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Population Ecology
Professor Andrea Garrison
Biology 3A
Illustrations ©2014 Cengage Learning unless
otherwise noted
Population Dynamics
• Patterns of change seen in a population over
time and from place to place
– Why does a population grow exponentially in
some areas and remain stable in others
– How do interactions between biotic and abiotic
factors influence population characteristics
– How do populations respond over generations to
their interactions with the biotic and abiotic
environment
Population Ecology
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Population Dynamics
• Many population characteristics help
ecologists understand the dynamics of a given
population
Population Ecology
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Population Characteristics
• Geographic range
– Often determined by
climate
• Many species limited by
temperature and/or
rainfall
• Genetic variability often
plays large role in
determining the extremes
of climate a species can
survive
– Only a few individuals
live at the extreme
boundaries of the range
Population Biology; picture:
http://www.allaboutbirds.org/guide/California_Towhee/id
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Population Characteristics
• Habitat
– Every species is found in
specific habitat(s) which
provide it with required
resources (food, nesting
space, etc)
– Habitats differ for each
species
– California Towhee requires
chaparral and other
tangled, shrubby, dry
habitats within its range
Population Ecology; photo by Jamie Chavez, https://creativecommons.org/licenses/by-nc/2.0/
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Population Characteristics
• Population size = #individuals in the
population
• Population density = #individuals/volume (or
area) of habitat
– Measure of resources available for each individual
– Use to monitor and manage populations (esp.
endangered species)
• Less dense populations have more resources
• Less dense populations may have trouble finding a
mate
Population Ecology
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Population Characteristics
• Spatial dispersion (distribution)
– Random dispersion pattern
• Unpredictable distribution
• Environmental conditions don’t vary
within habitat
• Individual not attracted to or repelled by
other individuals of same species
• Some spiders, rainforest trees, burrowing
clams
Population Ecology
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Population Characteristics
• Spatial dispersion (distribution)
– Clumped dispersion pattern
• Most common dispersion pattern
• Individuals grouped together
• Resources often patchy
– EX: Plant growth in area around
cowpats
• Social animals cluster
• Plants and animals often cluster
because of reproductive patterns
– Asexually reproducing plants and
animals create clones
» EX: sea anemones create
cluster of clones
– Reproduction via seeds, eggs, larvae
that lack dispersal mechanisms
» Young grow near parents
Population Ecology
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Clumped dispersion
Anemones closed, bits of shell on outside
Anemones open under water
2 anemone clones w/ anemone-free
zone due to fighting
Population Ecology
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Population Characteristics
• Spatial dispersion
(distribution)
– Uniform dispersion pattern
• Individuals of a species may repel
each other
– EX: creosote bushes produce
toxins so less competition for
water
» Territorial behavior to
defend space/resources
produce uniform dispersion
pattern
Population Ecology
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Population Characteristics
• Population growth rate
– Determined by age structure, generation time and
sex ratio
Population Ecology
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Population Characteristics
• Age structure
– Break down of
population into
age groups based
on reproductive
capabilities
Population Ecology
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Population Characteristics
• Generation time
– Time required to reach sexual maturity
• Sex ratio
– Relative proportions of males and females
– #females usually has bigger impact than #males
• Females actually produce the offspring
• Males often mate with >1 female
• If pairs bond for life, then #males is important
Population Ecology
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Population Dynamics
• The demography of a population helps ecologists
understand the dynamics of a given population
• Demography = statistical study of the processes
that change a population’s size and density over
time
• Demographic analysis allows prediction of future
population growth patterns
– Allows federal and local govts to anticipate needs for
school, water and sewage lines, etc
– Allows conservation ecologists to develop plans to
protect endangered species
Population Ecology
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Demography
• Age structure, generation time and sex ratio
contribute to demographic understanding of a
population
Population Ecology
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Demography
• Life tables summarize demographic
characteristics of a population
– Similar to mortality charts for life insurance
companies
– Look at several aspects of population
• Age structure of population (#individuals within each
age group)
• What percentage of each age group in a population
survives to the next age group
• Average #offspring produced by each female in age
group
Population Ecology
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Life Table
Population Ecology
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Demography
• Survivorship Curves reflect info in life tables
– Survivorship data roughly match one of three
types of curves
Population Ecology
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Demography
• Type I curves
– Most individuals survive
until late in life
– High mortality late in life
– EX: Dall’s sheep have
only 1-2 offspring and
nurture them through
high-risk first year
Population Ecology
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Demography
• Type II curves
– Mortality relatively
constant throughout life
• Predation
• Disease
• Starvation
– Common among lizards,
songbirds, small
mammals
Population Ecology
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Demography
• Type III curves
– High juvenile mortality
– Lower mortality once
individuals reach critical
age/size
– Common among organisms
that broadcast gametes,
seeds (plants, marine inverts,
fish)
• Plants produce many seeds
• Only few survive to germinate
• Less than ½ of those become
established
– Good location
– Sturdy enough to avoid
predation
Population Ecology
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Population Dynamics
• Life history characteristics
govern fluctuations in
population size over time
– Characteristics vary for
different organisms
• r-selected species are
opportunists
– Take advantage of a
situation when it arise
– Type III survivorship
• K-selected species
equilibrium based
– Require more stable
environment
– Types I and II survivorship
Population Ecology
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Population Dynamics
• Population growth models help ecologists
understand the demography of any given
population
Population Ecology
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Population Growth Models
• Geometric growth
– Occurs among organisms
that reproduce via
binary fission
• Bacteria, protists
– Parent cell divides in
two, no longer exists
Population Ecology
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Population Growth Models
• Exponential growth
– Occurs among organisms
that reproduce offspring,
then live alongside their
offspring for period of
time
• Most plants and animals
• We will track growth
characteristics of these
organisms
Population Ecology
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Population Growth
• Births increase population size
• Deaths decrease population size
• Population growth = births – deaths
N = population size
t = time
B = births
D = deaths
• ∆N/∆t = B – D over the time span
Population Biology
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Population Growth
• ∆N/∆t = B – D over the time span
• Ecologists use per capita numbers
– Averaged over entire population
– Give an average number per individual
– B/N = b; D/N = d
• Therefore, per capita ∆N/∆t = b – d
• Per capita ∆N/∆t = r (per capita growth rate)
• r = (b – d)
Population Biology
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Population Growth
• Per capita growth rate r = b – d
If b > d, then r positive and population is growing
If b < d, r negative and population declining
If b = d, r = 0 and population stable
Population Biology
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Population Growth
• r=b–d
 r N = 0.4 X 1000 = 400 new mice
• Population of mice
 N = 1000
 500 births this month
 100 deaths this month
 b = 500/1000 = 0.5
 d = 100/1000 = 0.1
 r = (0.5 – 0.1) = 0.4 = per capita
growth rate
 N = 1000 + 400 = 1400 mice at
end of month
 The 2nd month 0.4 X 1400 = 560
new mice
 N = 1400 + 560 = 1960 mice at
end of month
Population Biology
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Population Growth
• Intrinsic rate of increase = rmax = rate at which
a population will increase if there are no limits
on its growth rate
– Important aspect of a population
– Varies with reproductive strategy
• r-selected species have higher intrinsic rates of increase
Population Biology
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Exponential Growth
Pattern over time
• Exponential growth
– Early periods of
population growth
– Plenty of resources
– Rmax is achieved
Population Biology; Illustration ©2010 Pearson Education, Inc.
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Population Growth
Pattern over time
• Logistic growth
– Later stages of
population growth
– Resources become
limiting
– growth slows as it
approaches maximum
population the
environment can
sustain (Rmax is no
longer achieved)
Population Biology; Illustration ©2010 Pearson Education, Inc.
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Population Growth
• Carrying capacity = maximum population the
environment can sustain indefinitely
Population Biology; Illustration ©2010 Pearson Education, Inc.
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Population Growth
• Some populations overshoot the carrying
capacity more rapidly than others
Population Biology; Illustration ©2010 Pearson Education, Inc.
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Population Growth
• When population overshoots, it will usually
decline and hover around carrying capacity
Population Biology; modified by A. Garrison
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Population Growth
• Carrying capacity is
determined by densitydependent limiting
factors
– Resources whose
availability decreases as
pop density increases
• Food, water, space
– Impact shown at A -- rate of
pop increase slows until
pop size is stable
Population Biology; Illustration ©2010 Pearson Education, Inc.
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Density-Dependent Regulation of
Population Growth
Population Biology; Illustration ©2010 Pearson Education, Inc.
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Density-Dependent Regulation of
Population Growth
• Density-dependent factors may also increase
death rates
– Disease transmission
– Accumulation of wastes
– Predation due to greater availability
• All these factors (& others) cause slowing and
leveling off of population growth
Population Biology
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Density-Independent Factors
• Distribution is determined by densityindependent factors
– Factors unrelated to population density which
limit the presence of the species
– Weather, soil type, other necessary biotic or
abiotic factors
Population Biology
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Density-Independent Factors
• Geographic range
– Often determined by climate
• Genetic variability often plays large role in determining the
extremes of climate a species can survive
– Only a few individuals live at the extreme boundaries of the range
• Global climate change is increasing impact of
density-independent factor
– Ranges changing for many species
– Altitude limitations changing within geographic ranges
• Moving upwards as climate warms
Population Biology
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Density-Independent Factors
• Distribution within the geographic range
– Abiotic factors
• You don’t find worms living on rocks! Or fish living on land!
– Soil factors (another abiotic factor)
• Nutrients present in soil play large factor in determining location
of plant species within their range
– Deciduous plant require more nutrients than evergreens
– Biotic factors
• Other species may play role in distribution of a species within its
range
– Herbivores cannot live where their plant food source doesn’t grow
– Predators cannot live where their prey are not found
– Plants cannot live where their pollinators are not found
Population Biology
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Population Cycles
• Some species show cyclical patterns of population increases and
decreases
– Arctic rodents have 4 year cycle
– Snowshoe hares, ruffed grouse, lynx have 10 years cycles
• Uncertain of reasons
• Hypotheses
– Intrinsic control
• Physiological, hormonal changes foster dispersal
– ↑ aggression
– ↓reproduction
– Extrinsic control
• Cycling of predators or prey
– Dense population may exhaust food supply
» ↑ mortality, ↓ population size
– Supplementing food supply doesn’t balance this, however
Population Ecology
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Human Population
Population Ecology
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Human Population
• Just starting to slow down it’s increase
• Hasn’t reached carrying capacity
– Why not?
Population Ecology
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Human Population
• Human population also expanded its range from tropics
and subtropics throughout most of the earth
• Every population has it’s own niche
– All resources used by that population and the manner in which
they are used
• Technology allows human population to constantly change
it’s niche
– Agriculture: used food sources differently
– Used different food sources
– Improved health care ↓ effects of density-dependent limiting
factors
– Allows humans population to change its carrying capacity
• At least for a while
Population Ecology
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THE END
Population Ecology
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Predator-Prey Interactions
• Density-dependent
growth factors
– Predator-prey model
– Predator pop changes
lag behind prey
population changes
Population Ecology
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Predator-Prey Interactions
• In reality, the Canada
lynx population doesn’t
lag behind
– Further studies show
that population cycles of
hare are related to
complex interactions
between the hare, its
food sources and its
predators
Population Ecology
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