Dispersal

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• Dispersal
• Movement of individuals out of a population
• 1. Range expansion
• 2. Gene flow if individuals are assimilated into
  a different population.
• Gene flow
• May be necessary to rescue some populations from
  extinction.

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Habitat patches and effects on dispersal
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Greater Prairie Chicken
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Habitat (blue area) loss
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.
Expansion of Africanized bees
  Source Winston 1992
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.
Northward expansion of tree species
     Source Davis 1981
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Numerical Response

• Increase in predator density in response to
  increasing prey density.
• Recruitment increase in density
• 1. More predators being born
• 2. Immigration of predators into the area
• e.g., Barrow Alaska
• Predatory birds
• Pomarine Jaeger
• Snowy Owl
• Short-horned Owl
• Prey Brown lemming

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• 1951
• 1-5 lemmings/acre
• no breeding by predators
• 1952
• 15-20 lemmings/acre
• Pomarine Jaeger and Snowy Owl reproduced
• 1953
• 70-80 lemmings/acre
• All three species bred
• Jaegers four times more abundant than in 1952
• Snowy owl density did not increase, but each pair
  fledged more young
• 2-4 eggs/nest in 1952
• up to 12 eggs/nest in 1953

Types of recruitment?
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Numerical response by predators
     Source Korpimäki and Norrdahl 1991