Transcript population size - Mrs. Brenner`s Biology

Chapter 5
Populations
Location of the ecosystem –
Aleutian Islands, Alaska
Estimated Population Size
Change in sea otter populations
around the Aleutian Islands
100000
80000
60000
40000
20000
0
1900
1920
1940
1960
Year
1980
2000
2020
WHY?
With your group, discuss as
many reasons that you can think
of that might be causing the sea
otter population decline
Change in sea otter populations
around the Aleutian Islands
Killer whale prey in the Aleutians
If you were a killer whale…
• With your partners, rank the order of the
three prey species that you would prefer to
eat. Give a reasoning for your ranking.
Great
Whales
Harbor
Seals
Steller Sea
Lions
Sea
Otters
1.0
Proportion of maximum
0.8
0.6
0.4
0.2
0.0
1950
1960
1970
1980
Year
1990
2000
So who cares if the otters
go extinct?
• With your partners, make a
prediction about what will happen
to the kelp forests of the Aleutian
Islands habitat if the sea otters
become extinct
What’s next?
• With your partners, sum
everything up. What will
eventually happen to all the
members of this ecosystem if the
sea otters become extinct?
5 – 1 How Populations Grow
• What characteristics are used to
describe a population?
• What factors affect population
size?
• What are exponential growth and
logistic growth?
Characteristics of Populations
• Three important characteristics of a
population are:
1. geographic distribution – The area
inhabited by a population
2. density - # of individuals per unit
area
3. growth rate – how fast the population
grows
Population Growth
• Three factors can affect
population size:
1. The # of births
2. The # of deaths
3. How fast the population numbers
grow
Exponential Growth
• Under ideal conditions with
unlimited resources, a population
will grow exponentially
• When the individuals in a
population reproduce at a
constant rate
Exponential Growth
Logistic Growth
• Obviously, the conditions for
exponential growth are rarely ever
met
• As resources become less
available, the growth of a
population slows or stops
Logistic Growth
• Occurs when a population’s
growth slows or stops following a
period of exponential growth
Q: Why does this happen?
A: Lack of resources, lack of space
Carrying Capacity
• The largest number of individuals
that a population can support
Q: What is the growth rate when a
population is at carrying
A: Zero
5 – 2 Limits to Growth
• What factors limit population
growth?
Limiting Factor
• A factor that causes population
growth to decrease
Ex.) Parasitism, disease, drought,
extreme weather
• A resource base that is limited
can also affect the long term
survival of a species
Density – dependent Factors
• Limiting factors that depend on population
size
• Density dependent factors become limiting
only when the population density reaches
a certain level
• These factors operate most strongly when
a population is large and dense
• Density dependent factors include:
Competition
Predation
Parasitism
Disease
Density – Independent Factors
• Affect all populations in similar
ways regardless of the population
• Ex.) unusual weather, natural
disasters, seasonal cycles,
human activities
Unusual Weather
Natural Disasters
Seasonal Cycles
Human Activities
Human Activities
Density – Independent Factors
• In response to such factors, many species
show a characteristic crash in population
size
• Environments are always changing, and
most populations can adapt to a certain
amount of change
• Populations often grow and shrink in
response to such changes
• Major upsets in an ecosystem can lead to
long term declines in certain populations
5 -3 Human Population Growth
• How has the size of the human
population changed over time?
• Why do population growth rates
differ in countries throughout the
world?
Historical Overview
• Like the populations of many other living
organisms, the size of the human
population tends to increase with time
• In the U.S. and other developed
countries, the current growth is low
• In some developing countries, the human
population is growing at a rate of 3
people per. second
• The human population is well on its way
to reaching 9 billion in your lifetime
Exponential Human Growth
• The following factors have caused
a rapid increase in population
growth:
Agriculture
Industry
Reliable food source
Shipping
Improved sanitation
Medicine
Technology
Patterns of Population Growth
Q: Why can’t the human population
keep growing exponentially
forever?
A: The resources on earth are
limited
Demography
• The scientific study of human
population
• Birthrates, death rates, and age
structure of a population help to
project why some countries have
a high growth rate while others
grow more slowly
The Demographic Transition
Age Structure
• Population growth depends, in
part, on how many people of
different ages make up a given
population
Age-structure diagrams
• (population profile) – show the
population of a country broken
down by gender and age group
World Population: 1950 - 2050
Year
Average Annual
Growth Rate (%)
Population
1950
1.47
2,555,360,972
1960
1.33
3,039,669,330
1962
2.19
3,136,556,092
1963
2.19
1970
2.07
3,708,067,105
1980
1.69
4,454,607,332
1990
1.58
5,275,407,789
2000
1.23
6,078,684,329
2010
1.06
6,812,009,338
2020
.87
7,515,218,898
2030
.68
8,127,227,506
2040
.54
8,646,671,023
2050
.43
9,078,850,714
3,206,072,286