Ecology: Lecture 8

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Ecology Lecture 8

• Intraspecific Competition

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Population growth rate (dN/dt) as a function of
population size (N)

• Intraspecific competition is one of the
  density-dependent factors that decreases
  population growth rate at higher population
  densities (especially gtK/2)

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What is intraspecific competition?

• DEFINED
• Struggle with members of ones own species to
  gain needed resources when those resources are
  limited either in abundance or access.
• IMPORTANCE
• Affects the birth, death and growth of
  individuals, and thus of the population as a
  whole.
• Key element of the process of natural selection.

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Scramble/exploitative competition

• DEFINED
• Each individual has approximately equal access to
  the limited resource ? reduction of fitness is
  approximately equal
• Scramble competition so severe that none of the
  competitors get enough ? all die prior to
  reproduction Blowfly example Fig. 12.1

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Example blow fly experiments Fig. 12.1

• Experimental design (key aspects)
• How can scramble competition lead to oscillation
  of the population?
• What causes the severe decline?
• Why doesnt the entire population die off?
• What causes the rapid rise?

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Scramble/exploitative competition

• Exploitative competition all individuals have
  approximately equivalent decreases in fitness,
  but may still survive/reproduce.
• Similar to, but less severe than, scramble
  competition.

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Contest/interference competition

• DEFINED
• Unequal access to a resource ? only fraction of
  the population suffers serious deleterious
  effects.
• Individuals with particular characteristics may
  be favored for growth and reproduction, leading
  to natural selection of those traits
• Example Competition among male elephant seals
  for beachmaster status ? access to females.

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Effects of intraspecific competition on growth
and fecundity

• Example 1 Effects of population density on frog
  (Rana tigrina) growth rates Fig. 12.2
• Compare growth curves of populations reared at
  different densities
• High density also reduces chances of successful
  metamorphosis.
• Example 2 Effects of population density on harp
  seal growth Fig. 12.3
• Minimum age of sexual maturity increases with
  population size
• Note that time actually goes backwards on the
  graph.

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• Fig. 12.1, 12.2, and 12.3 were not available as
  PowerPoint, but will be shown in class. Be sure
  you understand them!

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Fecundity vs. density (harp seals)

• Number of seal births is a function of population
  density.
• Note the time lag (x-axis)
• Has the population increased or decreased over
  time?

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Fecundity vs. density (elk)

• Is the relationship similar to that for the
  seals?
• Note again the built-in time lag

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Fecundity vs. density(bison)

• How does the graph for bison compare to that for
  seals and elk?
• Fowlers hypothesis
• Large mammals will maintain a high population
  growth rate beyond K/2 (to near K) and then
  overcompensate.
• Relate to long response time lag (w)

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Overshoot of K followed by crash (reindeer herd
on St. Paul I., Pribolof Islands)

• Possibly explained by Fowlers hypothesis/ long
  time lag (w)

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Role of stress in mediating density-dependent
responses

• Stress hormone secretion (especially
  adrenocorticoid hormones) may increase at high
  densities, affecting many body systems (gonads,
  immune systems, etc...)
• Increases in spontaneous abortion in females
• increased susceptibility to disease

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Role of stress in mediating density-dependent
responses

• Pheromones from older, mature members of a
  population may suppress reproduction in younger
  members
• Example Studies in wild house mice
• Basics of experiment with female urine (be able
  to explain!)
• Controls?
• Key results
• How did urine from high-density mature females
  affect the juvenile females?
• What form of competition is this?
• Basics of experiment with male urine
• How did male urine affect females in the
  low-density population?
• What might you expect the same urine do to
  juvenile males?