Chapter 8
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Chapter 8. Population Ecology. Southern Sea Otters: Are They Back from the Brink of Extinction?. 1 million befor e 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.

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Chapter 8

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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

  • controlsea urchins and other kelp-eating organisms.

  • Kelp forests provide habitat & prevent shore erosion

Figure 8-1


  • Populations change

    • Distribution

    • Numbers

    • Age structure

    • density

  • changes occur based on resource distribution & environmental conditions

Figure 8-2


  • Patterns occur based on resource distribution.

Figure 8-2

  • 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 decrease through deaths and emigration

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 vary in capacity for growth

    • Reproduce early & often = high potential

    • 1 fly = 5.6 trillion in 13 months

  • No 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



Carrying capacity (K)

Population size (N)





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


Carrying capacity

Number of sheep (millions)


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




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)


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).


  • 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 (r-selected species).

  • Fewer, larger offspring with higher invested parental care (K-selected species).

Figure 8-9

Carrying capacity


K species;


K selection

Number of individuals

r species;


r selection


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

r-Selected Species



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



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

Constant loss

Percentage surviving (log scale)

Early loss


Fig. 8-11, p. 169

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