Chapter Intro-page 90

What You’ll Learn

You will explain how populations grow.

You will identify factors that inhibit the growth of populations.

You will summarize issues in human population growth.

• Compare and contrast exponential and linear population growth.

Section Objectives:

• Relate the reproductive patterns of different populations of organisms to models of population growth.

• Predict effects of environmental factors on population growth.

• A population is a group of organisms, all of the same species, that live in a specific area.

• A healthy population will grow and die at a steady rate unless it runs out of food or space, or is attacked in some way by disease or predators.

Principles of Population GrowthPrinciples of Population Growth

• Scientists study changes in populations in a variety of ways.

• One method involves introducing organisms into an environment that contains abundant resources and then watching how the organisms react.


• Studies of populations of larger organisms, such as an elk population in a national park, require methods such as the use of radio monitors.


• The growth of populations is unlike the growth of pay you get from a job.

• Populations of organisms, do not experience linear growth. Rather, the graph of a growing population starts out slowly, then begins to resemble a J-shaped curve.

How fast do populations grow?How fast do populations grow?

How fast do populations grow?How fast do populations grow?Population Growth of Houseflies

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• The initial increase in the number of organisms is slow because the number of reproducing individuals is small.

• Soon, however, the rate of population growth increases because the total number of individuals that are able to reproduce has increased.

How fast do populations grow?How fast do populations grow?

• A J-shaped growth curve illustrates exponential population growth.

• Exponential growth means that as a population gets larger, it also grows at a faster rate.

Is growth unlimited?Is growth unlimited?

• Exponential growth results in unchecked growth.

• Limiting factors, such as availability of food, disease, predators, or lack of space, will cause population growth to slow.

• Under these pressures, the population may stabilize in an S-shaped growth curve.

What can limit growth?What can limit growth?

What can limit growth?What can limit growth?Characteristics of Population Growth

Exponential growth

Carrying capacity

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• The number of organisms of one species that an environment can support indefinitely is its carrying capacity.

• When a population overshoots the carrying capacity, then limiting factors may come into effect.

Carrying capacityCarrying capacity

Click image to view movie.

Carrying capacityCarrying capacity

• Deaths begin to exceed births and the population falls below carrying capacity.

Carrying capacity

• Biologists study the factor that determines population growth—an organism’s reproductive pattern, also called its life-history pattern.

Reproduction PatternsReproduction Patterns• In nature, animal and plant populations

change in size.

• A variety of population growth patterns are possible in nature.

• Rapid life-history organisms have a small body size, mature rapidly, reproduce early, and have a short life span.

Rapid life-history patternsRapid life-history patterns• Rapid life-history patterns are common among

organisms from changeable or unpredictable environments.

Slow life-history patternsSlow life-history patterns

• Large species that live in more stable environments usually have slow life-history patterns.

• Slow life-history organisms reproduce and mature slowly, and are long-lived. They maintain population sizes at or near carrying capacity.

Slow life-history patternsSlow life-history patterns

• Three patterns of dispersal are random, clumped, and uniform.

Density factors and population growthDensity factors and population growth• How organisms are dispersed can be

important.

Random Clumped Uniform

• Population density describes the number of individuals in a given area.

Density factors and population growthDensity factors and population growth

• Ecologists have identified two kinds of limiting factors that are related to dispersal: density-dependent and density-independent factors.

• Disease, for example, can spread more quickly in a population with members that live close together.

• Density-dependent factors include disease, competition, predators, parasites, and food.


• Most density-independent factors are abiotic factors, such as temperature, storms, floods, drought, and major habitat disruption.

• Density-independent factors can affect all populations, regardless of their density.


Organism Interactions Limit Population SizeOrganism Interactions Limit Population Size

• Population sizes are limited not only by abiotic factors, but also are controlled by various interactions among organisms that share a community.

Predation affects population sizePredation affects population size• When a predator consumes prey on a large

enough scale, it can have a drastic effect on the size of the prey population.

• Populations of predators and their prey are known to experience cycles or changes in their numbers over periods of time.

Predation affects population sizePredation affects population size• The data in this graph reflect the number of

hare and lynx pelts sold to the Hudson’s Bay Company in northern Canada from 1845 through 1935.

Lynx and Hare Pelts Sold to the Hudson’s Bay Company

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• In field studies, predation increases the chance that resources will be available for the remaining individuals in a prey population.

Predation affects population sizePredation affects population size

Competition within a populationCompetition within a population

• Competition is a density-dependent factor.

• When only a few individuals compete for resources, no problem arises.

• When a population increases to the point at which demand for resources exceeds the supply, the population size decreases.

The effects of crowding and stressThe effects of crowding and stress

• When populations of certain organisms become crowded, individuals may exhibit symptoms of stress.

• As populations increase in size in environments that cannot support increased numbers, individual animals can exhibit a variety of stress symptoms.

• These include aggression, decrease in parental care, decreased fertility, and decreased resistance to disease.

• They become limiting factors for growth and keep populations below carrying capacity.

The effects of crowding and stressThe effects of crowding and stress

Question 1 Exponential growth means that as a

population gets larger, it also _____.

D. stabilizes in an S-shaped growth curve

C. grows at a steady rate

B. grows at a faster rate

A. grows at a slower rate

The answer is B. A J-shaped growth curve illustrates exponential growth.

Population Growth of Houseflies1 million

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Question 2 Which of the following would you expect to

observe after a population exceeds its carrying capacity?

D. population growth rate is unaffected by limiting factors

C. deaths exceed births

B. births exceed deaths

A. population increases exponentially

The answer is C. Limiting factors may come into effect after a population exceeds its carrying capacity. Deaths begin to exceed births and the population falls below carrying capacity.

Characteristics of Population GrowthExponential

growth

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Question 3Offspring per IndividualOrganism Life Span

mosquito 250 1 monthelephanthumansoak tree

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D. stabilizes in an S-shaped growth curve

C. grows at a steady rate

B. grows at a faster rate

A. grows at a slower rate

The answer is A. Rapid life-history organisms have a small body size, mature rapidly, reproduce early, and have a short life span.

Offspring per IndividualOrganism Life Spanmosquito 250 1 monthelephanthumansoak tree

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Question 4 The number of organisms of one species that

an environment can support indefinitely is its _____.

D. carrying capacity

C. demographic

B. growth rate

A. life-history pattern

The answer is D. If population size rises above the carrying capacity, more organisms die than are born and the population drops back below the carrying capacity.

Carrying capacity

Question 5 Compare the terms “density-dependent factors”

and “density-independent factors”.

Both are limiting factors for organisms. Density-dependent factors have an increasing effect as the population increases and include disease, competition, parasites, and food. Density-independent factors can affect all populations regardless of density. Most are abiotic factors such as temperature, rainfall, and major habitat destruction.

• Identify how the birthrate and death rate affect the rate at which a population changes.

Section Objectives:

• Compare the age structure of rapidly growing, slow-growing, and no-growth countries.

• Explain the relationship between a population and the environment.

• In the United States, a census is taken every ten years.

World Population

• One of the most useful pieces of data is the rate at which each country’s population is growing or declining.

• These figures are the basis for demography, the study of human population size, density and distribution, movement, and its birth and death rates.

• Human population growth is different because humans have the ability to change their environment.

Human population growth

• People live longer and are able to produce offspring that live long enough to produce offspring, hence, a population grows.

• There are a number of factors that determine population growth rate.

Calculating growth rate

• These are births, deaths, immigration and emigration.

• Birthrate is the number of live births per 1000 population in a given year.

• Death rate is the number of deaths per 1000 population in a given year.

• Movement of individuals into a population is immigration.


• Movement out of a population is emigration.

• Birthrate – Death rate = Population Growth Rate (PGR)

• If the birth rate of a population equals its death rate, then the population growth rate is zero.


• If the PGR is above zero, more new individuals are entering the population than are leaving, so the population is growing.

• A PGR can also be less than zero.


• Another quantitative factor that demographers look at is the doubling time of a population.

• Doubling time is the time needed for a population to double in size.

• The time it takes for a population to double varies depending on the current population and growth rate.

Doubling time

• Doubling time can be calculated for the world, a country, or even a small region, such as a city.

Doubling time

Age structurePopulation Distribution Per Age Range for Several Countries

Age

Stable growth Rapid growth Slow growth

Male

Female

Reproductive years

Population (percent of total for each country)

• The needs of populations differ greatly throughout the world.

• Sometimes, a population grows more rapidly than the available resources can handle.

Ecology and growth

Ecology and growth• Resources that are needed for life, such as food

and water, become scarce or contaminated.

• The amount of waste produced by a population becomes difficult to dispose of properly.

• These conditions can lead to stress on current resources and contribute to the spread of diseases that affect the stability of human populations both now and to come.

Ecology and growth

What is the study of human population size, density and distribution, movement, and birth and death rates called?

Question 1

D. biodiversity

C. phylogeny

B. demography

A. ecology

The answer is B. When various demographic data are monitored, societies are able to improve environmental conditions and quality of life.

Question 2

D. Declining at a decreasing rate each year

C. Growing at a decreasing rate each year

B. Declining at a greater rate each year

A. Growing at a greater rate each year

Year Birthrate Death rate123

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The answer is C. In each of these years, the population growth rate is above zero, but is decreasing.

Year Birthrate Death rate123

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170190370

Which interval in the diagram below represents the population reaching equilibrium near carrying capacity?

Question 3

D. 4

C. 3

B. 2

A. 1

The answer is D. The number of organisms tends to rise above and fall below the carrying capacity due to limiting factors.

• Populations of some organisms do not exhibit linear growth. If there is nothing to stop or slow growth, a population’s growth appears as a J-shaped curve on a graph.

• Populations grow slowly at first, then more rapidly as more and more individuals begin to reproduce.

Population Dynamics

• Under normal conditions, with limiting factors, populations show an S-shaped curve as they approach the carrying capacity of the environment where they live.

Population Dynamics

• If a population overshoots the environment’s carrying capacity, deaths exceed births and the total population falls below the environment’s carrying capacity. The number of individuals will fluctuate above and below the carrying capacity.

Population Dynamics

• Density-dependent factors and density-independent factors affect population growth. Density-dependent factors include disease, competition for space, water, and food supply. Density-independent factors are volcanic eruptions and changes in climate that result in catastrophic incidents such as floods, drought, hurricanes, or tornadoes.

Population Dynamics

• Demography is the study of population characteristics such as growth rate, age structure, and movement of individuals.

Human Population

• Birthrate, death rate, immigration, emigration, doubling time, and age structures differ considerably among different countries. There are uneven population growth patterns throughout the world.

Question 1

The answer is B. The graph of exponential growth is a J-shaped curve.

Question 2

What shape of age structure graph represents a rapidly growing population?

D. square

C. circle

B. thin rectangle

A. steep triangle

The answer is A.

Population Distribution Per Age Range for Several Countries

Age

Stable growth Rapid growth Slow growth

Male

Female

Reproductive years

Population (percent of total for each country)


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Question 3

What type of growth is shown in this graph?

D. equilibrium

C. exponential

B. slowly decreasing

A. slowly increasing

The answer is C. Exponential growth is rapid and is represented on a graph by a J-shaped curve.


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growth

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Question 4

D. 9 years

C. 5 years

B. 4 years

A. 2 years

Assume that each time interval on the graph is equal to one year. How long did it take this population to reach carrying capacity?

The answer is D. After 9 years, this population has nearly reached carrying capacity.


growth

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Question 5

Which of the following is characteristic of a species having a slow life-history pattern?

D. mature rapidly

C. small body size

B. long life span

A. short life span

The answer is B. Rapid life-history organisms have a small body size, short life span, and mature rapidly.

Question 6

During which time period was population growth the most rapid?

B. 1930 to 1960A. 1800 to 1930

Question 6

During which time period was population growth the most rapid?

D. 1975 to 1987C. 1960 to 1975

The answer is D. World population grew by 1 billion in just 12 years.

Question 7

If the birthrate is 125 and the death rate is 135, what is the population growth rate?

D. -10

C. 10

B. -260

A. 260

The answer is D. Use the formula:

Birthrate – Death rate = Population Growth Rate

• Corbis

• Carolina Biological Supply Co.

• Digital Stock

• Matt Meadows

• PhotoDisc

• Alton Biggs

Photo CreditsPhoto Credits

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Chapter Intro-page 90