Island Biogeography

1
Island Biogeography
2
Why study Islands?

• First biologists and geographers studied them
  like Wallace (East Indies), Darwin (Galapagos
  Islands) and Hooker (Southern Ocean).
• Natural experimental plots which offer
  differences in sizes, number of species,
  isolation, number of predators.
• Interaction much less complex than in mainland
  habitats.
• Due to their isolation evolutionary processes
  work at different rates
• Little or no gene flow to dilute the effect of
  selection and mutation causing a very high level
  of endemism

3
Why study Islands?

• Depending on scale and dispersal ability many
  habitats can be Islands (lakes, mountaintops,
  etc.)
• Islands can serve as natural field laboratories
  to study the relationship between area and
  species diversity
• Part of unintentional experiments are habitat
  loss and introductions of invasive species by
  humans, often detrimental consequences
• Only with a better understanding of species-area
  relationships can we design optimum conservation
  areas

4
What types of islands are there

• Oceanic islands which are located over oceanic
  plates and have never been connected to the
  continental shelf
• Continental shelf islands which are part of the
  continental shelf and can be connected to the
  mainland during periods of lower sea level
• Habitat islands distinct patches of terrestrial
  habitat surrounded by very different habitats but
  not water
• Non-marine islands which are somewhere between
  habitat and continental shelf islands in their
  level of isolation

5
Natural disturbances of islands

• Any relative discrete event in time that removes
  organisms and opens up space which can be
  colonized by individuals of the same or a
  different species
• Disturbances can be short term and frequently
  reoccurring like high winds or high rainfall
• Some disturbances like ENSO events and hurricanes
  occurring every decade or more with larger
  impacts on islands
• Other events occur only between 100 -1000 years
  for example volcanic eruptions, tsunamis or
  earthquakes

6
Implications of small founding populations

• Typically the number of organisms arriving by a
  chance event on a remote island is small
• Small founding populations containing only a
  subset of the source populations biodiversity
  can cause a genetic bottleneck
• Studies on Hawaiian fruit flies suggest that
  following the arrival of a single female with
  eggs on one of the islands, strong selection for
  females with less strict mate selection genes
  were more successful
• Leading to a significant shift in gene
  frequencies allowing better adaptation to the new
  environment (Carson 2002)

7
Implications of small founding populations

• The reduced genetic diversity in the founder
  population can also give rise to random genetic
  drift
• Genetic drift by can lead to significant changes
  in a species genetic makeup even without further
  adaptation

8
Giants and dwarfs

• The Galapagos and Indian Ocean tortoises were
  long regarded as typical island giants, but there
  have been large mainland species, only many are
  extinct due to humans
• But a study on insular species of mammals found
  that 85 of island rodents are larger, possibly
  due to the absence of predators (Foster 1964,
  Arnold 1979)

9
Giants and dwarfs

• On several islands in the Mediterranean dwarf
  hippopotami, elephants and deer existed several
  thousand years ago (Reyment 1983).
• The record is the Maltan elephant which stood
  1.5m shoulder height (Lister 1993)
• The untested hypothesis is that on small islands
  there are less resources available for large
  herbivores and often no predators, therefore size
  reduction is an advantage
• Maybe even human dwarf species Homo florensis on
  the Island of Flores (Brown et al 2004)

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Giants and dwarfs

• Three hypothesis for gigantism of island species
  (Schwaner Sarre 1988)
• 1. Predation hypothesis either there is
  selective release if no predation occurs or there
  is selective advantage to escape a window of
  vulnerability
• 2. Social-sexual hypothesis due to high
  densities that occur among island populations,
  intraspecific competition among males and females
  selects for larger body size
• 3. Food availability hypothesis increase in the
  mean and variance in food supply/demand ratio
  selects for giants

11
Loss of disperseability

• An interesting aspect of many species which
  dispersed to islands is, that in many cases they
  lost their dispersal ability afterwards
• Many birds became flightless, e.g. Aldabran
  rails, Dodos, Kakapo
• Plants lost their ability of wind dispersal on
  near shore islands in BC (Cody and Overton 1996)
  and elsewhere
• Flies lost their wings on Tristan da Cunha and
  Gough islands elsewhere wing sizes are reduced
• Original theory was this occurred due to
  preventing wind loss particular in insects, but
  Roff (1990,1994) found no clear relationship.

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Ecological release on islands

• Due to reduced competition or from other
  interacting organisms, like predators leads to
  two main changes in newly arrived species
• The loss of now unnecessary features (defensive
• traits, bold pattering, flight loss in many
  birds)
• Examples are the Solomon Island rails which lost
  bold patterning and the ability to fly (Diamond
  1991)
• Many birds also reverted to simpler song patterns
• (Otte 1989)
• Unfortunately many species also lost all fear of
  humans

13
Ecological release on islands

• The second form of release is from close
  competitors, allowing the colonist to occupy not
  only different niches but also a wider array than
  its ancestral form (Cox Ricklefs 1977)
• Its an important part for many scenarios of
  island evolution (e.g. adaptive radiation)
• Examples are the Fijian fruit bats, that are more
  diurnal on islands without predatory eagles
  (Lomolino 1984)
• Also the meadow vole is indiscriminate of habitat
  type on islands without predators (Lomolino 1984)
• Nesting sites of several bird species on the
  Orkney Islands shifted from cliffs and trees to
  shrubs and flat ground

14
Adaptive radiation

• Most well known examples are the Galapagos
  finches and the Hawaiian honey-creepers
• The availability of empty niches is very
  important to adaptive radiation, allowing the
  diversification which sometimes leads to new
  species
• There are also cases of non-adaptive radiation
  like the land snail genus Albinaria on the Island
  of Crete, which diversified without occupying
  different niches (Gittenberger 1991)

15
Island endemics

• Many endemics to islands used to have a much
  wider distribution, but were replaced in other
  habitats, hence not all endemics have evolved in
  situ (palaeo-endemics)
• One example is the St Helena Ebony originates
  from a more widespread species 9 million years
  ago. Since then the family on the mainland has
  developed away from this species (Cronk 1987)

• Whereas species evolved on islands are called
  neo-endemics
• The issue whether palaeo-endemics are more
  important for conservation due to a higher
  contribution to global biodiversity

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Island endemics

• The number of plant species endemic to the
  islands below (36,500) contribute 13.8 of the
  worlds higher plant species
• About 7,000 of these are only found within a
  single island or island archipelago
• The percentage of endemics are the highest for
  ancient continental islands like Madagascar and
  New Zealand
• Islands contribute a
  disproportionate amount
  for their land
  area to global
  plant biodiversity

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Island endemics

• Land snails only 8 archipelagos account between
  7.7-9.0 of the world land snail species. In
  particular larger islands with higher elevation
  harbour many species (Groombridge 1992)
• Insects in Hawaii are alone about 1000 species
  of fruit flies (Wagner Funk, 1995).
• Lizards Caribbean anoles are small arboreal
  insectivores and one of the larger and better
  studied vertebrate taxa. Out of 300 known Anolis
  species half occur on Caribbean islands (Losos
  1994, 2004)
• Birds Galapagos finches and Hawaiian
  honeycreepers. 1750 species of birds are confined
  to islands, 17 of described species.

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Island endemics
19
Species-isolation relationships

• Another key factor determining the number of
  species on an island is the level of isolation
• Islands of comparable sizes have a lower number
  of species if they are more isolated than habitat
  islands which are on continents (Wilson 1961)

20
Species-isolation relationships

• Williams (1981) found a decrease in the number of
  mainland bird species with increased distance
  from the mainland

21
Species-isolation relationships

• Reasons for decline of species diversity with
  distance
• Dependant on dispersal pathway, terrestrial
  mammals except bats can only disperse very
  limited distances (Lomolino, 1982)

22
Species-isolation relationships

• Bird species can disperse over larger distances,
  as seen in the example of resident land birds
  (Diamond 1972)

23
Species-isolation relationships

• Dispersal abilities are also dependant on the
  type of reproduction a organism uses
• Different estimates for ocean dispersal without
  human assistance is freshwater fish 5km,
  elephants and other large mammals 50km,
  tortoises, snakes and rodents reached the
  Galapagos 1100km, bats and land birds reached
  Hawaii 3600km (Menard 1986)
• Therefore the further an island is from the
  mainland the less species can disperse to it

24
Species-isolation relationships

• Isolation from the mainland can also be changing
  over time
• Example of lizard species on Islands in the Gulf
  of California (Wilcox 1978)

25
Species-area relationships

• One of the most obvious traits of Islands are a
  limited number of species, more countable than on
  the mainland
• The area available for species is also easier
  defined than on continents
• Darlington (1957) found an empirical relationship
  between Island area and number of reptile and
  amphibian species in the West Indies

26
Species-area relationships

• Darlington (1957) found an empirical relationship
  between Island area and number of reptile and
  amphibian species in the West Indies

27
Species-area relationships

• As a log-log plot, it is not a curve but a
  straight line
• As a rule of thumb with every 10 fold increase in
  size double the number of species are present.
• S is number of Species
• C is a constant which varies with the taxonomic
  group under study (taxa which consist of good
  dispersers (these species also typically have
  rapid population growth) will logically
  accumulate more species on an isolated island,
  all else being equal).
• A is the area of the island, and the exponent z
  has been shown to be fairly constant for most
  island situations
• Z represents a parameter for the slope of S and A
  on a log scale

S C A Z
28
Species-area relationships

• Geographic variation in C has been observed and
  'loosely' reflects the isolation of island groups
  typically studied
• The presence or absence of major air or water
  circulation pathways nearby increases C
• There are also effects of gross climatic
  difference, C is higher in the tropics than for
  islands at high arctic latitudes
• C is also regarded as the the scaling factor

29
Species-area relationships

• z in an all out treatment, is related to the
  distribution of abundances of species
• Therefore the number of species expected if the
  total number of individuals increases, as it
  would on a larger island, and those species
  follow a Preston log-normal distribution of
  abundance (see May 1975)
• Interpretation of these constant can be
  misleading (Lomolino 1989)

30
Species-area relationships

• Many studies have looked at and compared z-values
  for different habitats
• An early comparison (MacArthur and Wilson, 1967)
  found Islands to have z between 0.20-0.35 whereas
  non-isolated samples on continents or within
  large islands had a z of 0.12-0.17
• This suggests that any reduction in island area
  lowers the diversity more than a similar
  reduction of sample area in a contiguous mainland
  habitat
• Other studies (Williamson 1988) have found a less
  clearly marked difference in z between mainland
  habitats and islands

31
Species-area relationships

• Why might there be a difference in the
  species-area relationship between islands and
  isolated habitat areas on larger islands or
  continents?
• The inclusion of transients in species counts
  from small 'islands on continents
• Species with large home ranges for example wolf
  with 400 square km, or even larger areas for
  seasonal migrants like caribou or large predatory
  birds
• Such species might contribute to the number of
  species present but could not survive there if it
  would be a true island

32
Species-area relationships

• Species-area curves have been generated for a
  large variety of places and taxa, and the range
  of z values is remarkably small (Preston 1957,
  Williams 1953).
• Normally the relative abundance of species within
  a local biota fit log normal distribution

33
Species-abundance relationships

• The curves indicate the presence of a few common
  species (the right hand end of the curve) and a
  larger number of species of intermediate
  abundance
• The left hand end of the curve (the very rare
  species) are rarely included in studies, as they
  require a very high sampling effort

34
Species turnover

• The Krakatau story and its lessons
• A good record of recolonisation, particularly by
  bird species for the Krakatau Islands after the
  big volcanic eruption in 1883
• A rapid increase in bird species until 1920,
  after that number of species remained constant,
  but newcomers replaced already present species

35
Equilibrium theory of island biogeography

• Its based on the combination of species-area
  relationship, species-isolation relationship and
  species turnover (MacArthur and Wilson 1967).
• It proposes that the number of species inhabiting
  an an island is based on the dynamic equilibrium
  between immigration and extinction.
• The model is one of a dynamic equilibrium between
  immigration of new species onto islands and the
  extinction of species previously established.

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Equilibrium theory of island biogeography

• The formula is St1 StIV-E
• St is number of species at time t
• I is the Immigration rate
• V is additions through evolution
• E is losses by extinction
• The immigration rate is decreasing as there are
  fewer and fewer potential immigrant species
  remaining in the species pool P. This decrease is
  non-linear as the rate at which different species
  can disperse is different (e.g. tortoise vs bat)
• The extinction rate increases non-linearly as
  factors like competition, predation, and
  parasitism become more important at higher
  species densities.

37
Equilibrium theory of island biogeography
38
Equilibrium theory of island biogeography
39
Tests of the equilibrium theory

• In an experiment Simberloff (1976) censused
  terrestrial insect species on mangrove islands,
  and then cut the islands into smaller ones by
  creating 1m divides. This was sufficient to
  require jump dispersal from many insects
• The smaller islands maintained a lower species
  number according with the equilibrium theory
• Therefore in this study area as the only variable
  was a key determinant of number of species.

40
Is the world that simple?

• Here are many criticisms of the ETIB
• The theory ignores autoecology-but species are
  not exchangeable units (Armstrong 1982, Sauer
  1969)
• Data is rarely adequate for testing turnover
  (Lynch and Johnson 1974)
• Most turnover involves transients (Simberloff
  1976)
• Turnover equilibrium has not been demonstrated
  (Gilbert 1980)
• Immigration, extinction, and species pool are
  poorly defined (Williamson 1981,1989)
• Ignores successional effects and pace, and the
  hierarchical links between taxa (Bush and
  Whitacker 1991)

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Summary

• Islands provide interesting study areas for the
  speciation, dispersal, colonization, evolution,
  radiation etc.
• The simplified island world allows easier
  hypothesis testing than more connected
  continental habitats
• Islands harbour a disproportional part of
  biodiversity

42
References

• Main Sources
• Whitaker RJ 1998. Island Biogeography, Ecology,
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• Brown JH, Lomolino MV 1998 Biohepgraphy, second
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43
References

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