Factors that limit distributionsDispersal

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Factors that limit distributions-Dispersal

• Chapter 4

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Geographic distributions are determined by
suitable habitats.

• The distribution of a species is its geographic
  range.
• Geographic range of a species can be studied at
  several spatial scales from large to small
• Spatial patterning (dispersion) will depend on
  the scale of the study

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Influence of Spatial Scale on Distribution Pattern
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Geographic Distributions

• If an organism can survive outside of its actual
  range (as determined by a transplant experiment),
  the question is why does it not normally occur
  there?
• Simplest explanation is the species does not
  disperse to these areas

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Dispersal

• Dispersal is defined as the movement or transport
  of individuals away from their place of birth
• Generally defined as within a population, not
  between populations (this would be immigration,
  emigration), but not all authors define it this
  way

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Problems studying dispersal

• Most species actual ranges are not well known
• Difficult to detect if it does occur
• Most species do not have marked individuals that
  are under study
• Is both an ecological process determining
  geographic range and a genetic process
  determining genetic structure of populations

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Genetic effects of dispersal

• Dispersal of individuals will introduce
  alleles-maintaining gene flow
• New populations established from dispersers may
  be biased sample-founder effect
• Can study using different genetic markers

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Barriers to long-range dispersal limit geographic
distribution.

• Introduced species often expand successfully into
  new regions
• 160 European starlings were introduced near New
  York City in 1890 and 1891 within 60 years, the
  North American population of starlings covered
  more than 3 million square miles

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Spread of an Invasive Species-Starling
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Introduced species

• Other examples of successful introductions
• dogs in Australia, pigs and rats in Pacific
  islands
• fast-growing pines and eucalyptus trees worldwide
• Zebra mussels, gypsy moths
• Chestnut blight reference 1, reference 2

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Recovery of overexploited species

• California Sea Otter
• Single population found in California
• Expanded to south 3.1 km/year, north 1.4km/year
• Why the difference?

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Three modes of dispersal

• Diffusion
• Gradual movement of a population across
  hospitable terrain for a period of several
  generations (Gypsy moth, chestnut blight)
• Jump Dispersal
• Movement of individual organisms across large
  distances followed by successful establishment of
  a population in the new area intervening areas
  are usually not suitable habitat
• Includes island colonization, human assisted
• Rare bee I found in Miami, may have jumped from
  Cuba/Bahamas

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Three modes of dispersal

• Secular dispersal
• Dispersal over evolutionary time, may result in
  speciation as natural selection creates different
  selection in different parts of the range

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Geological evidence of dispersal

• Tree movements northward after the end of the
  last ice age have been estimated using pollen
  cores
• Distance moved Dn vlogeR0
• D average dispersal distance, R0reproductive
  rate per generation, n number of generations
• Tree movement is several orders higher than
  measured distances
• What explains this?

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Geographic variation in Time-due to glaciation
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Empirical Studies of dispersal

• Nearly all studies show very local dispersal
• Most tree seeds fall within a few m of parent
• Only a few seeds make it any distance
• Difficulty is in detecting haphazard
  long-distance dispersal events that are spatially
  and temporally rare
• Rapid post-glacial advance of tree species seems
  to fit long-distance dispersal model

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Dispersal Distance Eelgrass Seeds-Experimental
Arrays
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Dispersal Curve for Eelgrass
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Testing barriers to dispersal

• Examining patterns or abilities of movement
  relative to hypothesized barriers
• Water may be a barrier to land animals
• Can experimentally test in lab or field
  experiments

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Introduced species and dispersal

• Many non-native species have been introduced
• Most cannot survive on their own
• Thus, dispersal is not a barrier
• Lack of data on failures means harder to show
  this is true
• Rule of tens
• 1 out of every 10 species imported will become
  introduced, 1 of every ten introduced
  established, and 1 of every ten established a pest

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Local dispersal mechanisms

• Many species have good local dispersal mechanisms
• Often young stages can establish but they do not
  survive for long
• Factors other than dispersal must limit these
  species

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Colonization/Extinction

• Natural experiments that wipe out entire areas of
  all life can reveal rates of colonization as well
  as extinction
• Krakatoa volcano in Indonesia well studied
  example
• Colonization proceeded rapidly but varied by life
  forms with more mobile forms more rapidly
  colonizing the area than more sluggish forms
• Mount St. Helens
• Dispersal of larger seeded species is limiting

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Arrival of plants on Krakatoa by dispersal mode
and of species
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Global distribution patterns

• Continental drift and geological events such as
  ice ages determine broad scale distribution
  patterns
• More evolutionary history rather than ecological
  patterns
• Dispersal explanations Organism dispersed across
  preexisting barriers, then underwent speciation.
  Hard to scientifically test.
• Vicariance speciation Species present
  everywhere, barrier forms later, then speciation.
  Can combine geological data on barrier formation
  with specific taxon phylogeny to test
  scientifically

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Continental Drift

• The continents are islands of low-density rock
  floating on the denser material of the earths
  interior and carried along by convection
  currents
• the movements of the continents over time are
  called continental drift
• These movements have two important ecological
  consequences
• positions of continents, ocean basins influence
  climate
• continental drift creates and breaks barriers to
  dispersal

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Continental Drift Mesozoic to Present

• In the early Mesozoic era, 200 Mya, continents
  formed a single giant landmass called Pangaea
• By 144 Mya (beginning of the Cretaceous period)
  the northern continents (Laurasia) had separated
  from the southern continents (Gondwana)
• at this time Gondwana itself was also breaking
  apart
• By the end of the Mesozoic era (65 Mya), South
  America and Africa were widely separated, and
  many other patterns were emerging.

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Positions of Continents in Time
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Routes of Dispersal
Time in MY, black broken routes, red new routes
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Consequences of Continental Drift

• Details of continental drift have yet to be
  resolved, but implications for evolution of
  animals and plants are clear for example
• the distributions of the flightless ratite birds
  (such as ostriches) are the results of connection
  between the southern continents that made up
  Gondwana
• these birds are descended from a common Gondwanan
  ancestor
• splitting of a widely distributed ancestral
  population by continental drift is called
  vicariance

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Evolution of flightless birds explained by
vicariance speciation
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Evolutionary advantages of dispersal

• Natural selection will favor those individuals
  who leave the most offspring
• Dispersal allows offspring to colonize new areas
  that are not crowded enhances offspring survival
  and reproduction
• Examples of non-dispersing species are few and
  tend to be found in very isolated habitats such
  as alpine peaks (ecological island) and oceanic
  islands

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Evolution of dispersal

• Fugitive species
• Extreme selection for high offspring dispersal,
  usually coupled with large offspring production
• Designed to allow organisms to specialize on
  rare, but highly productive habitats that vary
  both spatially and temporally
• Most weeds can be considered fugitives, would die
  out if fields were no longer cultivated due to
  poor competitive abilities.