Ecology: Populations
Organizational Hierarchy of Life
Most Complex
Least Complex
biosphere
ecosystem
community
population
organism
organ system
organ
tissue
cell
organelle
macromolecule
molecule
atom
sub-atomic particles
Living
Non-living
Term
Population
Definition
Community
All populations in a common
environment
Organisms in community plus
nonliving factors
Ecosystem
Biosphere
Individuals of one species that
occupy a given environment
Zones of air, land and water at
earth’s surface occupied by living
things
What Is a
Population?
All members of a
particular
species who live
within an
ecosystem and
can potentially
interbreed
What Factors Influence
Population Size?
Change in population size =
(births – deaths) +
(immigrants – emigrants)
Migration is often less of an influence
than birth and death rates.
What Factors Influence
Birth and Death Rates?
• Biotic potential
– maximum rate at which population
could increase, given ideal conditions
• Environmental Resistance
– limits on growth set by living and nonliving components of the environment
What Factors Cause
Exponential Growth?
• Exponential Growth: number of
individuals increases at a constant rate
– continuously accelerating increase in size
– occurs when a species is introduced into a
new, unoccupied habitat
– resources are unlimited
– births consistently exceed deaths
Exponential
Growth Curve
(J-curve)
Occurs when biotic potential is unlimited
What Factors Influence
Biotic Potential?
• Age of first reproduction
• Frequency of reproduction
• Average number of offspring produced
with each reproductive event
• Length of reproductive life span
• Death rate of individuals under ideal
conditions
Exponential
Growth Curve
Affected by Start of
Reproduction
Reproduction
began at 4
years (i)
Reproduction
began at 6
years (ii)
Effect of Death Rates on Population Growth
bacteria
No
deaths
10% die
between
doublings
25% die
between
doublings
Exponential Growth Is Part of a
Boom-and-Bust Population Cycle
Cycle: Exponential growth when conditions are good (boom)
Population rapidly declines when conditions are bad (bust)
Lemming Population Cycles
Population decline caused by
overgrazing of tundra
increasing numbers of predators
social stress
What Factors Limit Population Growth?
Environmental Resistance is caused by
• Density-Independent Factors
– unrelated to population density
– weather and other natural events
– human activities, including pollution
• Density-Dependent Factors
– related to the number of individuals per
given area
– involve interactions within the community,
including predation, parasitism, competition
Population Cycles in Predators and Prey
The S-Curve of Population Growth
Number of Individuals
(environmental resistance)
Carrying Capacity
(biotic
potential)
Equilibrium
Exponential
Growth
Time
1800
Carrying Capacity
Population Size
1600
1400
Growth rate = 0
Growth rate slows down
1200
1000
800
Pleasant Island Pheasants
600
•1937 – 8 pheasants introduced
400
•1945 – 1325 pheasants on island
200
0
1936
•1947 – 1600 pheasants on island
1941
1946
Year
1951
Why Did the Pheasant Population
Stabilize?
• Females were laying the same
number of eggs
• Death rate increased, number
of surviving chicks decreased
Carrying Capacity
• Maximum population size indefinitely
supported by available resources
• Determined by sustained availability of
–renewable resources such as
nutrients, water and light
–space
• Influenced by environmental resistance
120,000
Grand Canyon Kaibab Deer
Population Size
100,000
80,000
•1907 – 4000 deer on Kaibab Plateau
• beginning in 1907– removal of
predators
•1924 – 100,000 deer
60,000
40,000
• very severe winter in 1925-26
•1927 – 10,000 deer
20,000
0
1907
1912
1917
Year
1922
1927
What Were the Effects of the
Deer Population at Its Peak?
• Forest was over
browsed leading to
– Loss of ground
cover
– Soil compaction –
less water and
increased erosion
• Carrying capacity
reduced for years
Applying Your Knowledge
1.
2.
3.
4.
5.
Biotic Potential
Environmental Resistance
J-Curve
S-Curve
Carrying Capacity
A. Does choice 3 or 4 show exponential
growth?
B. Does choice 1 or 2 represent the effects of
pollution on population size?
C. Which choice (1-5) represents the maximum
population size that can be supported by an
environment over a long period of time?
Survivorship Curves
R-selected Versus K-selected Strategies
Continuum from
Examples
Curve
Number of
young
Size of young
Maturation
Parental care
Reproduction
r-selected
k-selected
weeds, insects mammals
J-curve
S-curve
many
few
small
rapid
little or none
once per
lifetime
large
slow
extensive
multiple
times
Population Distributions
Random
Clumped
Uniform
Human Population Growth
Human population reached 6 billion on October 12, 1999
Population Growth Rates:
Developed & Developing Countries
Developing
Countries
Developed Countries
Generalized Age-Structure Diagrams
Post-Reproductive
(46-100 y old)
Reproductive
(15-45 y old)
Pre-Reproductive
(0-14 y old)
Expanding
Stable
Contracting
Age Structures Compared
Age Structures Compared
Population Growth by World Regions
World Average 1.4%
Developed
Countries 0.1%
Developing Countries 1.7%
Africa 2.4%
Latin America/Caribbean 1.8%
Asia (excluding China) 1.7%
China 0.9%
North
America 0.6%
Europe -0.1%
-0.5
0
0.5
1
1.5
2
Natural Increase (annual %)
2.5
3
U.S. Population Growth
Factors contributing to
U.S. population growth
– “baby boom”
– immigration
U.S. has 5% of world’s
population but
– uses 25% of world’s
energy
– releases more
greenhouse gases than
16x more people in Asia
Carrying Capacity Is
Affected by Lifestyle
Type of Diet
Vegetarian
Diet
Developing
World Diet
American
Style
Percent of
Could Feed
calories from
animal sources
0%
6-7 billion
25%
4 billion
60%
2.5 billion
Population Predictions
• Extreme (Paul Ehrlich) – Population will
probably crash with 50%-80% dying due to
war, famine, disease, & ecological collapse
(density dependent factors).
• Middle - Widespread famine and disease
will hit some areas of the world, but basic
food supply probably sufficient through
2020.
• Optimistic – Improving technology will
avert these disasters.