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Islands
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.
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.
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.
There are three types of
islands:
a. Islands that were
originally part of a nearby
continent, but were
separated by rising sea
levels (land-bridge
islands).
b. Islands that are part of a
volcanic island arc.
c.
Seamount chains which
formed over geological
“hotspots”.
The islands of
Indonesia were once
part of a larger land
bridge.
The Marianas – part of an
volcanic island arc in the
Pacific
The Hawaiian Islands have
formed as a plate passed
over a geological hot spot.
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.
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.
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.
The ratio of observed species to the expected number
declines with distance from New Guinea.
For conifers and flowering plants in the Pacific,
diversity is much lower in the more isolated island
groups of the central and eastern Pacific.
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.
Some flying animals,
such as birds and
bats, are capable of
reaching even very
distant islands.
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.
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.
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.
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.
We may often see adaptive radiation in island
populations
Hawaiian
honeycreepers
Darwin’s
finches of
the
Galapagos
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.
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?
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.
Major human impacts on islands
• Hunting
• Destruction of native vegetation
• Introduced species
Hunting has impacted several native species.
Galapagos tortoises. Ships might take hundreds
at a time. At least three races hunted to extinction.
Galapagos tortoise
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.
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.
Hawaiian Islands – even richer than the Galapagos.
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.
What leads to the number of species
found on an island?
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.
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.
Ants
Lizards
Island populations are more likely to go
extinct than those on mainlands, for
several reasons:
1. Populations are typically smaller.
2. They have less genetic diversity.
3. They were not originally adapted to the
island habitat.
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.
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.
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.
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.
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
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.
This should result in a difference in the equilibrium
number of species, with Nnear > Nfar
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.
So, the two approaches (nonequilibrium and
equilibrium) make very different
predictions about the nature of island
species.
1. 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.
2. The equilibrium species predicts that,
although the number of species will
remain relatively constant, the actual
makeup of those species will change.
Several datasets have been developed
that support the equilibrium theory. Jared
Diamond looked at bird species on the
Channel Islands off the California coast.
In 1969, E.O.
Wilson and
Daniel
Simberloff
conducted an
experiment
employing
mangrove
islets in the
Florida Keys.
They surveyed a series of islands of
differing sizes and distances from shore,
concentrating on the arthropod fauna
found on the islands.
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.
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.
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.
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.
So we see a different pattern for the number
of species as a function of time for a:
land-bridge
island..
… or an oceanic
island.