Transcript Island Biogeography - University of Windsor

Island Biogeography
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
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
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
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
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 population’s 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)
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
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)
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)
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
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,
Dodo’s, 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.
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
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)
• It’s 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
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)
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 palaeoendemics are more important for
conservation due to a higher
contribution to global biodiversity
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
Island endemics
• Land snails: only 8 archipelagos account between 7.79.0% 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.
Island endemics
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)
Species-isolation relationships
• Williams (1981) found a decrease in the number of mainland
bird species with increased distance from the mainland
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)
Species-isolation relationships
• Bird species can disperse over larger distances, as seen in
the example of resident land birds (Diamond 1972)
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
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)
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
Species-area relationships
• Darlington (1957) found an empirical relationship
between Island area and number of reptile and
amphibian species in the West Indies
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.
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S = C AZ
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
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
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)
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
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
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
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
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
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.
Equilibrium theory of island biogeography
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The formula is: St+1 = St+I+V-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.
Equilibrium theory of island biogeography
Equilibrium theory of island biogeography
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.
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)
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
References
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Main Sources
Whitaker RJ 1998. Island Biogeography, Ecology, Evolution, and Conservation, Oxford University Press. BOOK
Vitousek PM, Loope LL, Adsersen H (eds) 1995. Islands, biological diversity and ecosystem function. Springer. BOOK
Brown JH, Lomolino MV 1998 Biohepgraphy, second edition BOOK
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Further reading
Carson HL Female choice in Drosophila: evidence from Hawaii and implications for evolutionary biology GENETICA 116 (2-3):
383-393 NOV 2002
Foster JB 1964 Evolution of mammals on Islands, Nature, 202, 234-5
Arnold EN 1979 Indian Ocean giant tortoises: their systematics and island adaptations. Philosophical Transactions of the Royal
Society of London, series B, 286, 127-145
Reyment RA 1983 Paleaontological aspects of island biogeography: colonization and evolution of mammals on Mediterranean
islands. OIKOS, 41, 299-306
Lister AM 1993 Mammoths in miniature. Nature, 362, 288-289
Brown P, Sutikna T, Morwood MJ, Soejono RP, Jatmiko, Saptomo EW, Due RA A new small-bodied hominin from the Late
Pleistocene of Flores, Indonesia NATURE 431 (7012): 1055-1061 2004
Schwaner TD, Sarre SD, 1988, Body size of Tiger Snakes in Southern Australia, with particular reference to Notechis ater
serventyi (Elapidae) on Chappell Island. Journal of Herpatology, 22, 24-33
Cody ML, Overton JMcC 1996 Shortterm evolution of reduced dispersal in island plant populations. Journal of Ecology, 84, 5361
ROFF DA THE EVOLUTION OF FLIGHTLESSNESS - IS HISTORY IMPORTANT EVOLUTIONARY ECOLOGY 8 (6): 639-657
1994 .
ROFF DA THE EVOLUTION OF FLIGHTLESSNESS IN INSECTS ECOLOGICAL MONOGRAPHS 60 (4): 389-421 1990
Diamond JM 1991 A new species of rail from the Solomon islands and convergent evolution of insular flightlessness, The Auk,
108, 461-470
Otte D, Endler JA (eds) 1989 Speciation and its consequences, Sinauer BOOK
Cox GW, Ricklefs RE, 1977 Species diversity, ecological release, and community structuring in Caribbean land bird faunas,
Oikos, 28, 113-122
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References
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Lomolino MV 1984 Mammalian island biogeography: effects of area, isolation, and vagility. Oceologia, 61, 376-382
Lomolino MV 1984 Immigrant selection, predation, and the distribution of Microtus pennsylvanicus and Blarina brevicauda on
islands. The American Naturalist, 123, 468-483
Gittenberger E 1991 What about non-adaptive radiation? Biological Journal of the Linnean Society, 43, 263-272
Cronk, QCB 1989 The past and present vegetation of St. Helena. Journal of Biogeography, 16, 47-64
Groombridge E (eds) 1992 Global biodiversity: status of the Earth’s living resources.BOOK
Wagner WL, Funk VA (eds) 1995 Hawaiian biogeography: evolution on a hot spot archipelago. Smithonian Press BOOK
Losos JB, Schoener TW, Spiller DA Predator-induced behaviour shifts and natural selection in field-experimental lizard
populations NATURE 432 (7016): 505-508 NOV 25 2004
LOSOS JB INTEGRATIVE APPROACHES TO EVOLUTIONARY ECOLOGY - ANOLIS LIZARDS AS MODEL
SYSTEMS ANNUAL REVIEW OF ECOLOGY AND SYSTEMATICS 25: 467-493 1994
Wilson EO 1961 The nature of the taxon cycle in the Melanesian and fauna. American Naturalist, 95, 169-193
Williams MH 1981 Island populations BOOK
Lomolino MV 1993 Species-area and species-distance relationships of terrestrial mammals in the Thousand Island Region.
Oecologia 54 72-75
Diamond JM 1972 Biogeographic kinetics: Estimation of relaxation times for avifaunas of Southwest Pacific Islands. PNAS 69,
3199-3203
Menard HW 1986 Islands BOOK
Wilcox BA 1978 Supersaturated island faunas: A species-age relationship for lizards on post-Pleistocene land-bridge islands.
Science. 199, 996-998
Darlington P.J.Jr 1957. Zoogeography: The Geographical distribution of animals. BOOK
May, R.M. 1975. Patterns of species abundance and diversity. in M.L. Cody and J.M. Diamond (eds.) Ecology and Evolution of
Communities. Harvard Univ. Press, Cambridge, MA. pp.81-120.
References
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Lomolino MV 1989 Interpretation and comparison of constants in the species-area relationship: An additional caution. American
Naturalist 133, 71-75
MacArthur RH, Wilson EO 1967 The theory of island biogeography. Monographs in population biology, no 1 BOOK
Williamson M 1989 The equilibrium theory today: True but trivial. Journal of Biogeography 16 3-4
Preston FW 1957 Analysis of Maryland statewide bird counts. Maryland Birdlife 13, 63-65
Williams CB 1953 The relative abundance of different species in a wild animal population. Journal of Animal Ecology 22, 14-31
MacArthur RH Wilson EO 1963 An equilibrium theory of insular zoogeography. Evolution 17, 373-387
Simberloff, D. 1976. Experimental zoogeography of islands: effects of island size. Ecology 57:629.
Armstrong P 1982 Rabbits (Oryctolagus cuniculus) on islands: a case study of successful colonization. Journal of Biogeography
9 353-362
Sauer JD 1969 Oceanic islands and biogeographic theory: a review. The geographical review. 59, 582-593
Lynch JD Johnson NV 1974 Turnover and equilibria in insular avifaunas, with special reference to the Califronian Channel
Islands. The Condor, 76, 373-387
Gilbert FS 1980 The equilibrium theory of island biogeography, fact or fiction? Journal of biogeography, 7, 209-235
Bush MB Whittaker RJ 1991 Krakatau: colonization patterns and hierarchies, Journal of biogeography, 18 341-356