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5–1 How Populations Grow

A. Characteristics of Populations

B. Population Growth

C. Exponential Growth

D. Logistic Growth

Section 5-1

Section Outline

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5-1 How Populations Grow

Population - group of organisms belonging to a single species that lives in a given area.

3 Characteristics of any Population

1. Geographic distribution – area inhabited by population

2. Density - # per unit of space

3. Growth rate – how fast it grows

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Population Growth

Four factors that influence population size

1. Birth – duh.

2. Mortality - death

3. Emigration – out of an area

4. Immigration – into an area

• When these factors are balanced = no growth (negative or positive)

• What factors would increase size? Decrease size?

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What environmental factors influence these?

•Birth and immigration?– Abundance of resources (food, mates, shelter)– Organisms reproduce and move into the area for the

resources•Mortality and emigration?

– Resources on the decline– Organisms faced with dilemmas

• Adapt to other resources• Death/extinction• Move out - emigrate

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Exponential Growth

•Bacteria reproduce on average every 20 minutes

•In a matter of 1day, one bacteria reproduce into….

– 4720000000000000000000 bacteria

– mass of bacteria size of earth in 3 days

•Represented with a “J-curve©”

•Only occurs under ideal conditions (unlimited resources/no predation)

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0

1000000

2000000

3000000

4000000

5000000

6000000

7000000

8000000

9000000

1 3 5 7 9 11 13 15 17 19 21 23

Hours

Num

ber

of b

acte

ria

Exponential Growth Chart

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Logistic Growth

•Normal growth curves•Populations that grow exponentially use resources exponentially!

– “carrying capacity”– The limit to which any population ceases to grow due to

depletion of resources• Run out of food• Nowhere to live

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Nu m

ber

of Y

east

Cel

l s

Time (hours)

Carrying capacity

Section 5-1

Figure 5-4 Logistic Growth of Yeast Population

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PopulationGrowth

can be

represented bycharacterized by characterized by represented by

which cause a

Exponentialgrowth

Logisticgrowth

Fallinggrowth rate

S-shapedcurve

Limits ongrowth

No limits on growth

J-shapedcurve

Constantgrowth rate

Unlimitedresources

Section 5-1

Concept Map

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Name That Resource!

A situation that causes the growth of a population to decrease is called a limiting factor. Some limiting factors depend on the size of the population. Other limiting factors affect all populations in similar ways, regardless of the population size.

Section 5-2

Interest Grabber

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1. Imagine a small island that has a population of five rabbits. How might each of the following factors affect the rabbit population?

a. climate

b. food supply

c. predation

2. Now imagine another small island that has a population of 500 rabbits. How would the same factors affect this population?

3 Which of the factors depend on population size? Which factors do not depend on population size?

Section 5-2

Interest Grabber continued

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5–2 Limits to Growth

A.Limiting Factors

B.Density-Dependent Factors

1.Competition

2.Predation

3.Parasitism and Disease

C. Density-Independent Factors

Section 5-2

Section Outline

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Limiting Factors

• Factors that influence negative population growth• can be anything from….

– Climate– Predators– Food availability– Parasites– Human influences

• Clearing of forests – destroy natural habitat

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Density-Dependent Factors

•Any factor that is influenced by the density of the population.

•Competition– When populations become crowded,

resources are used up at a faster rate.– Occurs within species as well as among

groups of species.– Species that compete will change to ease

the competition– Drives evolution

• Ex: plants and sunlight (canopies/black walnut)

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Growth of Aphids

Exponential growth

Steady populationsize

Peak populationsize

Rapid decline

Steady populationsize

Section 5-2

A Density-Dependent Limiting Factor

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•Predation– Predator-prey relationship – regulation of populations by

predators– Most effective population control “technique”– Ex: wolves and moose in Isle Royale– Graph usually fluctuates often over time

• Predator and prey charts are inverse of each other

60

50

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01955 1960 1965 1970 1975 1980 1985 1990 1995

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2400

Moose Wolves

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Parasitism and Disease

•Disease and parasites very effective– Ability to spread directly related

to density of population• Higher density = faster

spread = higher death rate– May be what eventually causes

human population decline• Ex: Bubonic plague

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Density – Independent Factors

•Factors not caused by population size– Climatic change– Human disturbances

• Damming rivers, clearing forests•Most populations will shrink in response

– Unless these events are recurring or large in magnitude

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How Fast Are We Growing?

Until about 500 years ago, the world’s human population remained fairly stable. Then, as advances in medicine, agriculture, and technology occurred, the human population began growing very rapidly. Today, the world’s human population is greater than 6 billion people, and it continues to grow, but at a slower rate.

Section 5-3

Interest Grabber

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1. The human population is increasing by about 1.4 percent each year. Assume that the population is 6 billion (6,000,000,000). How large will the population be in one year?

2. If the human population continues to grow at a rate of 1.4 percent per year, the population would double in size (to 12 billion people) in only 51 years! What effect might this increase in population have on the environment and on other people?

Section 5-3

Interest Grabber continued

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5–3 Human Population Growth

A.Historical Overview

B.Patterns of Population Growth

1.The Demographic Transition

2.Age Structure

C.Future Population Growth

Section 5-3

Section Outline

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Historical Overview

• Populations of humans were regulated by the same limiting factors influencing other animals.

– Harsh climate– Food supply– Predators

•50% infant mortality rate as soon as 500 years ago.•8500 years ago – agriculture

– Now instead of nomads, humans domesticated in one area– Increased the food supply– Coupled with high birth rate already, population increased

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Agriculturebegins

Plowingand irrigation

Bubonicplague

IndustrialRevolutionbegins

Section 5-3

Human Population Growth

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Patterns of Population Growth

•Human populations will cease to grow exponentially because of lack of resources.•Today’s limiting factors:

– Disease– Famine– War

•Demography – study of human populations•Examine:

– Birth rates– Death rates– Age structure

•Can predict future trends in growth or decline of population

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Demographic Transition

•When the demography shifts in a population– Go from high birth rate/high death rate to….– Low birth rate/low death rate

•How does a society do that?

•Occurred in the United States, Japan, and Europe•Africa, S. America, and Asia, still have exponential growth….world overall still growing exponentially.

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United States Demographic Transition

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U.S. Population Rwandan PopulationMales MalesFemales Females

Section 5-3

Figure 5-13 Age Distribution

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What Can Age Structure Diagrams Tell Us?

•Trends by….– Age group– Gender– Birth rate– Life Expectancy

•Overall quality of life

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Future Population Growth

•By 2050 population may increase to 9 billion•Growth rate will slow to 0.43% from 1.4% currently and 2.1% in the ’60’s.•Some feel this is very bad for the environment•Others feel technology and changes in society will allow us to cope.

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Links from the authors on the gray wolf

Interactive test

For links on populations, go to www.SciLinks.org and enter the Web Code as follows: cbn-2051.

For links on population growth, go to www.SciLinks.org and enter the Web Code as follows: cbn-2053.

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