Transcript Document 7393936

Island Biogeography
Why study Islands?
• First biologists and geographers studied them like
Wallace (East Indies), Darwin (Galapagos Islands)
and Hooker (Southern Ocean).
• Islands are natural experimental plots which offer
differences in sizes, number of species, isolation,
number of predators.
• With fewer species interactions are much less
complex than in mainland habitats.
• Due to their isolation evolutionary processes work at
different rates
• There is little or no gene flow dilute the effect of
selection and mutation causing a very high level of
endemism in island species
• 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
• That led 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
The result of founder effects, drift, and selection on
islands is the occurrence of a number of extreme forms
that occur only on islands…
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)
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.
Size reduction is an advantage.
Maybe even human dwarf species
Homo florensis on the Island of
Flores (Brown et al 2004).
Three hypothesis have been proposed for gigantism of
island species (Schwaner & Sarre 1988):
1. Predation hypothesis: either a) there is selective
release if no predation occurs or b) 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 (sexual selection) 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 on Juan
Fernandez Island.
Centauria (A-C)
and Senecio (D-F)
seeds. A-Argentina,
B,C-Juan Fernandez
• Flies lost their wings on Tristan da Cunha and Gough
islands.
‘wings’
balancers
The original theory was that this occurred (was
selected) to prevent wind loss in insects and seeds, but
Roff (1990,1994) found no clear relationship.
Ecological
release on islands
Due to reduced competition or from other interactions
including predation, two main changes in newly
arrived species are commonly observed.
1. 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.
2. The second form of release is from close competitors,
allowing the colonists to occupy not only different
niches but also a wider array than its ancestral form
(Cox & Ricklefs 1977).
It’s an important part of many scenarios of island
evolution (e.g. adaptive radiation).
Examples are Fijian fruit bats, that are more diurnal
on islands without predatory eagles (Lomolino 1984).
The meadow vole uses habitat types indiscriminately
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
• The 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 (paleoendemics)
• One example is the St Helena Ebony; it originated
from a more widespread species 9 million years ago.
Since then the family on the mainland has developed
away from this species (Cronk 1987)
• Species evolved on islands are called neo-endemics.
• The issue: whether paleo-endemics are more
important for conservation due to a higher
contribution to global biodiversity.
• The number of plant species endemic to 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 percentages 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.
Land snails: only 8 archipelagos account between 7.79.0% of the world land snail species. In particular larger
islands with a larger range of elevations harbour many
species (Groombridge 1992)
Insects: in Hawaii’ alone there are 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: e.g. Galapagos finches and Hawaiian
honeycreepers. 1750 species of birds are confined to
islands; that represents 17% of described species.
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
• The reasons for decline of species diversity with
distance relate to species’ dispersal capacity and
pathway
• terrestrial mammals (except bats) can only disperse
very limited distances (Lomolino, 1982)
• Bird species can disperse over larger distances, as
seen in the example of resident land birds (Diamond
1972)
Dispersal abilities are also dependant on the type of
reproduction a organism uses.
Estimates for ocean dispersal without human assistance
are:
• freshwater fish 5 km,
• elephants and other large mammals 50 km,
• tortoises, snakes and rodents reached the Galapagos
1100 km,
• 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.
• Isolation from the mainland can also change 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
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 = CAz
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 the
relationship between S and A on a log scale.
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 (decreases) 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 scaling factor.
z, in an all out treatment, is related to the distribution of
abundances of species.
Therefore the number of species expected increases if
the total number of individuals increases, as it would on
a larger island. We expect, from basic theory, that those
species follow a Preston log-normal distribution of
abundance (see May 1975).
According to that theory, the value of z should be .263.
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.
Why might there be a difference in the species-area
relationship between islands and isolated habitat areas
on larger islands or continents?
The answer is the inclusion of transients in species
counts from small 'islands‘ on continents. Those species
may have large home ranges, larger than the area of
whole islands, but still be occasionally present within
continental ‘islands’. They could not survive on the
isolated 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
a log normal
distribution.
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.
The truncation of the distribution at the left hand end is
called a “veil line”.
Species turnover
There is a good record
of recolonization,
particularly by bird
species for the Krakatau
Islands after the
volcanic eruption in
1883 formed the current
island system.
Bird species numbers
rapidly increased until
1920; after that the
number of species
remained constant, but
there was replacement
(turnover).
That general pattern – of turnover, but relative constancy
in species richness, resulted in the …
Equilibrium theory of island biogeography
• It’s 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
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.
The immigration rate decreases 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.
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, since known ‘islands’ were
fragmented and diversity on the ‘half-islands’ was
lower, area was the only variable key in determining
the number of species.
Is the world that simple?
The theory ignores autoecology - species are not
exchangeable units (Armstrong 1982, Sauer 1969).
Data is rarely adequate for testing turnover, and most
turnover involves transients (Simberloff 1976).
Observed turnover may depend on the interval between
censuses (Diamond 1969, Diamond and May 1977,
Gilbert 1980)
Immigration, extinction, and species pool are usually
poorly defined (Williamson 1981,1989). Census interval
has been shown to be important.
The theory ignores all events involving species
interactions (e,g, succession) and evolutionary change
on islands.