Island Biogeography

1
Island Biogeography

• Chapter 19

2
Formation of a New Island

• Island of Krakatau
• Massive volcanic eruption in 1883.
• Destroyed two-thirds of island. Also, eradicated
  life on neighboring islands of Rakata, Sertung,
  and Panjang.

3
Formation of a New Island

• 1930, a new island was formed from volcanic
  activity (Anak Krakatau).
• Recolonization studies
• Nine months after 1883 eruption first colonist
  of Rakata was a spider.
• 1896, 11 species of ferns and 15 species of
  flowering plants. 16 species were dispersed by
  wind and another 8 by sea.

4
Formation of a New Island

• Recolonization of Rakata was greatly affected by
  how well plants were able to disperse.
• Early plant communities were dominated by
  grasses.
• 25 years later, plant communities were dominated
  by Cyrtandra bushes.
• In the 1920s, the plant communities were
  dominated by Neonauclea trees.

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Formation of a New Island

• Initially, wind and sea dispersed plants were
  more easily dispersed than those that required
  animals.
• After 40 years, animal dispersed species became
  as common as wind and sea dispersed.

6
Formation of a New Island

• Recolonization of islands was based on the size
  of the island and the distance of the island to
  source of colonists, and the ability of an
  organism to disperse.

7
Theory of Island Biogeography

• Equilibrium theory of insular zoogeography -
  first comprehensive theory of island
  biogeography Robert MacArthur and E.O. Wilson
  (1963, 1967).
• The number of species on an island tends toward
  an equilibrium number.

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Theory of Island Biogeography

• This equilibrium number is the result of a
  balance between the rate of immigration and the
  rate of extinction.
• Rate of immigration is highest when there are no
  species present on the island.
• Rate of extinction is low at the time of first
  colonization.
• Eventually, rate of extinction will equal rate of
  immigration.

9
Theory of Island Biogeography

• Both immigration and extinction lines should be
  curved.
• Species arrive at an island at different rates.
• Extinctions rise at accelerating rates.
• As more species arrive, competition increases.
• r-selected species arrive first (poor competitors
  followed by K-selected species (better
  competitors).

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Theory of Island Biogeography

• The equilibrium number of species is determined
  only by the islands area and position, which
  influences the rate of immigration and extinction.

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Theory of Island Biogeography

• Equilibrium is dynamic hence following
  colonization of an island
• Number of species remains constant.
• Extinction immigration.
• Results in a turnover of species.

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Theory of Island Biogeography

• Major modifications to MacArthur and Wilsons
  theory of island biogeography.
• Target effect (Whitehead and Jones 1969)
• The rate of immigration depends on an islands
  size.

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Theory of Island Biogeography

• The rescue effect (Brown and Kodric-Brown 1977).
• The distance from an island to a source pool of
  potential colonists affects both rate of
  extinction and rate of immigration.

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Theory of Island Biogeography

• Target and rescue effects complete MacArthur
  model.

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Theory of Island Biogeography

• Concept of an island
• Patches of particular habitat on continents are
  viewed as islands in a sea of other unsuitable
  habitat.

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Theory of Island Biogeography

• Strength of MacArthur-Wilson model generated
  falsifiable predictions.
• Prediction 1 the number of species should
  increase with increasing island size.

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Theory of Island Biogeography

• Prediction 2 the number of species should
  decrease with increasing distance of the island
  from the source pool.

18
Theory of Island Biogeography

• Prediction 3 the turnover of species should be
  considerable the number of species on the
  island might remain the same, but the identities
  of those species should change.

19
Species-Area Effects

• Oceanic islands - Studies of biogeography Lesser
  Antilles
• Islands enjoy a similar climate, surrounded by
  deep waters, and no historical connections to the
  mainland.

20
Species-Area Effects

• Ricklefs and Lovette (1999) summarized species
  richness for birds, bats, reptiles amphibians,
  and butterflies over 19 islands that varied in
  area (13 1,510 km2).

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• Significant relationship between area and
  richness.

22
Species-Area Effects

• Habitat islands
• James Brown (1978) - mountain ranges of the Great
  Basin.
• Mountain ranges are essentially isolated from one
  another.
• Significant relationship between species and area
  for mammals and birds.

23
Species-Area Effects

• As larger areas are sampled, fewer new species
  are added on continents than on islands.
• Continents have more transient species.

24
Species-Area Effects

• In this study, the results for mammals were
  consistent with island results, while the results
  for birds showed less of an effect.

25
Species-Area Effects

• Species as islands
• Species of host plants act as islands in a sea of
  other vegetation for the herbivores that eat from
  the plants.

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Species-Area Effects

• Elaborated by Donald Strong (1974)
• Found a speciesarea relationship between
  geographical area of distribution of British tree
  species and the number of insect herbivore
  species.

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Species-Area Effects

• Entire island of Great Britain was divided up
  into 10-km2 grids.
• Area the tree occupied in Britain was determined.
• Number of insect herbivores per species of tree
  was determined.

28
Species-Area Effects

• Reasons for a species-area relationship (Hart and
  Horwitz 1991).
• Extinction rates are greater on small islands.
• The passive effect of increased sampling effort
  in bigger areas increases the number of rare
  species found.
• Speciation may be more likely in bigger areas, an
  explanation also given for greater species
  richness in the tropics.

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Species-Area Effects

• Larger areas contain more core areas, which are
  less affected by disturbances.
• Perimeter areas contain more species that are
  sensitive to these disturbances.
• The species-area relationship may more likely be
  the result of an increased diversity of habitats
  on large islands than just an increase in area
  relationships.

30
Species-Area Effects

• Larger areas often contain greater diversity of
  habitats.
• Barry Fox (1983) investigated the relationship
  between species, area, and habitat diversity in
  Australian mammals.
• Classified habitats into seven broad types.
• Larger areas include more types of habitats.

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• Number of mammalian species is well predicted by
  area.

32

• However, species richness was better predicted
  from the number of habitats than from area.

33
Species-Area Effects

• Dan Simberloff (1976a,b) investigated the effect
  of area alone on the richness of species.
• Chose habitats that do not change as you sample
  bigger islands.
• Studied islands of pure mangroves of varying size
  in the Florida Keys.
• Collected every species that fed on the islands.

34
Species-Area Effects

• Reduction in area caused a reduction in the
  richness of invertebrate species.
• Area of islands was reduced experimentally.
• Seven months later, the insects became
  reestablished at equilibrium.
• Insect densities dropped on all experimental
  islands.

35
The Effect of Distance on Island Immigration

• MacArthur Wilsons best evidence for the effect
  of distance on island immigration came from a
  study of the numbers of land and freshwater bird
  species on four groups of islands.

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The Effect of Distance on Island Immigration

• The relationship between area and number of
  species is clear.
• There is also a distinct effect of distance
  nearer islands support more species.

37
The Effect of Distance on Island Immigration

• Jared Diamond (1972) relationship between
  distance and number of species.
• Tabulated land birds on islands close to the
  source area (New Guinea), and assumed these
  islands had 100 of the available birds.

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• He documented drop-off in species with increasing
  distance from New Guinea.

39
The Effect of Distance on Island Immigration

• Degree of saturation richness of bird species as
  a proportion of the number found on New Guinea.
• Strong decline with increasing distance.
• Supports MacArthur-Wilsons predictions.

40
Species Turnover

• Francis Gilbert (1980) investigations of
  turnover.
• Found 25 investigations to demonstrate turnover
  determined that most of them suffered from fatal
  flaws.
• Methodology, statistics, or quality of data.

41
Species Turnover

• Ex. Jared Diamond (1968) studied birds of
  Californias Channel Islands National Park.
• Compared his list to that of A. B. Howell (1917).
• Diamond reported that 5-10 species per island
  were no longer present, but just as many species
  not listed by Howell had apparently colonized the
  island indicating turnover.

42
Species Turnover

• Results were challenged Lynch and Johnson (1974)
  pointed out that Howells list was not exhaustive
  and just a summary of all known breeding records
  (some as old as 1860).
• Comparing old list with new lists can be
  problematic.

43
Species Turnover

• Simberloff and Wilson (1969, 1970) only study of
  turnover with any merit.
• Censused small (11 to 25 m in diameter) red
  mangrove islands in Florida Keys for all
  terrestrial arthropods.

44
Species Turnover

• Fumigated their experimental islands with methyl
  bromide to kill all arthropods.

45
Species Turnover

• Periodically after fumigation, they censused all
  islands for several years.
• After 250 days, most islands had similar number
  of arthropod species that they began with.
• Supporting MacArthur-Wilson theory.

46
Species Turnover

• Colonization and extinction rates were observed.
• Colonization rates during the first 150 days were
  higher on nearer islands than far islands.
• Supporting MacArthur-Wilson theory.
• Calculated rates of turnover were very low (1.5
  extinctions per year).
• Data was weak support for MacArthur-Wilson theory
  of turnover .

47
Species Turnover

• Same species returned to island.
• Indicates the existence of biological processes
  that shape the final community structure the same
  way every time the island is recolonized.
• Contrary to the theory of biogeography.
• Treats the dynamics of different colonizing
  species as equivalents.
• Community properties unimportant.

48
Species Turnover

• Conclusion
• Turnover involves only a subset of transient or
  unimportant species, with more important species
  becoming permanent after colonization.

49
Species Turnover

• Take home message turnover rates are low, which
  gives little support to this part of
  MacArthur-Wilson theory.

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Theory of National Park Design

• Shape, design and management of nature reserves.
• Centered on island biogeography theory, which
  suggests that large parks hold more species than
  smaller ones.

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Theory of National Park Design

• International Union for Conservation of Nature
  and Natural Reserves (IUCN) stated that refuge
  criteria and management practice should be based
  on the equilibrium theory of island biogeography.
• Recommendations are on shaky ground.
• Large areas cost a lot of money.

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Theory of National Park Design

• Which is better, single large areas or several
  small ones?
• Single large preserves may buffer populations
  against extinction.
• Many studies show that multiple small sites
  contain more species (broader range of habitats).

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Theory of National Park Design

• Fauna were shown to be richer in collections of
  small national parks than in large parks.
• Smaller parks are better for maintaining
  diversity.
• Implications for future land purchases.

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Summary

• Island biogeography theory predicts that the
  equilibrium number for species on an island is
  determined by a balance between immigration of
  species onto that island and extinction of
  species already there.

55
Summary

• The theory suggests that the number of species is
  determined by an island's size and position
  relative to a source pool of colonists.
• Extinction should increase on small islands,
  because of their smaller populations, and
  immigration should decrease on far islands,
  because colonists have a difficult time reaching
  distant places.

56
Summary

• Island biogeography theory also suggests that
  there is much turnover on islands as new species
  arrive and old ones become extinct.
• There is little evidence, however, to support
  this prediction.
• Most turnover that has been documented suggests
  that rates of turnover are low and center mainly
  on transient species.

57
Summary

• Island biogeography theory may be applied to
  "habitat islands" as well as real islands.
• In the relationship between species richness and
  area, the slope of the line may be steeper for
  true islands than habitat islands and steeper for
  poor dispersers like mammals than for good
  dispersers like birds.