Population Ecology
• A population is a group of individuals of a single
species living in the same general area
• Population Ecology = study of how population size of a
species changes over time and space
– Due to biotic and abiotic interactions
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Population Density v. Size
• Population size = number of individuals
• Population density = number of individuals in a given
unit of space (area or volume).
– Ex: minnows per Liter of pond water
A
B
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Calculating Population Change
r = (b – d) + (i – e)
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r = intrinsic rate of increase = the rate at which a population increases in
size if there are no limitations on population growth
r = per capita rate of increase = increase in population size (number
of offspring that survive to reproduce)
“Biotic potential”
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Exponential Growth
• Intrinsic Rate of Growth (Biotic Potential)
– Growth rate under ideal conditions—no limits
• Expansion into new habitat, re-colonization post-disturbance, rebound after
population decline
• Limited time span
– J- Shaped Curve (exponential growth)
© 2012 John Wiley & Sons, Inc. All rights reserved.
© 2012 John Wiley & Sons, Inc. All rights reserved.
Environmental Resistance
• Environmental limits (resistance)
– Prevent indefinite reproduction
– Density dependent factors
• Carrying Capacity (K)
– Maximum # of individuals an environment can support
• Can change over time
– Causes S- shaped curve (logistic population growth)
resistance
resistance
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• Carrying capacity (K) is the maximum population size
the environment can support
• Carrying capacity varies with the abundance of limiting
resources which can change over time
© 2012 John Wiley & Sons, Inc. All rights reserved.
Environmental Resistance
• Environmental limits (resistance)
– Prevent indefinite reproduction
– Unfavorable food, water, shelter, predation, etc.
• Carrying Capacity (K)
– Maximum # of individuals an environment can
support
– Causes leveling off of exponential growth
– S- shaped curve of logistic population growth
© 2012 John Wiley & Sons, Inc. All rights reserved.
© 2012 John Wiley & Sons, Inc. All rights reserved.
Factors That Affect Population Size

Density Dependent Factor
 Factor
whose effect on population changes as
population density changes
 Examples:
 Predation
 Disease/parasitism
 Competition
(for limited resources)
 Waste accumulation
 Intrinsic physiologic factors
© 2012 John Wiley & Sons, Inc. All rights reserved.
Number of Daphnia/50 mL
Number of Paramecium/mL
Figure 53.11
1,000
800
600
400
200
0
0
5
10
Time (days)
15
(a) A Paramecium population in the lab
180
150
120
90
60
30
0
0
20
40
60
80 100 120 140 160
Time (days)
(b) A Daphnia population in the lab
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Boom-Or-Bust Population Cycles
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Factors That Affect Population Size
• Density Independent Factors
– Factors that affects population size, but is not
influenced by changes in population density
– Examples:
•
•
•
•
•
Killing frost
Severe blizzard
Fire
Landslide/avalanch
flooding
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Evolution and Life History Diversity
• A life history entails three main variables
– The age at which reproduction begins
– How often the organism reproduces
– How many offspring are produced per
reproductive episode
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• Semelparity--big-bang reproduction--reproduce once and die
– Unpredictable environments
– Probability of adult survival, low
– Relatively low parental investment in offspring
• Iteroparity--repeat reproduction--produce offspring repeatedly
– Predictable environments
– Probability of adult survival high
– Relatively high parental investment in offspring.
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Life History Strategies are trade-offs
• Resources are finite:
– Adult survival v. reproductive output
– High output; low investment v. low output; high investment
• r v. K
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• 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
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Reproductive Strategies
r-selected species
K-selected species
- Smaller body size
-
- Early maturity
- Short life span
- Large broods (# of offspring
each reproductive event)
- Little or no parental care
- Probability of long term
survival is low
- e.g., Mosquitoes,
Dandelions,
Larger body size
- Slow development
-Late reproduction
- Long life span
- Small broods (# of offspring
each reproductive event)
-Significant
often prolonged
parental care
-Low reproductive rate
- e.g., Redwood trees,
elephants, human beings
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Number of survivors (log scale)
Figure 53.6
1,000
I
100
II
10
III
1
0
50
100
Percentage of maximum life span
© 2012 John Wiley & Sons, Inc. All rights reserved.
• Survivorship curves can be classified into three
general types
– Type I: Low death rates during early and middle
life and an increase in death rates among older
age groups
– Type II: A constant death rate over the organism’s
life span
– Type III: High death rates for the young and a
lower death rate for survivors
• Many species are intermediate to these curves
© 2012 John Wiley & Sons, Inc. All rights reserved.
Survivorship
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