Chapter 5: Populations
Population – a group of organisms belonging to
a single species that lives in a given area.
There are 3 main characteristics of any
population...
• Geographic distribution
– Area inhabited by a populations
• Density
– Number of individuals/unit area
• Growth rate
– How fast population grows
Range of mountain lion
Population Growth
There are four factors that influence population
size:
Birth
Mortality (death)
Emigration (leaving an area)
Immigration (entering an area)
When these factors are balanced, no growth occurs (positive or negative)
Question...
What factors would increase the size of a population?
What factors would decrease the size of a population?
What causes growth?
• Birth and Immigration...
• Abundance of resources
like food, shelter, mates
Decrease in population
• Mortality and Emigration...
–
–
–
Lack of resources.
Organisms must find new resources, or emigrate (move out)
May result in extinction of species
Exponential Growth
Occurs under ideal conditions... Unlimited resources or no predation
Bacteria reproduce on average every 20 minutes.
Starting with 1...
20 minutes – 2 bacteria
40 minutes – 4 bacteria
1 hour – 8 bacteria
2 hours – 64 bacteria
4 hours – 512 bacteria
1 day – 4,720,000,000,000,000,000,000 bacteria!
If this population growth went unchecked, bacteria
would cover the planet!
Bacterial division can be represented by an exponential growth chart
9000000
Exponential Growth Chart
Number of bacteria
8000000
7000000
6000000
5000000
4000000
3000000
2000000
1000000
0
1
3
5
7
9 11 13 15 17 19 21 23
Hours
Logistic growth
Exponential growth cannot possibly continue for long.
What might cause population growth to slow down or stop?
Birthrate decreases
Death rate increases
Immigration decreases
Emigration increases
(or a combination)
Concept Map
Section 5-1
Population
Growth
can be
Logistic
growth
Exponential
growth
characterized by
No limits on
growth
Unlimited
resources
represented by
Constant
growth rate
characterized by
Limits on
growth
J-shaped
curve
which cause a
Falling
growth rate
What might cause a population to stop increasing?
represented by
S-shaped
curve
What happens to organisms
when resources are scarce, or
populations are too large?
In 1911, 25 reindeer were introduced on
one of the Pribilof Islands near Alaska.
By 1938, the population had grown to over
2000 animals!
The reindeer ate mostly lichens,
which grow back very slowly.
What happened to the reindeer?
By 1950
There were only
8 reindeer alive
on the island
Carrying capacitylargest number of individuals
that a given environment can support.
5-2
Limits to Growth
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 a similar way,
regardless of the population size.
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
Now imagine another small island that has a population of 500 rabbits.
How would the same factors affect this population?
Which of the factors depend on population size?
Which factors do not depend on population size?
Limiting factors -
Factors that influence negative population growth
Some limiting factors:
• climate
• predators
• food availability
• parasites
• disease
• human influence
What kind of human influences might limit populations?
Density – Dependent Factors
Any factor that is influenced by the density of the population
1. Competition
•
•
•
•
When population become crowded, resources are used up at a faster rate
Occurs within a species as well among groups of species
Species that compete will change to ease the competition
Drives evolution
A Density-Dependent Limiting Factor
Exponential growth
Peak population size
Rapid decline
Steady population size
Steady population size
2. Predation
•
•
Predator-Prey relationship: regulation of population by predators
Most effective population control
3. Parasitism and Disease
•
Disease and parasites are very effective
Their ability to spread is directly related
to the density of the population
High density... Spreads faster with
Higher death rate
The cause of most great human declines in population
Bubonic plague of Europe – 1346-1350
China – 25 million (30% of population)
Europe – 50-60% of population
Spanish flu in 1918 killed 20-40 million
Density – Independent Factors
Factors not caused by
population size.
Climatic change. or
Human activities
Most populations
will shrink in
response
to the change, unless
these events are
recurring or large
in magnitude
Density-Independent factors:
Unusual weather, natural disasters, heavy rain, or drought
Human disturbance... Roads, clear-cutting, dams and invasive species
Invasive species
Causes the Extinction of native species
•
Burmese Python – Introduced into the Florida
everglades, this species has no natural predators, no
competition for resources, and can grow upwards of
20 feet in length. Today, there are an estimated 30,000
Burmese pythons feeding on endangered birds, deer
and alligators.
•
Asian Carp – Growing up to 4 feet long, and weighing
over 100 lbs, this fish has spread from fish farms
(during flooding) to the Mississippi River basin. They
leap out of the water, injuring fishermen. With no
natural predator, they out compete the native fish,
eating all the plankton. They may soon (or already) be
in the Great Lakes, the world’s largest freshwater
ecosystem, threatening a multibillion fishing industry.
Asian Carp Video
•
Kudzu – native to Japan, brought to U.S. in 1876 to be
used to inhibit soil erosion. It was fast growing and
spread across the U.S. as fast as 150,000 annually (can
grow up to 1 foot/day)
•
Invading the Everglades Video
•
Starling – 60 birds were released in Central Park in 1890 (Eugene
Schieffelin wanted to release every bird mentioned by Shakespeare
into North America). These birds roost in flocks up to a million,
devour tons of food in a day, and their droppings spread disease.
Many attempts have been tried to reduce their numbers (poison,
live wires, itching powder, bombs, etc.) without success. In 1960 a
flock of 10,000 starlings flew into an airliner, causing it to crash. 62
people were killed.
•
Asian Tiger Mosquito – Native to SE Asia, it is now believed to be the
most widely-distributed species in the world. Carries disease like
Dengue fever and West Nile, and feeds 24 hours a day (rather than
dawn and dusk like other mosquitoes)
•
Rabbit – Native to Europe and South Africa, the common rabbit
tends to overproduce, and has led to an unchecked expansion on to
every continent but Antarctica. In Australia, 24 rabbits were
introduced by an English farmer, Thomas Austin, who wanted them
to remind him of home and provide a bit of hunting. Within 10
years, the rabbit population had grown to numbers that “2 million
could be shot or trapped annually without a noticeable impact on
the population”. By 1900, the rabbits reflected an exponential
population growth, contributing to the erosion of soil across the
continent, millions in damage, and a significant factor of species
extinction.
•
Black rat – Originally from Asia, it was distributed by
Europeans around the world in ships. It thrives in rural and
urban environments and does so at the expense of many
other species. It is responsible for the extinction of
countless birds, reptiles, and other small vertebrates
throughout the world (May have contributed to the
destruction of the Easter Island community... ate seeds of
trees, no trees, no boats)
•
Asian Longhorned Beetle – Native to Asia and Japan, this
beetle tunnels and feeds under the bark of trees in its larval
stage, killing the tree. It spreads quickly and threatens 3035% of the trees on the Atlantic coast. It is also in California,
Ontario, and parts of Europe. The economic toll is estimated
in the tens to hundreds of billions of dollars.
•
Nile perch – From Africa, this species was introduced into
Lake Victoria in the 1950’s (largest lake in Africa, and
reservoir for the Nile River). Without any natural predators,
the population exploded in the 1980s, leading to the
extinction of hundreds of native species. It has destroyed
ecosystems since it eats primary consumers like fish,
crustaceans, insects, and zooplankton. This fish can grow up
to 2 meters in length and weigh over 200 kg.
• Snakehead Fish – Originally from Asia, these
fish have shark-like teeth, can grow over 3 feet
long, lay up to 75,000 eggs a year, and can
breathe and migrate on land, searching for
other bodies of water for up to 4 days at a
time. These fish inhabit U.S. lakes from Maine
to California, decimating native species.
Snakehead Video
• Cane Toad – Introduced purposefully as a
form of pest control, these toads ate beetles
that were destroying sugarcane. Native to
South and Central America, they were
introduced to Hawaii, the Philippines, and
Australia. They eat not only pests, but just
about any animal they can catch. They secrete
toxins that can kill any animal they come into
contact with, decimating any population that
tries to eat them. Cane Toad Video
• Zebra mussels – (like a clam) found in many
inland lakes and rivers, they reproduce
quickly on any surface, clogging water pipes
and intake screens, causing problems for
power plants and municipal water supplies.
5-3
Human Population Growth
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 7 billion
people, and it continues to grow, but at a slower rate.
Human Population Growth
Section 5-3
Turn back to the Growth chart on page 129 of your text
A personal way to think of population growth...
Year
10000 BC ...
5000 BC ...
1000 BC ...
0 .........
1000 .........
1500 .........
1800 .........
1830 .........
1850 .........
1900 .........
1927 .........
1959 .........
1975 .........
1987 .........
1999 .........
2011 .........
Human population
1 million
5 million ... 5000 years to add 5 million
50 million ... 4000 years to add 45 million
200 million ... 1000 years to add 150 million
400 million ... 1000 years to add 200 million
500 million ... 500 years to add 100 million (plague)
900 million ...500 years to add 400 million
1 billion (1000 million) took 30 yrs to add 100 million
1.2 billion
... 20 years to add 200 million
1.6 billion
2 billion
Mrs. Zalisko’s dad was 3 years old when the population
hit 2 billion (He is now 88 years old)
2.9 billion
Mrs. Zalisko was born (passes 3 billion in 1960)
4 billion
... 15 years to add 1 billion
5 billion
... 12 years to add 1 billion
6 billion
... 12 years to add 1 billion
7 billion
... 12 years to add 1 billion
Population Projections for 2012
We are projected to hit 8 billion in 2025 ........................ 14 years (13 years from now)
We are projected to hit 9 billion in 2043 ....................... 18 years (31 years from now)
We are projected to hit 10 billion in 2083 ..................... 40 years (71 years from now)
New York, U.S.
Beijing, China
Mumbai, India
Historical Overview
• For most of human history, we were hunter-gatherers,
regulated by the same limiting factors influencing other
animals.
– Harsh climate, inconsistent food supply, and
predation
•50% infant mortality rate as recently as 500 years ago.
•About 8,500 years ago – agriculture revolution
– Instead of nomads, humans grew crops and
domesticated animals, staying in one area
– Increased the food supply;
– Continued high birth rate, food supply more
consistent, and population increased
Patterns of Population Growth
Human populations will cease to grow exponentially because of lack of resources.
Today’s limiting factors: Disease, Famine, and War
Demography –
the study of human populations
Examine birthrates, death rates,
and age structure
Demographers try to predict
future trends in populations
Demographic Transition – When the demography of a population shifts
Example: Go from a
High birth rate : High death rate
To...
Low birth rate : Low death rate
An age structure diagram can tell us
trends...
by age group
by gender
by birth rate
by life expectancy
This tells us about the quality of life
How does a society do that?
These Age
Distribution
Pyramids show the
distribution of age
groups between a
developing country
and 2 developed
countries
What do these charts show?
There is a single factor that predicts a decline in the
birth rate in any country...
The education and economic independence of women
Why?
Educated women find that they do not need to have as many children to
ensure some survive. They learn family planning, contribute to the family
income, and spending less time bearing and caring for children.
The birth rate in developed countries is 1.6 children/woman
The birth rate in developing countries is 3.1 children/woman
World Population Growth & Reading
• http://www.prb.org/Publications/Datashee
ts/2011/world-population-datasheet/video-7-billion.aspx
Back in 1972, a photograph was taken of the Earth from Apollo 17
This is the only picture ever taken that reveals
the entire planet.
The environmental movement used this photo
as an example of the limitations of our resources.
Garret Hardin’s essay, (p. 144)
“The tragedy of the Commons”,
was used as an example to preserve our resources
“Spaceship Earth” photo