Chapter 8
Population Ecology
Southern Sea Otters: Are They Back
from the Brink of Extinction?

1 million before settlers
 They were over-hunted to
the brink of extinction by
the early 1900’s for fur
 Put on endangered
species list in 1977

300 increased to 3000
Figure 8-1
Core Case Study:
Southern Sea Otters: Are They Back from
the Brink of Extinction?

Sea otters are an
important keystone
species
 control sea urchins
and other kelp-eating
organisms.
 Kelp forests provide
habitat & prevent
shore erosion
Figure 8-1
POPULATION DYNAMICS AND
CARRYING CAPACITY
 Populations





change
Distribution
Numbers
Age structure
density
changes occur based on resource distribution
& environmental conditions
Figure 8-2
POPULATION DYNAMICS AND
DISTRIBUTION
 Patterns
occur based on resource distribution.
Figure 8-2
(a)Clumped:
Most common distribution
Resources are clumped
Herds/packs: provide protection, help hunting,
raising young
Fig. 8-2a, p. 162
(b) Uniform (creosote bush):
even spread out to make best use
Of scarce resources like rain in dessert
Fig. 8-2b, p. 162
(c) Random (dandelions):
randomly scattered - rare
Fig. 8-2c, p. 162
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. (majority here = growing pop)
Reproductive age: those capable of
reproduction.
Postreproductive age: those too old to
reproduce. ( majority here = declining pop)
Even distribution in age structure = stable pop
Limits on Population Growth:
Biotic Potential vs. Environmental
Resistance
 Populations


Reproduce early & often = high potential
1 fly = 5.6 trillion in 13 months
 No

vary in capacity for growth
pop can grow indefinitely
Limiting factors:
• Sunlight, water, nutrients, living space
• Predators, competition, disease
Limits on Population Growth:
Biotic Potential vs. Environmental
Resistance


The intrinsic rate of increase (r) is the rate at
which a population would grow if it had unlimited
resources = BIOTIC POTENTIAL
Carrying capacity (K): the maximum population
of a given species that a particular habitat can
sustain indefinitely without degrading the habitat.
Exponential & Logistic Curves
Biotic potential
J – curve
S - curve
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
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.
Number of reindeer
Population
overshoots
carrying
capacity
Population
Crashes
Carrying
capacity
Year
Fig. 8-6, p. 165
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
The number of individuals per
unit area (for terrestrial
organisms) or volume (for
aquatic organisms)
At low population densities,
individuals are spaced well
apart. Examples: territorial,
solitary mammalian species
such as tigers and plant species
in marginal environments.
At high population densities,
individuals are crowded
together. Examples: colonial
animals, such as rabbits, corals,
and termites.
Low density populations
High density populations
Population Density and Population
Change: Effects of Crowding
 Environmental
resistance = all the factors
that act to limit the growth of a population.

Some population control factors have a greater
effect as the population’s density increases.
• e.g. biotic factors like disease

Some population control factors are not affected
by population density.
• e.g. abiotic factors like weather
Density Dependent Factors
 The
effect increases as population density
increases




Competition for resources
Predation
Parasitism
Infectious disease
 These
factors tend to regulate pop at fairly
consistent size, often near carrying capacity
Density Independent Factors
‣
The effect doesn’t depend on
population’s density – doesn’t
matter if crowded together or
spaced far apart:
Physical (or abiotic) factors
temperature
precipitation
humidity
acidity
salinity etc.
Catastrophic events
floods and tsunamis
fire
drought
earthquake and eruption
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.
Irruptive: populations explode and then crash to a
more stable level.
Cyclic: populations fluctuate and regular cyclic or
boom-and-bust cycles.
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
Population size (thousands)
Hare
Lynx
Year
Fig. 8-7, p. 166
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, 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:
Opportunists and Competitors
 Large
number of
smaller offspring with
little parental care (rselected species).
 Fewer, larger
offspring with higher
invested parental
care (K-selected
species).
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
Survivorship Curves:
Short to Long Lives
 The
way to represent the age structure of a
population is with a survivorship curve.



Late loss population live to an old age.
Constant loss population die at all ages.
Most members of early loss population, die at
young ages.
Survivorship Curves:
Short to Long Lives
 The
populations
of different
species vary in
how long
individual
members typically
live.
Figure 8-11
Late loss
Early loss
Age
Fig. 8-11, p. 169