Islands

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Islands
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Islands can serve almost as a laboratory for the
study of biogeography. The biota of an island is
simpler than that of a continental area, and the
interactions are easier to understand.
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Islands are often depauperate in species numbers
relative to mainland areas. Only 28 land bird
species are found on the Galapagos, the result of
maybe 13 colonization events. Some
undifferentiated, others apparently derived by
speciation within the archipelago. Equivalent
area in South America would have a bird fauna 10
to 20 times as rich.
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Barro Colorado Island in Panama was formed in
1913 by the damming of the Chagres River. 23
species of forest birds have disappeared since
the island was formed.
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• There are three types of islands
• Islands that were originally part of a nearby
  continent, but were separated by rising sea
  levels (land-bridge islands).
• Islands that are part of a volcanic island arc.
• Seamount chains which formed over geological
  hotspots.

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The islands of Indonesia were once part of a
larger land bridge.
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The Marianas part of an volcanic island arc in
the Pacific
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The Hawaiian Islands have formed as a plate
passed over a geological hot spot.
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The types of islands have different
characteristic flora and fauna. Islands formed
by isolation from continents would have a biota
which would be a subset of that on the continent.
It would have changed, however, as the result
of independent evolution and extinction. The
biota of island arcs and hotspot island chains
originally arrived by trans-ocean dispersal. In
both cases, several islands exist at one time,
creating the possibility for inter-island
dispersal and a more complex pattern of
evolutionary change.
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There is no doubt that the degree of isolation of
an island or island group is a factor in
determining the biota that it will support.
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Jared Diamond showed that, on very remote
islands, the number of species may be less than
that predicted by equilibrium theory. This is
because of the great difficulty in dispersing to
these islands.
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For conifers and flowering plants in the Pacific,
diversity is much lower in the more isolated
island groups of the central and eastern Pacific.
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If we plot the number of genera vs. island area,
it becomes clear that the two are related. The
more isolated islands (represented by ?) have
fewer genera that less isolated islands of the
same size.
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Some flying animals, such as birds and bats, are
capable of reaching even very distant islands.
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However, most land animals must rely on dispersal
mechanisms like drifting on masses of debris.
Although this process is likely rare, it
certainly happens and has been documented for
organisms like iguanas.
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Dispersal to islands is typically by a
sweepstakes route,. The dispersing organisms
share adapations that allow them to reach the
island, rather than adaptations allowing them to
live there once they reach it. This is one
factor that restricts the diversity of life on
islands.
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Long distance dispersal in plants is much more
likely. A great many plants are adapted for such
dispersal. In addition, the long distance
dispersal of a plant species can typically be
accomplished by a single spore or seed, where in
animals it typically requires a pair of organisms
or a pregnant female.
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Islands also show high endemicity. All native
land birds of the Hawaiian Islands are endemic.
Over 40 of plants on isolated oceanic islands
are endemic.
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We may often see adaptive radiation in island
populations
Hawaiian honeycreepers
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Darwins finches of the Galapagos
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Certain ecological groups are underrepresented.
Large predators. Flightless mammals tend to be
absent. Certain types of plants adapted for
disturbed sites tend to be absent.
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There is a tendency toward certain modified
behaviors. Flightlessness in
birds Fearlessness Probably a response to
reduced predation. Plants tend to have lost
defenses against herbivory. Why?
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Another characteristic that may be seen on
islands is ecological release, leading to niche
expansion. This may lead to groups of organisms
playing ecological roles different from those
they might fill on the mainland.
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Major human impacts on islands

• Hunting
• Destruction of native vegetation
• Introduced species

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Hunting has impacted several native species.
Galapagos tortoises. Ships might take hundreds
at a time. At least three races hunted to
extinction.
Galapagos tortoise
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High degree of endemicity 42 of native
plants All of mammals and reptiles. Human impact
dates from 16th Century. Four islands have been
settled. Total human population is now 9,000. In
1959, uninhabited portions were declared a
national park. Tourism now major industry
60-70,000 visitors annually.
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On island of Pinta, one male and two female goats
were introduced in 1959. In 1973 the populaton
was estimated to be about 30,000 (almost
200/square km). Feral cattle, donkeys, horses
and pigs also a problem. Introduced rats have
probably led to the extinction of native rice
rats on seven islands.  Introduced plants also a
problem. Guava now dominant plant in many areas.
Lantana, quinine tree.  
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Hawaiian Islands even richer than the
Galapagos.
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Over 1200 species of flowering plants 95
endemic. 22-24 colonizations by land snails
have led to over 1000 species. 47 species and
subspecies of songbirds.   Biggest difference is
that the Hawaiian Islands had been heavily
impacted by man before Cook got there in
1778.   Banana poka vine from SA. Sort of a
Hawaiian kudzu.   Hawaiian Islands contain more
than a quarter of the threatened and endangered
species in the US.
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What leads to the number of species found on an
island?
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Island life is probably more hazardous than that
on the mainland. For one thing, catastrophic
events have more severe effects. There is
typically no place to hide.
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Also, when a species is lost by extinction, it is
more difficult to replace it be immigration than
in a mainland situation. For these, and other
reasons, islands tend to support fewer species
than mainland areas of similar size.
Lizards
Ants
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• Island populations are more likely to go extinct
  than those on mainlands, for several reasons
• Populations are typically smaller.
• They have less genetic diversity.
• They were not originally adapted to the island
  habitat.

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How do we explain the fact that islands are
typically depauperate in species richness
relative to mainland areas of comparable size.
Originally, this was explained by a
nonequilibrium theory of island biogeography
which stated that islands are depauperate because
they have not had sufficient time to accumulate
species by immigration.
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In 1963, Robert MacArthur and E.O. Wilson
presented a new hypothesis to explain patterns of
species richness on islands. Their equilibrium
theory of island biogeography proposed that the
lower number of species on islands was not the
result of insufficient time, but rather the
result of an equilibrium process peculiar to all
islands.
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The theory is based on the idea that, at any
given time, the number of species on an island is
the result of a balance between two processes
extinction and colonization.
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When a new island forms, species begin to
colonize. As more and more species accumulate,
the colonization rate begins to decline. The
extinction rate, on the other hand, begins to
increase with increasing diversity.
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At some point, the two processes balance each
other, and the number of species on the island
should stabilize. This equilibrium number is
known as S
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The equilibrium theory can also be used to
explain the effect of size and distance on the
number of species found on islands. Consider
two islands of similar sizes but different
distances from the mainland pool. Since
extinction rates are a function of the available
resources and should be related to the size of
the island, we would expect them to be similar on
the two islands. Colonization rates, however,
should be greater for the island near the
mainland than for the more distant island.
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This should result in a difference in the
equilibrium number of species, with Nnear gt Nfar
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A similar argument can be used to explain the
effect of island size. If two islands are of
relatively equal distance from the mainland, we
can expect colonization rates to be similar.
Extinction rates, however, should be greater on
the smaller island. Therefore, we expect a
higher equilibrium number of species on the large
island.
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• So, the two approaches (nonequilibrium and
  equilibrium) make very different predictions
  about the nature of island species.
• The equilibrium theory predicts that the number
  of species will not change over time. The
  nonequilibrium theory predicts that the number of
  species should increase with time.
• The equilibrium species predicts that, although
  the number of species will remain relatively
  constant, the actual makeup of those species will
  change.

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Several datasets have been developed that support
the equilibrium theory. Jared Diamond looked at
bird species on the Channel Islands off the
California coast.
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In 1969, E.O. Wilson and Daniel Simberloff
conducted an experiment employing mangrove islets
in the Florida Keys.
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They surveyed a series of islands of differing
sizes and distances from shore, concentrating on
the arthropod fauna found on the islands.
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Then, they defaunated the islands by enclosing
them in plastic and pumping in methyl bromide to
kill all the arthropods.
They found that species increased for a while,
then reached an asymptote approximately equal to
the original number. But the makeup of the
species had changed.
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Following the publication of the theory, a number
of other studies were conducted to examine its
validity. A study on plant species on a group of
islands off Britain showed that, in that case,
the effect of size was indirect. Large islands
had a greater degree of habitat heterogeneity,
and therefore greater diversity.
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Another factor is the nature of the islands. As
mentioned earlier, some islands are of the land
bridge type while others arose at sea and have
never had a connection to the mainland. Oceanic
islands confirm pretty closely to the patterns
predicted by island biogeographic theory. Land
bridge islands are a different story.
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Land bridge islands begin with the species
complement to be expected of a mainland area.
Remember that this is typically more species than
would be expected on an island of that size. So,
over time, we expect the number of species to
diminish. This is referred to as a relaxation
fauna.
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So we see a different pattern for the number of
species as a function of time for a