Ch. 46 - Part 2 Regulation of Population Size
Regulation of Population Size
• The study of population dynamics
– Focuses on the complex interactions
between biotic and abiotic factors
that cause variation in population
size
Population Change & Population Density
• In density-independent populations
– Birth rate and death rate do not
change with population density
• In density-dependent populations
– Birth rates fall and death rates rise
with population density
•
•
•
•
Density-independent factors
Population limiting factors whose
occurrence is not affected by population
density
Abiotic factors such as climate and
weather, fire, physical disruption of
habitat
Limit population size well before
resources or other density-dependent
factors become important
Can cause rapid crashes in populations
Density-independent Effect
Impact of flash flood not affected by density:
3/5 die in low-density area = 60%
12/20 die in high-density area = 60%
Density-dependent factors
• Population limiting factors whose
effects depend on population density
• As the number of individuals increases,
so does the percentage of individuals
affected
• Generally are biotic factors like:
- Competition
- Predation
- Parasitism
Competition:
• Intraspecific competition is competition
between members of the same species
for resources that are limited in supply
(food, shelter, mates)
- Populations usually compete for
resources unless population is held
below carrying capacity by other
factors
• Interspecific competition is competition
between members of different species
Density-dependent Effects - Competition
When density is low (a) every bird has access to
nesting hole
When density is high (b) there is competition & not
all birds gain a nest hole
Competition for Resources
• In crowded populations, increasing population
density intensifies intraspecific competition for
resources
4.0
3.8
Average clutch size
Average number of seeds per
reproducing individual (log scale)
10,000
1,000
100
3.6
3.4
3.2
3.0
2.8
0
0
10
100
Seeds planted per m2
(a) Plantain. The number of seeds produced by
plantain (Plantago major) decreases as density
increases.
0
10
20
30
40
50
60
Density of females
(b) Song sparrow. Clutch size in the song sparrow on
Mandarte Island, British Columbia, decreases as density
increases and food is in short supply.
70
80
Density-dependent Effect on Reindeer
Individuals in a large
population have a
smaller share of the
limited food supply.
Reindeer on St. Paul
Island, Alaska
underwent a sharp
decline as a result of
overgrazing of the
land
Territoriality
• In many vertebrates and some invertebrates
– Territoriality may limit density
– Cheetahs mark their territories with chemicals
• Oceanic birds
– Exhibit territoriality in nesting behavior
Health
• Population density can influence the
health and survival of organisms
• In dense populations pathogens and
disease can spread more rapidly
Predation
• As a prey population builds up predators
may feed preferentially on that species
• The major limiting factor for predator
populations is the availability of its prey
• Predation is probably a significant factor
for the prey population
Density-dependent Effects - Predation
Harder to find prey at low densities
Easier to find prey at high densities
• Most populations are probably
regulated by a mixture of densityindependent and density dependent
factors
• Many populations are fairly stable and
near carrying capacity that and are
regulated by density-dependent factors
• Many others show short-term
fluctuations due to density-independent
factors
Life History Patterns
Populations vary on factors such as:
• number of births per reproduction
• age of reproduction
• life span of individuals
• probability of living entire life span
• These factors are part of a species life
history.
- There are trade-offs to be made
regarding how much energy to use
for which purposes
• Natural selection shapes the life
histories of species
Two Types of Life History Patterns:
• K-selection, or density-dependent
selection
– Selects for life history traits that are
sensitive to population density
• r-selection, or density-independent
selection
– Selects for life history traits that
maximize reproduction
Opportunistic life history (r-strategists)
• Small bodied species, reproduce when
young, produce many offspring, populations
tend to grow exponentially when conditions
are favorable
• Such populations typically live in
unpredictable environments and are
controlled by density-independent factors
• Emphasis on quantity of reproduction rather
than on individual survivorship or quality
• Exhibit type III survivorship curves
• Use a tactic called r-selection
• Examples: bacteria, insects, rodents, annual
plants
Equilibrium life history (K-strategists)
• Larger bodied species, produce few
slowly maturing offspring but provide care
for their young
• Population size stable, near carrying
capacity, held there by density-dependent
factors
• Natural selection resulted in production of
better-adapted offspring
• Exhibit type I survivorship curve
• Use tactic called K-selection
• Examples: Saguaro cacti, oaks, pines,
birds of prey, large mammals, humans
Life History Strategies
Human Population Growth
• Human population has gone through a
period of rapid exponential growth
• Went around “the bend” of the J-shaped
curve around 1650.
- It had taken millions of years to reach
that point.
• Once around the bend, the time it takes
to double the population (doubling time)
shrinks rapidly. Now about 53 years.
• No population can grow indefinitely
– And humans are no exception
The Global Human Population
6
4
3
2
The Plague
1
0
8000
B.C.
4000
B.C.
3000
B.C.
2000
B.C.
1000
B.C.
0
1000
A.D.
2000
A.D.
Human population (billions)
5
Though the global population is still growing
– The rate of growth began to slow approximately
40 years ago
2.2
2
Percent increase
1.8
1.6
2003
1.4
1.2
1
0.8
0.6
0.4
0.2
0
1950
Figure 52.23
1975
2000
Year
2025
2050
Human Population Milestones
Pop. Size
(Billions)
1
Years
1804
Time Elapsed since
previous billion
~ 4 million years
2
1927
123
3
1959
32
4
1974
14 years, 9 months
5
1987
13 years, 3 months
6
1999
12 years, 3 months
Predicted 7
2012
12 years, 4 months
Predicted 8
2025
13 years
Predicted 9
2040
15 years
World Population Growth
Country Development:
• More-Developed Countries (MDCs)
– Slow population growth (0.1% overall) (0.6% in
United States due to high immigration levels)
– High standard of living
– North America, Europe,Japan, Australia
– Doubled populations between 1850 & 1950.
Due to:
1. Decline in death rate due to modern
medicine & improved socioeconomic
conditions.
2. Decline in birthrates shortly thereafter.
This is sequence of events is called the
demographic transition.
Country Development;
• Less-Developed Countries (LDCs)
– Rapid population growth; current rate is
1.6%
– Low standard of living
– Latin America, Africa and Asia
– Continuing increases in population due to:
1. Steep decline in death rate after WWII
due to importation of modern medicine
2. Demographic transition occurred later
- 82% of world’s population lives in Africa, Asia
& Latin America
Infant Mortality and Life Expectancy
• Infant mortality and life expectancy at birth
– Vary widely among developed and developing
countries but do not capture the wide range of the
human condition
60
80
60
40
Life expectancy (years)
Infant mortality (deaths per 1,000 births)
50
30
20
20
10
0
0
Developed
countries
Figure 52.26
40
Developing
countries
Developed
countries
Developing
countries
Age Distributions
• A country’s age structure can be used to predict
future population growth.
• LDC’s have larger population growth because
they have more women entering the reproductive
years than older women leaving them.
• Zero Population Growth:
If each couple has two children to replace
themselves, eventually the human population will
level off but NOT immediately. Why?
• Replacement Reproduction:
The greater number of women entering their
reproductive years compared to older women
leaving them, will cause most countries to
continue to grow for at least one or two
generations.
Age Structure Diagrams (2002)
Age Structure Pyramids
Male
Decrease
Italy
Slow growth
United States
Rapid growth Afghanistan
Male
Age
Female
Female
85
80–84
75–79
70–74
65–69
60–64
55–59
50–54
45–49
40–44
35–39
30–34
25–29
20–24
15–19
10–14
5–9
0–4
8
6 4 2 0 2 4 6
Percent of population
Figure 52.25
8
Male
Age
Female
85
80–84
75–79
70–74
65–69
60–64
55–59
50–54
45–49
40–44
35–39
30–34
25–29
20–24
15–19
10–14
5–9
0–4
8
6 4 2 0 2 4 6
Percent of population
8
8
6 4 2 0 2 4 6
Percent of population
8
Environmental Impact
• Environmental impact of a population is
measured in terms of:
1. Population size
2. Resource consumption per capita
3. Resultant pollution caused by
population size and increased resource
consumption
Thus, there are two types of overpopulation:
1. Due to population growth (obvious in
LDCs)
2. Due to resource consumption (obvious in
MDCs)
Average person in US = 30 in India
• Population growth is putting extreme
pressure on each country’s social
organization, the Earth’s resources,
and the biosphere
• MDCs consume a much larger
proportion of the Earth’s resources
than do LDCs
Environmental Impact Caused by MDCs & LDCs
U.S.Population Statistics
• 281,000,000 (April 2000);
• 309,000,000 (March 2010)
– Most people are Caucasian (66%)
– Hispanic (15%) - 60% from Mexico
– Black (12%)
– Asian (4%) - ~36% live in CA
– Mixed (2%)
– Native Americans (1%)
• 40% of population under 18 belong to a
minority group - minorities increasing
Average Density of People in U.S. &
Diamond Bar, CA
• U.S. Density = 73 people/mi2
• Diamond Bar area = 14.9 mi2 or 38.8 km2
• Population = 58,763 people
• D.B. Density = 3,944 people/mi2 or
1,515 people/km2
Mumbai, India - World’s Most Densely
Populated City
• 14.35 million people/484 km2
• Density Mumbai = 29,650 people/km2
• Let’s get the Diamond Bar equivalent:
29,650 people/km2 (density-Mumbai) X 38.8 km2
(D.B.) would be 1,150,420 people in DB
• Others:
– Shenzhen, China (5th) - 466 km2 / 8 million
people (17,150 people/km2)
– Seoul, South Korea - 16,700 people/km2
– Taipei, Taiwan - 15,200 people/km2
Mumbai, India