Demography and Population Growth

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Demography and Population Growth

• Chapter 4

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Demography

• Quantitative description of a population
• Size, age, sex ratio, distribution

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Population

• Members of a species at a specific time and place
• Time usually defined by the population (fruit
  flies days, turtles many years)
• Place, physical boundary, can be difficult to
  define
• Must be an ecological or genetic discontinuity
  correlated to a physical barrier
• 2 difficulties distribution movement

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Population (continued)

• Genetically and ecologically defined populations
  do no necessarily coincide
• Genetically similarity in alleles
• Ecologically similarity in demography
• E.g. populations on both sides of ravine may be
  demographically very similar but genetically
  different, and vice versa for forest habitats

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Population Density

• of individuals per unit area
• Crude density regardless of habitat quality
• Ecological density density in appropriate area
• To measure density a direct census, or relative
  abundance, or proportion method

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• of animals marked at t1
• Total of animals in population
• of marked animals captured at t2
• Total of animals captured at t2

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Dispersion

• Dispersion Vs dispersal (distribution of
  organisms Vs movement away from home)
• Random
• Aggregated
• Hyperdispersed

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Dispersion

• Vagility ability to move relatively long
  distances
• Coarse-grained Vs fine-grained habitats
• Habitat is fine-grained for an organism if that
  organism has high vagility relative to the
  habitat (and vice versa for fine-grained)

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Age Structure

• Distribution of individuals among age classes
• Each individual has different probability of
  dying based on age
• Probability determines distribution of
  individuals among age classes
• Ways to determine age growth rings, teeth wear,
  scale rings, horn rings, etc
• Data can be used to make life tables
  survivorship curves

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Sex Ratio

• Sex ratio can be determined in of population
• Can be determined per age classification
  (primary, secondary, tertiary, quaternary)
• Primary at fertilization
• Secondary at birth
• Tertiary at sexual maturity
• Quaternary of adult population
• More difficult for plants

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Other factors

• Reproductive success (some males reproduce much
  more than others, or vice versa)
• Differential survivorship
• In reptiles, sex can be determined by
  environmental temperature
• Traits which help reproductive success may be
  detrimental to survivorship
• Some sexes can change within a lifetime

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Population Growth

• Net reproductive rate factor by which the
  population would increase if unchecked
• Exponential growth
• Discrete reproduction N(t) N0R0t
• Continuous reproduction dN/dt rN (or Nt
  N0ert)
• Populations cannot grow exponentially forever
  (limited resources)

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Population Growth

• Upper limit of population K, the carrying
  capacity
• A population stabilizes at K, or exceeds it, and
  then crashes
• If population stabilizes at K, growth is density
  dependent, which yields the logistic equation
  (logistic growth)
• dN/dt rN (K - N) or, dN/dt rN (1 N)
• K
  K

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