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Chapter 8
Population Ecology
Core Case Study:
Southern Sea Otters: Are They Back
from the Brink of Extinction?
 They
were overhunted to the
brink of extinction
by the early
1900’s and are
now making a
comeback.
Figure 8-1
Core Case Study:
Southern Sea Otters: Are They Back
from the Brink of Extinction?
 Sea
otters are an
important
keystone species
for sea urchins
and other kelpeating organisms.
Figure 8-1
POPULATION DYNAMICS AND
CARRYING CAPACITY
 Most
populations live in clumps although other
patterns occur based on resource distribution.
Figure 8-2
Why Live In Clumps?
 Provides
animals with better protection from
predators and population decline
 Gives some predator species better chance
of getting a meal
 Provides temporary groups from mating and
caring for young.
Changes in Population Size:
Entrances and Exits
 Populations
increase through births and
immigration
 Populations
emigration
decrease through deaths and
Age Structure: Young Populations
Can Grow Fast
 How
fast a population grows or declines
depends on its age structure.




Prereproductive age: not mature enough to
reproduce.
Reproductive age: those capable of
reproduction.
Postreproductive age: those too old to
reproduce.
Populations made up of reproductive age will
increase
 Populations
dominated post reproductive age
will tend to decrease
 Fairly even distribution will stay stable births
equal deaths
Limits on Population Growth:
Biotic Potential vs. Environmental
Resistance
 No
population can increase its size
indefinitely.


The intrinsic rate of increase (r) is the rate at
which a population would grow if it had unlimited
resources.
Carrying capacity (K): the maximum population
of a given species that a particular habitat can
sustain indefinitely without degrading the habitat.
 Population
reaches some size limit due to
limiting factors (light, water, living space,
nutrients, competitors, infectious diseases)
 Growth rate of population decreases as size
reaches carrying capacity due to resources
dwindling
Growth
 Populations
typically show 2 types of growth
patterns
 Exponential growth= Increase at a fixed rate
each year no limitation to resources
 Logistically=
Rapid increase of a population
then will level off because of declining
resources or competition
Exponential and Logistic Population
Growth: J-Curves and S-Curves
 Populations
grow rapidly with
ample
resources, but
as resources
become limited,
its growth rate
slows and levels
off.
Figure 8-4
Environmental
Resistance
Carrying capacity (K)
Biotic
Potential
Exponential
Growth
Time (t)
Fig. 8-3, p. 163
Exponential and Logistic Population
Growth: J-Curves and S-Curves
 As
a population
levels off, it
often fluctuates
slightly above
and below the
carrying
capacity.
Figure 8-4
Logistic Growth
 Rapid
exponential growth followed by steady
decrease in population until becoming steady
 Population slows due to environmental
resistance
 Example brown tree snake
Overshoot
Number of sheep (millions)
Carrying capacity
Year
Fig. 8-4, p. 164
Exceeding Carrying Capacity: Move,
Switch Habits, or Decline in Size
 Members
of
populations which
exceed their
resources will die
unless they adapt or
move to an area with
more resources.
Figure 8-6
Number of reindeer
Population
overshoots
carrying
capacity
Population
Crashes
Carrying
capacity
Year
Fig. 8-6, p. 165
• Exceeding carrying capacity due reproductive
time lag
• Examples: Reindeer introduced to the Bering
Sea
• Can reduce lands carrying capacity (amount of
resources needed to sustain the population)
• Develop adaptive traits that reduces
environmental resistance
• Migrating to other areas
Exceeding Carrying Capacity: Move,
Switch Habits, or Decline in Size
 Over
time species may increase their carrying
capacity by developing adaptations.
 Some species maintain their carrying
capacity by migrating to other areas.
 So far, technological, social, and other
cultural changes have extended the earth’s
carrying capacity for humans.
Population Density and Population
Change: Effects of Crowding
 Population
density: the number of individuals
in a population found in a particular area or
volume.

A population’s density can affect how rapidly it
can grow or decline.
• e.g. biotic factors like disease

Some population control factors are not affected
by population density.
• e.g. abiotic factors like weather
 Higher
population densities may help
sexually reproducing individuals find mates
but can also lead to competition
 Help shield members from predators
 Population density decreases opposite effect
happens
 Density dependent factors regulate a
population at a constant size. (Plagues)
 Density independent factors (can kill
members of a population
Types of Population Change
Curves in Nature

Population sizes may stay the same, increase,
decrease, vary in regular cycles, or change erratically.




Stable: fluctuates slightly above and below carrying
capacity. Undisturbed areas
Irruptive: populations explode and then crash to a more
stable level. Algae, Insects (Seasonal changes)
Cyclic: populations fluctuate and regular cyclic or boomand-bust cycles. (Figure 8-7) snowshoe hare and lynx
Irregular: erratic changes possibly due to chaos or drastic
change.
Types of Population Change
Curves in Nature
 Population
sizes often vary in regular cycles
when the predator and prey populations are
controlled by the scarcity of resources.
Figure 8-7
Case Study: Exploding White-Tailed
Deer Populations in the United States
 Since
the 1930s the white-tailed deer
population has exploded in the United States.

Nearly extinct prior to their protection in 1920’s.
 Today
25-30 million white-tailed deer in U.S.
pose human interaction problems.


Deer-vehicle collisions (1.5 million per year).
Transmit disease (Lyme disease in deer ticks).
REPRODUCTIVE PATTERNS
 Some
species reproduce without having sex
(asexual).

Offspring are exact genetic copies (clones).
 Others



reproduce by having sex (sexual).
Genetic material is mixture of two individuals.
Disadvantages: males do not give birth, increase
chance of genetic errors and defects, courtship
and mating rituals can be costly.
Major advantages: genetic diversity chance of
offspring protection.
Sexual Reproduction: Courtship
 Courtship
rituals
consume time and
energy, can transmit
disease, and can
inflict injury on males
of some species as
they compete for
sexual partners.
Figure 8-8
Reproductive Patterns
 Species
use different reproductive patterns to
help ensure survival.
 R selected= Have many offspring with little
parental protection. They produce so many
that a few will survive
 Reproduce when conditions are favorable
 Figure 8-10
 K selected= Have small number of offspring.
Offspring more likely develop inside mother’s
womb
Reproductive Patterns:
Opportunists and Competitors
 Larger
in size and
card for by mother
until reproductive age
 Tend to do well in
competitive
conditions
 Populations follow a
logistic growth curve
 Figure 8-10
Figure 8-9
Carrying capacity
K
K species;
experience
K selection
r species;
experience
r selection
Time
Fig. 8-9, p. 168
Reproductive Patterns
 r-selected
species tend to be opportunists
while K-selected species tend to be
competitors.
Figure 8-10
Cockroach
r-Selected Species
Dandelion
Many small offspring
Little or no parental care and protection of offspring
Early reproductive age
Most offspring die before reaching reproductive age
Small adults
Adapted to unstable climate and environmental
conditions
High population growth rate (r)
Population size fluctuates wildly above and below
carrying capacity (K)
Generalist niche
Low ability to compete
Early successional species
Fig. 8-10a, p. 168
K-Selected Species
Elephant
Saguaro
Fewer, larger offspring
High parental care and protection
of offspring
Later reproductive age
Most offspring survive to reproductive age
Larger adults
Adapted to stable climate and environmental
conditions
Lower population growth rate (r)
Population size fairly stable and usually close to
carrying capacity (K)
Specialist niche
High ability to compete
Late successional species
Fig. 8-10b, p. 168