BIOL 115 Environmental Science

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BIOL 115Environmental Science

• Fall Quarter 2007
• Professor
• Dr. E. Binney Girdler

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The major causes of biodepletion we have studied
thus far can be called systematic pressures

• Habitat destruction, fragmentation, and
  degradation
• Exotic species
• Overexploitation

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Problems with Small Populations

• Systematic pressures reduce abundance
• Species decline as population by population goes
  extinct, individual by individual dies
• Once a population is small enough,extinction may
  be the result of chance events

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Consider an individual organism

• Death is inevitable when it occurs it will
  always be for some reason
• Exposure, lack of food, disease, aggression,
  accident, age
• Often involves an element of chance, like being
  hit by a car, or being born during an extreme
  drought

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In real populations
In theory
N2 N1 (b d)

• Individuals do not produce the average number of
  offspring, and death is probabilistic, too
• This demographic variation can have real effects
  on the fate of a small population

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An exercise in chance

• Why rich people get richer and poor people go
  broke

Don't go to Vegas with just a little bit of money
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An exercise in chance

• Two populations one with five individuals, one
  with twenty individuals

• For each time step, or year, 50/50 chance of an
  individual surviving and reproducing, or dying

• In a strictly deterministic world, both
  populations should remain steady.
• Formulate a hypothesis about the likelihood of
  persistence for the two populations in a chancy
  world.
• In 20 years, which population is more likely to
  still be around?

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Computer simulations of populations
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What if your population had a positive growth
rate?

• A population with a single asexually reproducing
  individual
• 60 chance of survival
• If it survives, can produce two individuals
  before dying

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If there were 1000 individuals, the population
would almost certainly increase exponentially
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• But with our population of one, theres a very
  real chance of extinction, even with a positive
  growth rate.

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Demographic processes affected by chance events

• Age structure (how many adults, juveniles)
• Sex ratio
• Social structure/ability to find mates
• Variability in birth and death rates
• Spatial structure of the population

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Real examples of sex ratio problems in small
populations

• Dusky seaside sparrow last five individuals were
  all male
• Lakeside daisy last three individuals were of
  the same self-infertile mating type

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An example of chance factors at work

• Once ranged from New England to Virginia
• Steady decline as Europeans encroached (habitat
  loss, exploitation)
• 1876, only one population left on Marthas
  Vineyard
• 1900, less than 100 individuals

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An example of chance factors at work

• 1907, reserve set up, predator control
  implemented
• 1916, population had increased to more than 800
  birds, but a fire swept through breeding habitat

• 1917, heavy predation by an unusually high
  density of goshawks left fewer than 150 birds

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An example of chance factors at work

• 1920, numbers increase to about 200, but disease
  (transmitted by introduced domestic turkeys)
  reduced population to less than 100
• 1928, 13 birds left, 2 females
• 1930 A single bird remaining
• 1932 None surviving