BIO 402: Field Biology

ISLAND BIOGEOGRAPHY

DEFINITIONS

Biogeography = the study of a species’ distribution in space and time

Factors limiting and influencing species distribution:

• physical and biological factors limit dispersal
• species distribution can change with time

Islands provide an unusual barrier to dispersal

• large distance of unsuitable habitat
• oceans are the prime example of these barrier, but there are others (eg. rivers, mountains)

EARLY STUDIES ON ISLAND BIOGEOGRAPHY

Studies off the coast of California on small islands
    compared bird species present between 1917 & 1968
    noted a number of interesting patterns:
      a) the larger the island, the greater the number of species present
      b) the greater the distance from the mainland, the fewer number of species
      c) a number of additions (immigrations) occurred between 1917 to 1968
      d) a number of losses (extinctions) were recorded between 1917 to 1968
      e) the total number of species stayed about the same 1917 to 1968

Originally thought the changes were due to change in the islands
    only observed few changes on the islands
      a) a fire in 1959 on Santa Barbara Island
      b) poor sampling of some islands in 1917

Could not understand the extinctions and immigrations
    small number of individuals?
    competition with new immigrants?

Suggested that other extinction and immigrations may have occurred

Wilson & Simberloff studied islands in the Florida Keys in 1969

Fumigated a small island to kill all insects

• recorded initial species present
• eliminated all species
• counted species found immigrating for two years
• had a control island (no fumigation) which was also checked

Findings were similar to those of California investigation

• fewer species than on mainland
• large number of extinctions from month to month
• large number of immigrations
• by the end the same number of species as before
• only 6 months to get to the pre-fumigation numbers, then remained constant

EQUILIBRIUM THEORY OF ISLAND BIOGEOGRAPHY

Information from studies to formulate a theory of how an island is colonized:

1. initial immigration of species
   only a portion of those species on the mainland
2. immigration rate decreases as number of species present increases
3. extinction rate increases from zero with time
   a) few number of individuals of a species
   b) species decrease in number due to competition
4. eventually get number immigrations = number extinctions to produce equilibrium

Several interesting variations on basic theme noticed:

• Effect of island distance from mainland
  greater distance means fewer immigrants which can make journey
  no effect on extinction rate
  results in decreased number of species on island further away
• Effect of island size
  larger island with more habitats
  more immigrants can find & settle on the island  AND less likely to have extinctions
  result are more species on larger island

Example at Mt. Krakatau (volcanic island in East Indies)
    nearest neighbor island 25 miles away
    all life destroyed on island with explosion in August 1883

    Recolonization charted by scientists
      9 mo – 1 spider
      3 yrs – 1 species blue-green algae
              11 species ferns
              15 species flowering plants
      10 yrs – coconut trees
      25 yrs – 263 species animals
             dense forest
      50 yrs – 47 species vertebrates (36 birds, 5 lizards, 3 bats, 1 rat, 1 crocodile, 1 python

APPLICATIONS OF ISLAND BIOGEOGRAPHY TO HABITAT ISLANDS

Ocean is simply an area in which species cannot live (unsuitable habitat)
    to migrate, a species must be able to pass over or through unsuitable area

 Mountain islands in the Andes of South America:
    many species are restricted to higher elevations
    same type of distribution patterns exist as with oceans

 Carolina Bays – depressions in the coastal plain of NC, SC, & GA
    species restricted to depressions (wetter & cooler)