POPULATION ECOLOGY
Campbell & Reece Chapter 53
Population Ecology

the study of populations in relation to their
environment
Dynamic Biologic Processes that
Influence Population Density

Population: is a group of individuals of a
single species living in same generral area
 Members
of a population rely on same resources
& are influenced by same environmental factors
 They are likely to interact & breed with each
other
Populations


often described by their boundaries & #s
boundaries may be natural ones or
ecologists may arbitrarily define them
Population Density

# of individuals per unit area or volume
Dispersion

the pattern of spacing among individuals
w/in boundaries of the population
Mark-Recapture Method

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way to determine population size:
ecologists cannot count all individuals in a
population if organisms move too quickly or
are hidden from view
Technique: capture a random sample of
individuals & “mark” & then release them.
Some species can be identified w/out
physically capturing them: dolphin, whale
Mark-Recapture Method
Population Dynamics

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Population density
is not a static
property:
Births
Deaths
Immigration
Emmigration
Patterns of Dispersion
Patterns of Dispersion: Clumped

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*most common
plants & fungi clumped where soil conditions
& other environmental factors favor
germination & growth
animal clumping may have to do with being
successful in some way:
 Mayflies
swarm in great #s which increases their
chances of mating (only have ~2 days)
 Wolf pack more likely to kill a moose or deer than a
single wolf
Mayfly Swarm
Patterns of Dispersion: Uniform

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may result from direct interactions between
individuals in a population
some plants secrete chemicals that inhibit
germination & & growth of nearby
individuals that could compete for
resources
animals that show territoriality are spaced
apart: often as result of antagonistic
interactions
Patterns of Dispersion: Random


unpredictable spacing
1 individual’s position is unrelated to other
individuals
Demographics


study of the vital statistics of a population
& how it changes over time
especially important are birth rates & death
rates
Life Table


age-specific summaries of the survival
pattern of a population
construct one by following the fate of a
cohort: a group of individuals of the same
age, from birth until all of them are dead
Life Table of Belding’s Ground
Squirrels
Survivorship Curves


a graphic method of representing some of
the data in a life table
plots proportion or #s in a cohort still alive
at each age
Idealized Survivorship Curves
Type I Curve



flat @ first reflecting low death rate
during early & middle life
then steep drop as death rate increases
with increasing age
typical curve for large mammals that
produce few young but take good care of
them
Type III Curve



drops sharply at start reflecting high death
rate among the young
flattens out as death rate for those
individuals that make it out of early life
decreases
typical for species that produce many
offspring but provide little or no care for
them:
 fishes,
invertebrates
Type II Curve
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intermediate
have a constant death rate
typical of rodents, some invertebrates,
some lizards, & some annuals
Survivorship Curves

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not all species fall into 1 of the 3 types
some invertebrates have a “stair-step”
pattern: more vulnerable during periods
when molting, less vulnerable when not
molting
Reproductive Rates


Demographers tend to just look @ #s of
females & how many female offspring they
have
simplest way to describe the reproductive
pattern of a population is to ask how
reproductive output varies with the age of
females
Reproductive Table



aka a fertility schedule
an age-specific summary of the
reproductive rates in a population
constructed by measuring reproductive
output of a cohort from birth to death
Natural Selection

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favors traits that improve an organism’s
chances of survival & reproductive success
every species has trade-offs between
survival & traits such as frequency of
reproduction, # of offspring
traits that affect an organism’s schedule of
reproduction & survival make up its life
history
Life Histories


1.
2.
3.
are very divers but exhibit patterns in the
variability
have 3 basic variables:
when reproduction begins
how often the organism reproduces
how many offspring produced during each
reproductive episode
Big-Bang Reproduction

1 individual
reproduces large #
of offspring then
die: called
semelparity
Iteroparity

produce only a few offspring during
repeated reproductive episodes
Semelparity or Iteroparity?
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Which is better?
Critical factor is survival of offspring:
*when survival of offspring is low as in
highly variable or unpredictable
environments, semelparity is favored
*dependable environments where
competition of resources is fierce favors
iteroparity
Limited Resources
means Trade-Offs


sometimes see trade-offs between survival
& reproduction when resources are limited
example: red deer females have higher
mortality in winters following summers in
which they reproduce
Species whose Young have High
Mortality Rates


often produce large #s of relatively small
offspring
example: plants that colonize disturbed
environments usually produce many small
seeds  only a few reach suitable habitat
 smaller
seeds allow them to be carried farther
Parental Investment Increases
Survival of Offspring


example: oak, walnut, & coconut trees have
large seeds with large store of nrg &
nutrients to help seedlings become
established
example: primates provide an extended
period of parental care
Population Growth

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All populations have a tremendous capacity
for growth
Unlimited increase does not occur
indefinitely for any species, in lab or in
nature
Exponential Model
of Population Growth



unlimited growth does not occur for long in
nature but it is assumed to be true in this
model
Δ population = (# births + # immigrants ) (# deaths + # emigrants)
 Or, ignoring immigration & emigration:
Δ N/Δt = B - D
Exponential Model


now use average #s of births & deaths per
individual during a specified period of time
If there are 34 births per year in a
population of 1,000 then the annual per
capita birth rate is 34/1,000 = 0.034 = b
Exponential Growth Equation

ΔN/dt = TMax N: represents a population’s
potential growth in an unlimited environment
where TMax is the maximum per capita rate of
increase & N is the # of individuals in the
population
Exponential Growth
Exponential Model

http://www.slideshare.net/MrDPMWest/population
-growth-apbio
Logistic Model

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in nature, as any population density
increases: each individual has access to
fewer resources
eventually, there is a limit to the # of
individuals that can occupy a habitat
Carrying Capacity: (K) the maximum
population size that a particular
environment can sustain
Carrying Capacity

varies over space & time with the abundance
of limiting resources:
 Energy
 Shelter
 Refuge
from predators
 Nutrients
 Water
 Suitable nesting sites
Carrying Capacity

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per capita birth rate decreases if there are
not enough nutrients for adults to maintain
themselves or if disease or parasitism
increases with density
per capita death rate increases for same
reasons
either way: results in lower per capita rate
of increase
Logistic Growth Model

per capita rate of increase approaches 0 as
the carrying capacity is reached
Logistic Growth Equation
Logistic Model

fits few real populations perfectly, but it is
useful for estimating possible growth
Life History Traits are Products
of Natural Selection


traits that affect an organism’s schedule of
reproduction & survival make up its:
life history:
3
1.
2.
3.
main variables:
age @ 1st reproduction
how often the organism reproduces
# offspring produced/ reproductive episode
Semelparous Organisms

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reproduce once & die
aka “big-bang”
Iteroparous Organisms

produce offspring repeatedly
Semelparous vs. Iteroparous?

1.
2 critical factors:
survival rate of offspring

2.
low chance survival where environment
variable or unpredictable: semelparity
likelihood adult will survive to reproduce
again

adults less likely to survive: semelparity
“Trade-Offs” & Life Histories

trade-offs between reproduction & life
histories:
Trade-Offs

selective pressures influence the trade-off
between # & size of offspring
 Plants:
 those
that colonize disturbed environments have
small seeds…only few reach suitable habitat
 Animals:
 those
with high predation rate have larger #s
offspring (quail, mice, sardines)
Trade-Offs

other species put large investment to insure
survival of offspring
 trees
with large seeds (walnut, brazil nut)
provide nutrients that increases offspring’s
chances of survival
K-Selection/ R-Selection


selection for traits sensitive to population
density & are favored @ high densities is
known as K-selection = density dependent
selection
selection for traits that maximize
reproductive success in low density
environments is called r-selection
Density-Dependent Factors


refers to any characteristic that varies
with population density
birth or death rate changes based on
population density
 often
because water &/or nutrients become
scarce
Density-Independent Factors

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any characteristic not affected by ppulation
density
birth & death rates stable no matter what
the population density is
Determining Equilibrium for
Population Density

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red line = densityindependent death
rate
blue line = densitydependent birth
rate
junction =
equilibrium density
Mechanisms of DensityDependent Population Regulation

w/out some type of (-) feedback between
population density & rates of birth & death:
a population would never stop growing
Mechanisms of DensityDependent Regulation
1. Competition for
Resources

competing for
nutrients & other
resources decreases
the birth rate
 farmers add
fertilizer to reduce
competition for
nutrients
Mechanisms of DensityDependent Regulation
2. Predation
 as # prey increases
a predator may
develop preference
for that species
Mechanisms of DensityDependent Regulation
3. Toxic Wastes
 Brewer’s Yeast
used to convert
carbohydrates to
ethanol in winemaking but
anything > 13%
alcohol it toxic to
the yeast
Mechanisms of DensityDependent Regulation
4. Intrinsic Factors
 some species experience increased death
rate * decreased birth rate when population
density reaches certain point even when
there is sufficient nutrients
 white-footed
mice: immune system altered 
more deaths & reproductive maturity delayed
 fewer births when certain density reached
Mechanisms of DensityDependent Regulation
5. Territoriality
 when space becomes
resource competing for
it can limit population
size
 maintaining a territory
insures there will be
enough food to live &
reproduce
Mechanisms of DensityDependent Regulation
6. Disease
 if transmission rate
depends on certain
level of crowding
then it is densitydependent
 respiratory viruses
spread thru air: is
more easily spread in
large cities than in
rural areas
Mechanisms of DensityDependent Regulation
7. Population Dynamics
 the normal fluctuations in population size
from yr to yr
Mechanisms of DensityDependent Regulation
8. Stability & Fluctuation
 because changing environmental conditions
disrupt populations, they all experience size
fluctuations
Emigration


when a population becomes crowded &
resource competition increases often
emigration #s increase
example: when slime mold resources become
scarce some single celled individuals group
together (amoeba group) a & form a slug
Metapopulation

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form when a # of local populations are
linked
local populations in a metapopulation can be
thought of as occupying discrete patches of
suitable habitat in a sea of unsuitable places
Human Population Growth
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since ~ 1650 has been growing exponentially
w/in last 55 yrs rate of growth has fallen
by nearly ½
predictions: rate will continue to decline
until ~2050 & reach equilibrium ~2100
Human Population Growth
Demographic Transition

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movement from high birth rate & high death
rate  low birth rate, low death rate
occurred in human population with industrial
revolution
Age Structure
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relative # of individuals of each age
use “pyramids”
USA: #s fairly steady except for “baby
boomers”: increased birth rate that
followed up to 20 yrs after WWII
Age Structure: Rapid Growth
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bottom-heavy
pyramid
skewed toward
more young
individuals
ex: Afghanistan,
Congo
Age Structure: Slow Growth
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fairly steady
growth over time
can be do to steady
birth rate or
falling birth rate +
steady increase in
immigration
ex: USA
Age-Structure: No Growth
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smaller base: #s
under reproductive
age are underrepresented
ex: Italy, Germany
Infant Mortality
Life Expectancy
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Infant Mortality = # of infant deaths per
1,000 live births
Life Expectancy @ birth = predicted
average length of life
both vary widely among human populations
if infant mortality high parents may choose
to have more children to increase odds some
will reach adulthood
Comparing Industrialized &
Developing Nations
Global Carrying Capacity

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is uncertain
has changed over time due to advances in
technology
Ecological Footprint

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is the aggregate land & water area needed
to produce all the resources a person or
group of people consume & to absorb their
wastes
it‘s 1 measure of how close we are to Earth’s
carrying capacity
We‘re using many of Earth’s resources in
unsustainable manner
Annual per capita Energy Use
http://www.worldometers.info/wor
ld-population/

http://www.worldometers.info/world-population/