Insular biogeography or island biogeography
is a field within biogeography that examines
the factors that affect the species richness
and diversification of isolated natural communities.
The theory was originally developed to explain
the pattern of the species–area relationship
occurring in oceanic islands. Under either
name it is now used in reference to any ecosystem
(present or past) that is isolated due to
being surrounded by unlike ecosystems, and
has been extended to mountain peaks, seamounts,
oases, fragmented forests, and even natural
habitats isolated by human land development.
The field was started in the 1960s by the
ecologists Robert H. MacArthur and E. O. Wilson,
who coined the term island biogeography in
their inaugural contribution to Princeton's
Monograph in Population Biology series, which
attempted to predict the number of species
that would exist on a newly created island.
== Definitions ==
For biogeographical purposes, an insular environment
or "island" is any area of habitat suitable
for a specific ecosystem, surrounded by an
expanse of unsuitable habitat. While this
may be a traditional island—a mass of land
surrounded by water—the term may also be
applied to many nontraditional "islands",
such as the peaks of mountains, isolated springs
or lakes, and non-contiguous woodlands. The
concept is often applied to natural habitats
surrounded by human-altered landscapes, such
as expanses of grassland surrounded by highways
or housing tracts, and national parks. Additionally,
what is an insular for one organism may not
be so for others, some organisms located on
mountaintops may also be found in the valleys,
while others may be restricted to the peaks.
== Theory ==
The theory of insular biogeography proposes
that the number of species found in an undisturbed
insular environment ("island") is determined
by immigration and extinction. And further,
that the isolated populations may follow different
evolutionary routes, as shown by Darwin's
observation of finches in the Galapagos Islands.
Immigration and emigration are affected by
the distance of an island from a source of
colonists (distance effect). Usually this
source is the mainland, but it can also be
other islands. Islands that are more isolated
are less likely to receive immigrants than
islands that are less isolated.
The rate of extinction once a species manages
to colonize an island is affected by island
size; this is the species-area curve or effect.
Larger islands contain larger habitat areas
and opportunities for more different varieties
of habitat. Larger habitat size reduces the
probability of extinction due to chance events.
Habitat heterogeneity increases the number
of species that will be successful after immigration.
Over time, the countervailing forces of extinction
and immigration result in an equilibrium level
of species richness.
=== Modifications ===
In addition to having an effect on immigration
rates, isolation can also affect extinction
rates. Populations on islands that are less
isolated are less likely to go extinct because
individuals from the source population and
other islands can immigrate and "rescue" the
population from extinction; this is known
as the rescue effect.
In addition to having an effect on extinction,
island size can also affect immigration rates.
Species may actively target larger islands
for their greater number of resources and
available niches; or, larger islands may accumulate
more species by chance just because they are
larger. This is the target effect.
=== Influencing factors ===
Degree of isolation (distance to nearest neighbour,
and mainland)
Length of isolation (time)
Size of island (larger area usually facilitates
greater diversity)
The habitat suitability which includes:
Climate (tropical versus arctic, humid versus
arid, variability, etc.)
Initial plant and animal composition if previously
attached to a larger land mass (e.g. marsupials,
primates)
The current species composition
Location relative to ocean currents (influences
nutrient, fish, bird, and seed flow patterns)
Location relative to dust blow (influences
nutrients)
Serendipity (the impacts of chance arrivals)
Human activity
== 
Historical record ==
The theory can be studied through the fossils,
which provide a record of life on Earth. 300
million years ago, Europe and North America
lay on the equator and were covered by steamy
tropical rainforests. Climate change devastated
these tropical rainforests during the Carboniferous
Period and as the climate grew drier, rainforests
fragmented. Shrunken islands of forest were
uninhabitable for amphibians but were well
suited to reptiles, which became more diverse
and even varied their diet in the rapidly
changing environment; this Carboniferous rainforest
collapse event triggered an evolutionary burst
among reptiles.
== Research experiments ==
The theory of island biogeography was experimentally
tested by E. O. Wilson and his student Daniel
Simberloff in the mangrove islands in the
Florida Keys. Species richness on several
small mangroves islands were surveyed. The
islands were fumigated with methyl bromide
to clear their arthropod communities. Following
fumigation, the immigration of species onto
the islands was monitored. Within a year the
islands had been recolonized to pre-fumigation
levels. However, Simberloff and Wilson contended
this final species richness was oscillating
in quasi-equilibrium. Islands closer to the
mainland recovered faster as predicted by
the Theory of Island Biogeography. The effect
of island size was not tested, since all islands
were of approximately equal size.
Research conducted at the rainforest research
station on Barro Colorado Island has yielded
a large number of publications concerning
the ecological changes following the formation
of islands, such as the local extinction of
large predators and the subsequent changes
in prey populations.
== Applications in conservation biology ==
Within a few years of the publishing of the
theory, its potential application to the field
of conservation biology had been realised
and was being vigorously debated in ecological
circles. The idea that reserves and national
parks formed islands inside human-altered
landscapes (habitat fragmentation), and that
these reserves could lose species as they
'relaxed towards equilibrium' (that is they
would lose species as they achieved their
new equilibrium number, known as ecosystem
decay) caused a great deal of concern. This
is particularly true when conserving larger
species which tend to have larger ranges.
A study by William Newmark, published in the
journal Nature and reported in the New York
Times, showed a strong correlation between
the size of a protected U.S. National Park
and the number of species of mammals.
This led to the debate known as single large
or several small (SLOSS), described by writer
David Quammen in The Song of the Dodo as "ecology's
own genteel version of trench warfare". In
the years after the publication of Wilson
and Simberloff's papers ecologists had found
more examples of the species-area relationship,
and conservation planning was taking the view
that the one large reserve could hold more
species than several smaller reserves, and
that larger reserves should be the norm in
reserve design. This view was in particular
championed by Jared Diamond. This led to concern
by other ecologists, including Dan Simberloff,
who considered this to be an unproven over-simplification
that would damage conservation efforts. Habitat
diversity was as or more important than size
in determining the number of species protected.
Island biogeography theory also led to the
development of wildlife corridors as a conservation
tool to increase connectivity between habitat
islands. Wildlife corridors can increase the
movement of species between parks and reserves
and therefore increase the number of species
that can be supported, but they can also allow
for the spread of disease and pathogens between
populations, complicating the simple proscription
of connectivity being good for biodiversity.
In species diversity, island biogeography
most describes allopatric speciation. Allopatric
speciation is where new gene pools arise out
of natural selection in isolated gene pools.
Island biogeography is also useful in considering
sympatric speciation, the idea of different
species arising from one ancestral species
in the same area. Interbreeding between the
two differently adapted species would prevent
speciation, but in some species, sympatric
speciation appears to have occurred.
== See also ==
Disturbance (ecology)
Island ecology
Mammals of the Caribbean
Patch dynamics
Distance decay
Sky island
