PopulationsChapter 8

• Chapter 26 begins with the Mystery of Easter Island. At one time, Easter Island was forested, and supported a large population of humans who carved huge stone statues. Today the island has no forests, is sparsely populated, and the people have no memory of the culture that created the statues. What possible connections are there between the disappearance of the forests and the disappearance of the statue-carving culture?

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Why study populations?

• In the field, populations tend to be the unit of study. A population is a natural grouping, so studying populations reflects what is going on in nature.

• Even so – it’s not always easy to define a population!

Population Science• Studying growth rates of

populations helps us understand:

• the effects of rapid overpopulation.

• how population growth is regulated.

• We can derive important lessons for humans from studies of populations in nature.

General Principles• A population consists of

members of the same species living in the same ecosystem at the same time.

• Total population increases or decreases according to the number of births, deaths, immigration, and emigration that occurs.

Distribution

• Individuals distribute themselves in a population in three general patterns:

• Clumped

• Uniform

• Random

DistributionClumped

distribution is typical of organisms

that move in groups

(herds, flocks, etc.), or that

cluster around

resources, such as

plants near a water source.

DistributionUniform

distribution is typical where resources are

scarce. Individuals compete to

claim enough territory to

support them and keep a

distance from others.

DistributionRandom

distribution is rare. Organisms may distribute

randomly if resources are abundant and the organisms

do not form social groups.

Trees in a diverse forest may distribute

randomly.

Male marine iguanas are highly territorial. They also compete for females. Male iguanas tend to be

distributed uniformly throughout their territory. Why?

1 2 3

5% 5%

90%1. They live in social groups.

2. Each male has its own distinct breeding territory.

3. The iguana’s resources are localized.

Which pattern of distribution do human populations tend to show?

1 2 3

71%

17%12%

1. Clumped2. Uniform3. Random

Growth Rate

• To determine the actual change in numbers of a population in a given unit of time, we look at the difference between losses (deaths and emigration) and additions (births and immigration)

• (births - deaths) + (immigrants - emigrants) = change in population size.

What is the change in a population over a ten-year period if in that time there are 9,000 births,

2,000 deaths, 800 immigrants, and 400 emigrants?

1 2 3 4

0%

95%

3%3%

1. 12202. 66003. 70004. 7400

Growth Rate• If we want to know the

rate (r) at which a population is increasing, we need to know:

• Birth rate (b) = number of births in a population during a certain time period.

• Example: 150 births in a gull population of 1000 = 150/1000 = 0.15 per year.

Falklands Conservation

Growth Rate

• We also need to know:

• Death rate (d) = number of deaths in the same time period.

• Example: 50 deaths in a gull population of 1000 = 50/1000 = 0.05 per year.

Falklands Conservation

Growth Rate

• Growth rate (r) = birth rate – death rate

• r = b – d

• Ex: 0.15 – 0.05 = 0.1 (10% per year)

minus

=

percent increase

Growth Rate

• If we want to know the actual number of individuals by which the population increased, we use this formula:

• G = r x N

• G = 0.1 x 1000 = an increase of 100 individuals per year.

• Try this:

• You are studying a population of 30 ferns. This year you saw six new fern plants become established, and 3 fern plants died. Calculate the growth rate of the population.

• Remember:r = b – dG = r x N

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What is the annual growth rate of a population of 10,000 sea turtles if there are

500 deaths and 1,500 births per year?

1 2 3 4

3% 0%6%

91%

1. 5%2. 10%3. 15%4. 20%

• Suppose your eccentric uncle says that for your birthday, he will give you your choice between two presents:

• Choice A: $1,000,000 on your birthday.

• Choice B: A penny on your birthday, two pennies the next day, four the next, and so on for 30 days.

• Which would you take? Why?

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• Choice A yields $ 1,000,000

• Choice B yields $10,737,418.23 (Why? See: http://mathforum.org/dr/math/faq/faq.doubling.pennies.html)

• What happened? Why did Choice B give you so much more money?

• What does this have to do with population growth?

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Exponential GrowthBiotic potential = Maximum growth

possible.

This assumes a maximized birth and minimized death rate

Calculated as:

r = b - d

G = r x N

What happens to r and G when b gets big and d gets

little?

Exponential Growth

Exponential growth produces a J-shaped population graph.

Exponential Growth

Age of first reproduction affects the rate of

population growth. Why?

Exponential Growth

Death rates and average lifespan also affects growth rate. Why?

• Under what conditions can exponential growth occur in nature?

• You’ve probably guessed that exponential growth can’t go on forever. What factors limit population growth?

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Population Limits• Two opposing forces act on

population growth.

Biotic Biotic potentiapotentia

ll

EnvironmentaEnvironmental resistancel resistance

Environmental Resistance

• Species introduced to a new environment may experience exponential growth.

• Environmental resistance will eventually limit growth.

• Some populations experience “boom and bust” cycles.

• Others stabilize and show logistic growth.

Population Limits

The upper limit for population growth is determined by the carrying capacity of the environment.

K = Carrying Capacity

:# births

= # deaths

Population Limits

Where there are many natural controls, populations tend to demonstrate logistic growth.

Available space limits

barnacle populations.

If a population overshoots the carrying capacity of the environment, the result is a population crash.

Population Limits

Population Limits

Where there are few natural controls, a population may rise rapidly, exceed carrying

capacity, then crash as most of the population starves.

Which population is most likely to experience exponential

growth?

1 2 3

33% 33%33%1. Algae introduced

into a small pond in North Dakota.

2. A migrating herd of pronghorn antelope in Eastern Oregon.

3. Chinook salmon in the Columbia River.

Cyanobacteria

population boom

• In July growth conditions for cyanobacteria become

favorable

• By early September the nutrient supply has been

depleted and competition for what is left is fierce. Most cyanobacteria can’t get

enough and die.

• The population grows rapidly

How can this be represented graphically?

How can the cyanobacteria example be represented

graphically?

1 2 3

33% 33%33%1. _

2. _

3. _

time

nu

mb

er

What caused the cyanobacteria to crash was environmental

resistance that was:

1 2

50%50%

1. Dependent on the density of the population.

2. Independent of the density of the population.

• This graph shows human population over the last 14,000 years. What kind of curve is this? What implications does this have for humans?

Date

Technical andcultural advances

Industrial andmedicaladvances

Agricultural advances

123

Billions Time to addeach billion(years)All of humanhistory

1312131433

1804

201219991987197419601927

1

234567*

*projected bubo

nic

plag

ue

billi

ons

of p

eopl

e

2012*

1830

1975

1960

1930

1987

19992006

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Population Limits

A growing population

may become self-limiting.

In fruit flies, reproduction rate drops in response to

crowded conditions.

offs

prin

g pe

r da

y lifespan

population density

days

Population LimitsDensity-

independent factors limit populations

regardless of how large or small the

population is. Usually abiotic.

Examples: Seasonal weather

changesNatural disasters

Pollution

Population Limits

Density-dependent factors affect a population more strongly the

larger it grows. Usually biotic.

Examples: PredationParasitesDisease

Resource competition

Which of these is a density-dependent factor?

1 2 3 4

5% 3%

87%

5%

1. Harsh, cold winters with lots of snow and ice.

2. A sudden tornado.

3. An outbreak of cholera in a refugee camp.

4. A violent earthquake.

A population of Bluebirds is displaced when a new housing development destroys the meadow where they nested. They

move to another meadow where other nine male bluebirds live. The males compete intensely for nesting sites. At the end of the

season, there are still only nine successful males. Competition for nesting sites is a:

1 2

28%

72%

1. Density-dependent factor

2. Density-independent factor

A squirrel population is isolated on the Capitol grounds in Salem. Heavy traffic on all sides makes it

hard for squirrels to leave the grounds. Squirrel fatalities happen as squirrels try to cross the streets. Is traffic a density-dependent or density-independent

factor for these squirrels?

1 2

50%50%

1. Density-dependent factor.

2. Density-independent factor.

• For each of these scenarios, list both density-independent and density-dependent factors that could be involved.

• During a drought, a thick stand of young pine trees is attacked by pine bark beetles.

• A large herd of deer is caught by a winter storm that buries much of their food supply. Several of the deer, suffering from parasites as well as lack of food, are caught and killed by wolves.

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Survivorship• Populations show different patterns

in survival at different ages, which in turn can affect population growth.

• Early loss – many young die

• Constant loss – equal loss at all ages

• Late loss – high survivor of young, most deaths in old age

Survivorship curves very for different species, depending on their reproductive

strategy.

15 - 29

(a) Developed countries

0 - 14

75 and older

60 - 74

45 - 59

30 - 44

15 - 29

0 - 14

75 and older

60 - 74

45 - 59

30 - 44

age

femalemaleag

e

femalemale

(b) Developing countries

postreproductive (45–79 yr)

prereproductive (0–14 yr)

reproductive (15–44 yr)

millions of people

millions of people

2025 20502006

Different survivorship curves can have different consequences for populations, even of the same

species.

Birth rates do slow down as nations become more industrialized. However, the world population is not

evenly developed, and in developed nations, resource consumption per capita is high.

• Based on what you have learned in Chapter 26, how can you explain the disappearance of the ancient, statue-carving culture on Easter Island?

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Deer and Wolves

• Using the worksheets provided, calculate the population change in the deer population for each year.

• Population changes = births – deaths

• In this case, deaths are due to both starvation and predation.

Year Wolves Deer Deer offspring

Predation Starvation Deer population

change

1997 10 2,000 800 400 100 300

1998 12 2,300 920 480 240 200

1999 16 2,500 1,000 640 500 -140

2000 22 2,360 944 880 180 -116

2001 28 2,224 996 1,120 26 -150

2002 24 2,094 836 960 2 -126

2003 21 1,968 788 840 0 -52

2004 18 1,916 766 720 0 46

2005 19 1,952 780 760 0 20

2006 19 1,972 790 760 0 30

Total Deer and Wolf Populations by Year

0

500

1,000

1,500

2,000

2,500

3,000

1997 1998 1999 2000 2001 2002 2003 2004 2005 2006

Year

Nu

mb

er o

f D

eer

0

5

10

15

20

25

30

Nu

mb

er o

f W

olv

es

Deer Wolves

exponentialgrowth

populationcrash

Without wolves:

bean weevils (prey)

A high predator population

reduces the prey population

The prey populationpeaks when the

predator populationis low

braconid wasp (predator)

How does this graph relate to your deer/wolf graph?

(a) Predators often kill weakened prey

On the back of your graph:

1. Describe what happened to deer and wolf populations between 1997 and 2006.

2. What might have happened if wolves had NOT been introduced to the island?

3. Some people think it was cruel to introduce wolves. Some think it would have been cruel NOT to. Is there another management plan that would have been as good or better?

Recap• Population size changes through

birth, death, immigration, and emigration.

• Population size is regulated by environmental restraints that increase deaths or decrease births.

• Populations are distributed in various patterns for social reasons or because of resource availability.