Transcript Island Biogeography - University of West Alabama

Island Biogeography
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.
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 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 transocean 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.
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.
Some flying animals,
such as birds and
bats, are capable of
reaching even very
distant islands.
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.
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.
Some plants have developed seeds
or fruits that can be carried in the
sea without being harmed.
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.
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.
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.
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.
Biogeographers and ecologists became anxious
to apply these ideas to other island-like
situations, particularly “habitat islands” on the
mainland. A habitat island is a region of suitable
habitat that is surrounded by uninhabitable area
that serves as a barrier to dispersal.
The basin and range province in Nevada is an
example of such a habitat island.
Other habitats that have been investigated
using island biogeographic theory are
vertebrates on mountaintops, invertebrates
in caves, fish in lakes, and even
herbivorous insects on plants.
For the most part, the theory does not
translate well. Plant diversity on
mountaintops in the Cascade Range seems
to increase with the age of the peak, rather
than establish equilibrium.
There are some
situations,
however, where the
theory does seem
to apply. For
example, there are
species-area
effects for birds
and mammals
inhabiting mountain
ranges in the Basin
and Range
Province.
We can also see
the effect of
isolation. For
mammals on
montane habitat
islands, increasing
isolation (distance
from the nearest
habitat island)
We also see evidence of a
seems to lead to
relaxation effect, resulting
decreasing
from the fact that these
species richness.
habitats were formerly
more widespread.
Two important phenomena prevent habitat
islands from conforming well to island
biogeography theory:
1. Habitat diversity may be more
important than area. This is true on
oceanic islands as well, but the effect
seems to be magnified in mainland
habitat islands.
Bird species diversity in the Basin and Range
is more closely correlated with habitat diversity
than with area.
2. The formation of a habitat island in a
mainland area creates an “edge effect”. This
refers to a change in physical and biological
characteristics of a habitat as one moves
from the edge to the interior.
Island biogeography theory
has had serious ramifications
for conservation. One of the
most significant
anthropogenic influences on
natural habitats has been the
fragmentation of habitats into
smaller and smaller
“islands”. These fragmented
habitats often show
relaxation effects like some
islands.
Mammal diversity in Mt. Rainier National Park
has decreased by 26% between 1920 and
1976. This is probably due to the increasing
fragmentation of forest habitats around the
park due to logging.
We would like to use our increasing
understanding of island biogeography
theory to assist in solving practical
conservation problems. Two areas are of
particular interest:
1. Using the theory to predict the effect of
anthropogenic habitat fragmentation.
2. Using our knowledge of species-area
effects and relaxation faunas to design
nature preserves that will maximize longterm species diversity.