Factors Limiting Distribution: Dispersal Chapter 4

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Factors LimitingDistribution Dispersal
Chapter 4
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Dispersal

• The transport of animals to geographical areas
  not currently inhabited by that species.
  
• Simplest explanation as to why a particular
  species may not be located in a geographical
  are.
  
• If colonization is successful, dispersal will
  result in gene flow and thus affect the genetic
  structure of a population.
  
• Also result in a founder effect

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Example Zebra Mussel
Probably introduced from ship ballast water.
Spread throughout the majority of the Mississippi
drainage within 10 years.
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Example Gypsy Moth
Accidentally introduced by a French astronomer in
1868. Control programs ceased around 1900. Began
to spread again, and accidentally transported to
Michigan. Spread 21 km per year between 1966 and
1989.
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Example Chestnut Blight
Fungus that is lethal to Chestnuts.
First noticed in 1900, and apparently introduced
on nursery stocks from Asia. Search began in 1927
to find blight resistant trees.
Oak-chestnut forests have been replaced with oak
or oak-hickory forests.
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Example California Sea Otter
Thought to be extinct in 1911.
Small population found in 1914 was now
protected. Northern range spread about 1.4 km/yea
r and southern range spread about 3.1 km/year.
Southern otters move more as individuals and
northern suffering higher mortality?
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Three Modes of Dispersal

• Diffusion Gradual movement of a population
  across a hospitable terrain for a period of
  several generations.
  
• Jump Dispersal Movement of individual organisms
  across large distances of inhospitable habitat
  followed by the successful establishment of a
  population in the new area.
• Secular Dispersal Diffusion occurring in
  evolutionary time.

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Jump and Diffusion vs. Secular

• Most colonization's involve Jump dispersal first,
  followed by Diffusion.
  
• Secular diffusion occurs over geologic time.
  Although the geographic range is expanding,
  natural selection is causing migrants to diverge
  from the ancestral population.
• Not of immediate interest for ecologists working
  in ecological time
  

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Simple Diffusion Mathematically
Can this explain the spread of oak trees since
the last ice age?
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Oak Tree Dispersal Following Last Ice Age
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Measuring Tree Seed Dispersal
Seed traps can be use to determine seed dispersal
Distances are too small to account for dispersal
of trees after the ice age.
Answer to Reids paradox seems to lie in
haphazard, long range dispersal of seeds.
Colonization not driven by mean seed dispersal,
but extreme dispersal events. Wind, animals, John
ny Appleseed!
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Dispersal Can Be Affected by Barriers

• Freshwater organisms are prevented from
  dispersing by land and saltwater
  
• Local populations strongly affected by jump
  dispersal
  
• Water can be a barrier to some terrestrial
  animals
  
• Ruffed Grouse found only on three Michigan
  islands of the great lakes, all within 800 m of
  the mainland
  
• Palmer (1962) showed that these birds could not
  fly for more than 800 m cant colonize far
  islands by jump dispersal
  
• Artificial stockings have been successful
• Isnt it ironic that immobile trees colonized
  offshore islands that flight capable birds didnt!

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Is Dispersal The Only Limit To Distribution?

• Humans have moved many species around the globe
  often with disastrous consequences (think
  locally nutria, hydrilla and water hyacinth).
  
• Humans have allowed several species to bypass
  traditional geographic barriers
  
• However, many times it is not just
  inaccessibility that determines whether or not a
  species is found in a particular habitat.

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Terms for Introduced Nonnative Species
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How Successful Are Introductions?-actually, they
are usually failures
Overall, continental bird introductions are
successful about 10 30 of the time.
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Statistical Generalizations Tens Rule

• Williams and Fitter (1996) predicted that
• 1 species in 10 imported to a country becomes
  introduced
  
• 1 in 10 of introduced species becomes
  established
  
• 1 in 10 of the established species becomes a
  pest
  
• Of course exceptions occur
• Much of Hawaii has been cleared, so habitats have
  become unsuitable to native birds but not
  introduced ones
  

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Local Scale Dispersal

• Transport is rarely a limiting factor in plant
  dispersal
  
• Seeds/spores carried primarily by wind or
  animals
  
• Rumex crispus var. littoreus not limited by
  dispersion or seed-germination
  
• Wind dispersed seeds and spores usually colonize
  disturbed areas first
  
• Small animal are often dispersed by wind
• Spiders
• Mosquitos
• Salt marsh mosquitoes from LA found 74-106 km
  offshore!

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Colonization and Extinction Krakatau

• August 26, 1883 Krakatau exploded (25 km3)
• Two islands a few kilometers away were completely
  covered in ash
  
• The nearest island not destroyed by the explosion
  was 40 kilometers away

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Groups Colonizing Krakatau
Birds dependent on plant colonization
Most plants and animals probably colonized by
wind Large vertebrates may have arrived by floati
ng on driftwood rafts or possibly swam.
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Continental Drift Disjunct Distributions
Antarctic beech modern distribution
Dispersal before or after barriers were formed?
Continental drift takes some continents farther
apart, while bringing others closer together.
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Explanation Of Disjunct Distributions

• Dispersal explanations assumes organism
  dispersed across preexisting barriers (e.g.,
  mountains and rivers).
  
• Vicariance explanantions assume that a species
  was present on the entire area and subsequently
  was fragmented by the formation of barriers.

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Why Disperse?

• Pro Natural selection will favor those that
  disperse from a crowded area to an empty area.
  
• Con Most individuals that disperse die.

Two choices Stay at home and produce a few
descendants or take a chance to colonize a new
area and leave many descendants.
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Examples of Dispersal Abandonment

• Flightless birds on islands
• Insect Species
• Subantarctic islands 76 of the insects are
  flightless
  
• Ecologogical islands (alpine zone of tropical
  mountains)

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Fugitive Species

• Devote most of their effort to dispersal
• Weeds of plant (dandelion) and animal (water
  boatman) kingdoms
  
• Colonize temporary habitats
• Grow almost predominately on disturbed areas

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Summary

• Three methods of dispersal diffusion, jump,
  secular
  
• Transplant experiment inaccessibility
• Tens rule (most introduced sp. die out)
• Dispersal rarely limits local distribution of
  plants and animals
  
• Dispersal is adaptive if it allows successful
  colonization
  
• Fugitive species