Chapter 5: PopulationsPopulation – 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:

BirthMortality (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 GrowthOccurs 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!

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1000000

2000000

3000000

4000000

5000000

6000000

7000000

8000000

9000000

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

Hours

Num

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Exponential Growth Chart

Bacterial division can be represented by an exponential growth chart

Logistic growthExponential growth cannot possibly continue for long.What might cause population growth to slow down or stop?

Birthrate decreasesDeath rate increases

Immigration decreasesEmigration increases

(or a combination)

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

What might cause a population to stop increasing?

What happens to organisms when resources are scarce, orpopulations are too large?

In 1911, 25 reindeer were introduced onone of the Pribilof Islands near Alaska.

By 1938, the population had grown to over2000 animals!

The reindeer ate mostly lichens, which grow back very slowly.

What happened to the reindeer?

By 1950There were only8 reindeer alive on the island

Carrying capacity- largest 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. climateb. food supplyc. 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 FactorsAny 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 east the competition• Drives evolution

A Density-Dependent Limiting Factor

Exponential growth

Steady population size

Peak population size

Rapid decline

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 bypopulation size.

Climatic change. orHuman activities

Most populationswill shrink in responseto the change, unlessthese events arerecurring or largein 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.

• 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)

• 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, 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 30-35% 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. Without any natural predators, the population 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.

• 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.

• 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 GrowthUntil 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.

Section 5-3

Human Population Growth

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 million200 million ... 1000 years to add 150 million400 million ... 1000 years to add 200 million500 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 million1.2 billion ... 20 years to add 200 million1.6 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 billion6 billion ... 12 years to add 1 billion7 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 aHigh birth rate : High death rate

To...

Low birth rate : Low death rate

How does a society do that?

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

These Age DistributionPyramids 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

The birth rate in developed countries is 1.6 children/womanThe birth rate in developing countries is 3.1 children/woman

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.

Why?

World Population Growth & Reading

• http://www.prb.org/Publications/Datasheets/2011/world-population-data-sheet/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 revealsthe entire planet.

The environmental movement used this photoas an example of the limitations of our resources.

“Spaceship Earth” photo

Garret Hardin’s essay, (p. 144)

“The tragedy of the Commons”,was used as an example to preserve our resources