Ecology: Lecture 8

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Ecology Lecture 8

• Intraspecific Competition 2

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Role of dispersal in mediating density-dependent
responses

• Potential benefits to dispersers (individual
  fitness)
• Higher growth rates. Why?
• Higher fecundity. Why?
• Outbreeding ?
• New genetic combinations increased
  heterozygosity of offspring
• Always a benefit?
• Avoid competition with kin
• Combined inclusive fitness may be higher
• What does inclusive fitness include?

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Role of dispersal in mediating density-dependent
responses

• Potential costs to dispersers (individual
  fitness)
• Greater risk in movement (? food water, ?
  predation, unfamiliarity with terrain)
• Inability to find suitable habitat
• Individual not as well-adapted to new environment
• Outbreeding depression (disrupt co-adapted genes)
• Offspring less adapted to the environment
• Loss of kin associations
• Loss of adaptive social traditions

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Role of dispersal in mediating density-dependent
responses

• Benefits of dispersal depend upon current level
  of competition
• In some species, young adults cannot find
  breeding territories or mates unless they
  disperse (i.e. male lions)
• Some species adjust dispersal depending upon
  population density
• Example water striders (Gerris spp.)

Wingless
Winged
Images from cirrusimage.com
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Role of dispersal in mediating density-dependent
responses

• Effects on populations
• Can establish new populations within a
  metapopulation
• ? vulnerability to extinction. (Why?
• How will the genetic make-up of the new
  population compare to the original population?
• Founder effect
• But what if dispersers to new location arrive
  from multiple source populations?

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Types of dispersal

• Pre-saturation dispersal
• At what population densities does this type of
  dispersal take place?
• Below K/2 (density-independent)
• Which individuals disperse?
• Both genders
• Healthy individuals in their prime
• Genetically-based subset?

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Types of dispersal

• Saturation dispersal
• At what population densities does this type of
  dispersal take place?
• Above K/2 (density-dependent)
• Which individuals disperse?
• Juveniles and subdominants (Why?)

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Sink habitats

• Organisms permanently removed from the source
  population
• Unable to sustain itself (dgtb)
• Example Conch populations in Florida
• Is it possible for sink habitats to have high
  population densities?

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Queen conch metapopulations
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Role of social interactions in mediating
density-dependent responses

• Social dominance determines who will have the
  best access to limited resources
• What type of competition is this and why?
• Contest competition dominants achieve
  relatively high levels of fitness at the expense
  of submissives when resources are limited.
• How can population size be regulated by social
  dominance?
• Social dominance is more pronounced for
  high-density populations (Example wolf
  populations)

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Regulation of population size social dominance
in wolves

• Packs consist of relatives, with male and female
  hierarchies
• The alpha (and sometimes beta) male mates with
  the alpha female. Only the alpha female has
  young.
• All pack members cooperatively raise young.
• Packs become larger when population densities are
  high
• Fewer alpha and beta individuals
• Fewer offspring
• Packs break apart when population densities are
  low?
• More alpha and beta individuals
• More offspring produced at low density.

Photo courtesy of NOAA