Population Ecology

1
Chapter 8

• Population Ecology

2
Chapter Overview Questions

• What are the major characteristics of
  populations?
• How do populations respond to changes in
  environmental conditions?
• How do species differ in their reproductive
  patterns?

3
Core Case Study Southern Sea Otters Are They
Back from the Brink of Extinction?

• They were over-hunted to the brink of extinction
  by the early 1900s and are now making a
  comeback.

Figure 8-1
4
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 kelp-eating organisms.

Figure 8-1
5
POPULATION DYNAMICS AND CARRYING CAPACITY

• Most populations live in clumps although other
  patterns occur based on resource distribution.

Figure 8-2
6
(a) Clumped (elephants)
Fig. 8-2a, p. 162
7
(b) Uniform (creosote bush)
Fig. 8-2b, p. 162
8
(c) Random (dandelions)
Fig. 8-2c, p. 162
9
Changes in Population Size Entrances and Exits

• Populations increase through births and
  immigration
• Populations decrease through deaths and
  emigration

10
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.

11
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.

12
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
13
Environmental Resistance
Carrying capacity (K)
Population size (N)
Biotic Potential
Exponential Growth
Time (t)
Fig. 8-3, p. 163
14
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
15
Overshoot
Carrying capacity
Number of sheep (millions)
Year
Fig. 8-4, p. 164
16
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
17
Population overshoots carrying capacity
Population Crashes
Number of reindeer
Carrying capacity
Year
Fig. 8-6, p. 165
18
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 earths carrying
  capacity for humans.

19
How Would You Vote?

• Can we continue to expand the earth's carrying
  capacity for humans?
• a. No. Unless humans voluntarily control their
  population and conserve resources, nature will do
  it for us.
• b. Yes. New technologies and strategies will
  allow us to further delay exceeding the earth's
  carrying capacity.

20
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 populations 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

21
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.

22
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
23
Hare
Lynx
Population size (thousands)
Year
Fig. 8-7, p. 166
24
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
  1920s.
• 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).

25
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.

26
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
27
Reproductive PatternsOpportunists and
Competitors

• Large number of smaller offspring with little
  parental care (r-selected species).
• Fewer, larger offspring with higher invested
  parental care (K-selected species).

Figure 8-9
28
Carrying capacity
K
K species experience K selection
Number of individuals
r species experience r selection
Time
Fig. 8-9, p. 168
29
Reproductive Patterns

• r-selected species tend to be opportunists while
  K-selected species tend to be competitors.

Figure 8-10
30
r-Selected Species
Cockroach
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
31
K-Selected Species
Saguaro
Elephant
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
32
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.

33
Survivorship Curves Short to Long Lives

• The populations of different species vary in how
  long individual members typically live.

Figure 8-11
34
Late loss
Constant loss
Percentage surviving (log scale)
Early loss
Age
Fig. 8-11, p. 169