of plowing and irrigation.
Industrial Revolution.
development of the first cities.
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5-3
Which of the following is NOT a potential limiting factor of human population growth?
famine
medicine
war
disease
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5-3
After the demographic transition is complete, a population
grows rapidly.
grows slowly.
begins a period of rapid decline.
stays about the same size as time passes.
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5-3
An age-structure diagram shows a breakdown of a population by
location and age group.
age group and gender.
birthrate and death rate.
age group and emigration rate.
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5-3
Since the mid-1960s, the average annual growth rate of the human population has
remained about the same.
failed to show a consistent pattern.
increased.
decreased.
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