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54
Population
Ecology
54 Population Ecology
• 54.1 How Do Ecologists Study Populations?
• 54.2 How Do Ecological Conditions Affect Life
Histories?
• 54.3 What Factors Influence Population
Densities?
• 54.4 How Do Spatially Variable Environments
Influence Population Dynamics?
• 54.5 How Can We Manage Populations?
What’s a population?
all the individuals of a species in a
given area.
Population structure describes the
age distribution of individuals, and
how those individuals are spread
over the environment.
Photo 54.1 Rookery of northern elephant seals (Mirounga
angustirostrus), Baja California.
Population Density?
The number of individuals per unit
area or volume is the population
density.
Density has strong influence over how
individuals react with one another
and with populations of other
species.
demographic events: births, deaths,
immigration, and emigration
Change the pop density
demography
Factors population ecologists study:
• # and density of individuals
Tagging, marking
• rates of demographic events
• locations of individuals
Tracking
physiological & environmental data
Figure 54.1 By Their Marks You May Know Them
Figure 54.2 Hydrogen Isotopes Tell Where Migratory American Redstarts Molted Their Feathers
What are molecular marker examples? Useful?
H isotopes -- where American
redstarts molt during migrations
Isotopes in feathers reflect the latitude
at which the feathers grew
strong latitudinal gradient of these
isotopes in precipitation
Population Estimates & Statistics to Extrapolate
Mobile animals: capture, mark,
recapture method
Proportion of marked individuals in
the new sample is used to estimate
population size:
m2 n1

n2 N
Accurate?
ONLY if marked individuals randomly
mix with the unmarked ones, and
both are equally likely to be captured
Some animals learn to avoid traps, or
learn that traps provide food and
become “trap-happy.”
Equation for estimating N (pop size)
N1  N 0  B  D  I  E
Table 54.1 Life Table of the 1978 Cohort of the Cactus Finch on Isla Daphne (Part 1)
LIFE TABLE
life table: track a group of individuals
born @ same time (cohort)
Survivorship: # still alive at later dates
fecundity: # offspring produced in a time
interval
Table 54.1 Life Table of the 1978 Cohort of the Cactus Finch on Isla Daphne (Part 2)
Life tables can be used to predict future trends.
For cactus finch, mortality rate was high
during the first year, then dropped.
Mortality rate fluctuated year to year
because the birds are dependent on
seed production, which fluctuates with
rainfall.
Figure 54.3 Survivorship Curves: survivorship in relation to age---3 MAIN PATTERNS!
Figure 54.4 Age Distributions (Age Structure Diagrams) Change over Time
average
family
size
increased
from 2.5
to 3.8
children
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54.2 Life Histories
What’s a life history?
• describes how it allocates time and
energy among the various activities
throughout its life
• can vary dramatically
• single offspring per reproductive
episode....or lots!
• Some reproduce only once and then die
(salmon, agave)
Figure 54.5 Big Bang Reproduction
Figure 54.6 An Oil Droplet Is an Energy Kick Start
*Fishery Management*
Black rockfish females continue to grow
throughout their lives
large females produce many more eggs
than small ones
Eggs from older females contain oil
droplets that are food for developing
fish, so offspring have a better chance
for survival.
Cont.--
Intensive fishing off Oregon from 1996–
1999 reduced average age of females
from 9.5 to 6.5 years.
Age reduction decreased number of eggs
produced and average growth rates of
offspring.
Maintaining populations of this species
may require no-fishing zones where
females can grow to large sizes.
Local Red Drum Research
http://www.dnr.sc.gov/marine/mrri/insh_fish/
reddrum/reddrumportal1.html
Guppies in Trinidad?
Influence of predation
If predator fish are excluded (by
waterfalls)  fish have much lower
mortality rates
When reared in the lab, guppies from the
high predation site matured earlier,
produced more eggs, and produced
more offspring per brood.
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54.3 Population Densities
Photo 54.4 Dogwinkles and barnacles in the intertidal zone.
Photo 54.5 Dense colony of purple sea urchins
(Strongylocentrotus purpuratus).
Photo 54.6 Winter swarm of ladybugs (Hippodanina convergens).
dn dt equation...
N
r
 (b  d ) N
t
ΔN/Δt = rate of change of pop over time
r = net reproductive rate
rmax = intrinsic rate of increase
N
 rmax N
t
Is this intrinsic rate possible in nature?
YES!
Over short time periods, close to rmax
northern elephant seals were hunted
to near extinction:
populations grew exponentially on
some islands after hunting was
stopped
Figure 54.7 Exponential Population Growth
But then the population reaches the ....
The environmental carrying capacity (K)
# of any particular species that can
be supported in an environment
Environmental limits 
birth rates decrease, death rates increase
What determines K?
• availability of resources
•
Food, shelter
• Diseases and parasites
• Social interactions
• S-shaped curve = logistic growth
Figure 54.8 Logistic Population Growth
Logistic growth model
Growth stops when N = K
N
KN
 r
N
t
 K 
Density-dependent factors:
• pop density increases 
food supplies may be depleted
• Predators may be attracted to high
densities of prey, increasing death
rate.
• Diseases can spread more easily.
Density-Independent Factors:
Weather-related phenomena
Figure 54.9 Regulation of an Island Population of Song Sparrows (Part 1)
Figure 54.9 Regulation of an Island Population of Song Sparrows (Part 2)
Figure 54.9 Regulation of an Island Population of Song Sparrows (Part 3)
Song sparrows on Mandarte Island
12 yr study
Population size fluctuated significantly.
Death rates high during cold, snowy
winters, regardless of population
density.
Density-dependent factors were also important:
• # breeding males limited by territorial
behavior
• more breeding females 
fewer offspring each one fledged
• more birds alive in autumn  less
chance juveniles would survive the
winter
Photo 54.7 Thorny tree swarms with locusts, Burkina Faso.
Photo 54.8 Adult Chinook salmon is released into holding tank at
a hatchery in British Columbia.
General Trend:
More stable population # in species with
long-lived individuals and low
reproductive rates.
Insect pops fluctuate more than birds and
mammals.
Environmental factors can change carrying
capacity for species.
Figure 54.10 Individuals Born during Years of Good Reproduction May Dominate Populations (1)
Figure 54.10 Individuals Born during Years of Good Reproduction May Dominate Populations (2)
What if the pop dens depends on ONE resource?
likely to fluctuate
boreal forests: many birds and mammals
eat conifer seeds.
trees reproduce synchronously and
episodically
Mortality rates can be high in years
with poor seed production.
4 Factors Strongly Influence Variation of Pop Density:
• Resource abundance
• Size of individuals
• Length of time a species has lived in an
area
• Social organization
Animals that eat plants are generally
more abundant than animals that eat
other animals.
WHY?
What about body size?
small body size  higher pop densities
require less energy to survive vs larger
mammal species worldwide
Figure 54.11 Population Density Decreases as Body Size Increases
Oh, the invasive species!
No natural pathogens and predators 
very high population densities
Zebra mussels were introduced to the
Great Lakes in 1985.
They spread rapidly and reached
densities much higher than in their
native Europe.
Figure 54.12 Introduced Zebra Mussels Have Spread Rapidly
Figure 54.13 The Last Refuge
Giant sequoias are restricted
to a few groves in the
southern Sierra Nevada but
Douglas firs are widespread and
abundant
54.5 How Can We Manage Populations?
Numbers of births and growth of
individuals tend to be highest when
population is below carrying capacity.
If humans wish to maximize the number
of individuals harvested from a
population, we should try to maintain it
below carrying capacity.
Hunting seasons are established with
this goal in mind.
54.5 How Can We Manage Populations?
In fast reproducing populations,
harvest rates can be high.
Growth rates of individuals are often densitydependent, so harvesting pre-reproductive
individuals allows others to grow faster.
Some fish populations can be harvested on
a sustained basis because
a few females can produce enough
eggs to maintain the population!!!
54.5 How Can We Manage Populations?
Many fish have been overharvested and
population sizes reduced.
Cod and haddock on Georges bank were
so heavily exploited that fishing had to
be stopped to allow populations to
recover.
Figure 54.17 Overharvesting Can Reduce Fish Populations
54.5 How Can We Manage Populations?
Whaling has also resulted in declining
populations.
Most whale populations have failed to
recover.
Whales are large animals with slow
reproductive rates. Many adults are
needed to produce a small number of
offspring.
54.5 How Can We Manage Populations?
The International Whaling Commission
was established to guide recovery of
whale populations.
Member countries voted to ban all
commercial whaling, but some members
now lobby to restore harvest of nonendangered species.
Lack of a market for whale meat may in
the end cause the demise of commercial
whaling.
54.5 How Can We Manage Populations?
Humans wish to decrease the size of
populations of many pest species.
Reducing population numbers below
carrying capacity stimulates higher
birth rates and growth of the
population.
A more effective approach is to remove
the resources for the population, (e.g.,
making garbage unavailable for rats).
54.5 How Can We Manage Populations?
Humans introduce other species to
control pests, such as the cactus moth
to control Opuntia cacti in Australia.
Sometimes the introduced predator or
parasite fails to control the pest; or
worse, begins to attack other species.
54.5 How Can We Manage Populations?
Toads were introduced into Australia to
control cane beetles in sugar cane
fields.
The toads couldn’t reach the beetles high
on the sugar cane plants, but have been
an ecological disaster for other species.
They are poisonous, reproduce quickly,
and outcompete native amphibians.
Figure 54.18 Biological Control Gone Awry
54.5 How Can We Manage Populations?
The size of the human population now
contributes to most environmental
problems.
Human social organization and
specialization has allowed us to
increase the carrying capacity for
humans.
54.5 How Can We Manage Populations?
Earth’s current carrying capacity for
humans
biosphere’s ability to absorb our byproducts, especially CO2 from fossil
fuels;
water availability
our willingness to cause extinction of
other species to accommodate our
increasing use of Earth’s resources.