This document discusses island biogeography and provides examples of continental and oceanic islands. It describes how continental islands like Great Britain and Borneo have fauna more similar to nearby mainland areas due to past connections, while oceanic islands have fauna that arrived by air/water and is often endemic. It discusses species-area relationships and how larger islands support more species due to lower extinction rates. It summarizes Robert MacArthur and E.O. Wilson's equilibrium theory of island biogeography.

1. Khwopa College
BSc, Environment Science
Zoology
Island Biogeography
Saroj Raj Gosai
2014

2. Island Biogeography
 The study of animal life on islands forms one of the
fascinating chapters in the study of Zoogeography
 The biological environment on the island is different
from that of the continents and this has a far
reaching effect on its fauna
 Wallace has divided the islands into two distinct
categories namely Continental islands and Oceanic
islands
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3. Continental islands
 Islands that have been connected with the nearby
mainland at one time or other
 Through the sinking of the land or by a rise in the sea
level they must have become separated from the
continent by a stretch of the sea
 The sea, separating the two may be narrow like the
straits of Dover which separates the continental islands
of Great Britain form the continent of Eurasia or may be
wide like the Formosa strait which separates the island
of Formosa from mainland China
 Are close to the mainland and they resemble each other
geologically
 Great Britain is a young continental island
 Borneo, Formosa and Japan are older
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4. Continental islands
 The fauna of the continental island as well as the mainland
are more or less identical and always include certain
proportion of mammals and amphibia
 The terrestrial fauna of the continental island must have
reached the island across the dry land when the island was
still connected with the continent
 The difference that one notices in the faunal content of
the island and the mainland depends upon the length of time
a continental island has been independent form the mainland
 If the island is an old one it will be lacking in animals which
are comparatively new comers to mainland and secondly the
animals in the island would have undergone extensive
adaptive radiation resulting in the production of a wide
variety of species unknown in the mainland
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5. Continental islands
 Some of the peculiar forms present in the continent
islands are mainly due to the evolution of the new forms
under changed insular conditions
 Sometimes continental islands have preserved some of
the species which have become extinct on the mainland
 The survival of such forms on the continental islands is
due to lack of competition from the more progressive
forms and hence they are shielded from the hazardous
effects of natural selection
 The following are the important continental islands:
 1. Great Britain, 2. Bornea, 3. Java, 4. Philippines, 5.
Formosa and 6. Japan
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6. Oceanic Islands
 Islands which never had a connection with a continent
 They may be of volcanic origin or may be formed by the
building up of coral reef or by a combination of both
methods
 They are often far removed from the nearest mainland
 The oceanic islands always include a chance assemblage of
animals composed of a haphazard collection of diverse
animals groups
 Invariably the oceanic island fauna is quite conspicuous
by the entire absence of terrestrial mammals and
amphibians
 The fauna of an oceanic island must be derived from
across the sea
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7. Oceanic Islands
 The direction from which the fauna comes will be
determined to some extent by the prevailing wind and
ocean currents
 Only forms which can cross the ocean either by active
flight or with high natatory (swimming) capacity or by
some means of dispersal such as logs of wood, ice blocks,
etc., will be able to reach the oceanic islands colonise
them
 Hence the fauna of an Oceanic island is likely to be poor
in basic groups
 If inroduced by man animals tend to flourish in Oceanic
island because of the lack of competition
 The Oceanic island differs from the mainland in climate,
vegetation and fauna
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8. Oceanic Islands
 These environmental difference lead to divergence on
the part of the inhavitants when compared with the
mainland relatives
 The islands, among themselves, differ considerably in
their climate, vegetation and fauna but have certain
common features:
 A. Vegetation and fauna tend to be sparse,
 B. Mammals, amphibia and strictly fresh water fishes are
totally absent
 The absence of carnivorous forms leads to certain
general trends in the evolution of other island inhavitants
 From example the birds tend to loose the power of flight,
become large and flighless like the Dodo which lived on
the island of Mauritius where there were no native
mammals
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9. Oceanic Islands
 Animals tend to develop into sizes like the giant lizard of
Galapagos and giant tortoises found in Galapagos and
Seychelles
 The other evolutionary tendencies that are generally noticed in
the fauna of Oceanic island are the evolutionary the evolution
of wingless insects
 Birds tend to lose the bright colour of their mainland relatives,
evolving into either white or dark forms
 The following are the important Oceanic islands:
 1. Azores, 2. Bermuda, 3. Galapagos, 4. St. Helena
 It is not always easy to determine whether an island is Oceanic
or Continental. The difficulty is mainly due to the fact that in
the anicient islands differences between the two types of
faunae disappear or Oceanic island may receive animals by
transportation and this may alter the faunal relationships
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10. Oceanic Islands
 Galapagos Islands reoffered to as Oceanic islands by
Darwin, Wallace, Hesse, Allee and Schmidt while many
others regard it as Continental island
 New Zealand for example is regarded as Continental
island by Wallace while Wlikens, the well known geologist,
regards it as Oceanic
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11. Factors affecting the Insular fauna
 According to Hesse, Allee and Schmidt isolation,
competition, space restriction, special insular climates
which are common to all islands are responsible for the
special faunal characters of the islands
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12. Island Biogeography
 Islands are unique. Since they are isolated, evolutionary
processes work at different rates - there is little or no
gene flow to dilute the effects of selection and
mutation.
 Endemism is rampant. However, both in theory and
practice, that same isolation makes islands more
vulnerable to habitat change and extinction.
 Introduction of a single predator or herbivore can have
dramatic impact on the local community
 Since islands are isolated, and in many cases the species
found on them are endemic, extinction has been
particularly common on islands.
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13. Island Biogeography
 93% of the bird species whose extinction has been
recorded since 1600 have been island species.
 Historical records suggest a mean extinction rate
through the Pleistocene of approximately 1 species in
83.3 years. In 1980, that rate was one species every 3.6
years.
 Islands are excellent natural laboratories to study the
relationship between area and species diversity
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14. Species-Area Relationships
 Great Britain has 44 species of mammals, yet Ireland,
only approximately 20 miles further removed from
mainland Europe into the Atlantic, has only 22 species.
 Is 20 miles a sufficient distance to increase isolation and
decrease mammalian immigration by half? If so, then
flying mammals (bats) should show similar numbers of
species on both islands, since immigration and isolation
would be significantly less severe to a bat species. The
number of bats is not similar. Only 7 of the 13 bat
species resident in Great Britain breed in Ireland. What
factor accounts for the difference? The single factor
which provides the best explanation is island area (though
it is not the only contributing factor). It depicts the
relationship between area and the number organisms
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15.  Why do many more species of birds occur on the island of
New Guinea than on the island of Bali? One answer is that
New Guinea has more than fifty times the area of Bali,
and numbers of species ordinarily increase with available
space.
 This does not, however, explain why the Society Islands
(Tahiti, Moorea, Bora Bora, etc.), which collectively have
about the same area as the islands of the Louisiade
Archipelago off New Guinea, play host to many fewer
species, or why the Hawaiian Islands, ten times the area
of the Louisiades, also have fewer native birds.
 Two eminent ecologists, the late Robert MacArthur of
Princeton University and E. 0. Wilson of Harvard,
developed a theory of "island biogeography" to explain
such uneven distributions. 15

16.  They proposed that the number of species on any island
reflects a balance between the rate at which new species
colonize it and the rate at which populations of
established species become extinct.
 If a new volcanic island were to rise out of the ocean off
the coast of a mainland inhabited by 100 species of birds,
some birds would begin to immigrate across the gap and
establish populations on the empty, but habitable, island.
 The rate at which these immigrant species could become
established, however, would inevitably decline, for each
species that successfully invaded the island would
diminish by one the pool of possible future invaders (the
same 100 species continue to live on the mainland, but
those which have already become residents of the island
can no longer be classed as potential invaders). 16

17.  Equally, the rate at which species might become extinct
on the island would be related to the number that have
become residents.
 When an island is nearly empty, the extinction rate is
necessarily low because few species are available to
become extinct.
 And since the resources of an island are limited, as the
number of resident species increases, the smaller and
more prone to extinction their individual populations are
likely to become.
 The rate at which additional species will establish
populations will be high when the island is relatively
empty, and the rate at which resident populations go
extinct will be high when the island is relatively full.
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18.  Thus, there must be a point between 0 and 100 species
(the number on the mainland) where the two rates are
equal -- where input from immigration balances output
from extinction.
 That equilibrium number of species would be expected to
remain constant as long as the factors determining the
two rates did not change.
 But the exact species present should change continuously
as some species go extinct and others invade (including
some that have previously gone extinct), so that there is
a steady turnover in the composition of the fauna.
 That is the essence of the MacArthur-Wilson equilibrium theory of
island biogeography. How well does it explain what we actually
observe in nature?
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19.  One famous "test" of the theory was provided in 1883 by
a catastrophic volcanic explosion that devastated the
island of Krakatoa, located between the islands of
Sumatra and Java.
 The flora and fauna of its remnant and of two adjacent
islands were completely exterminated, yet within 25
years (1908) thirteen species of birds had recolonized
what was left of the island.
 By 1919-21 twenty-eight bird species were present, and
by 1932-34, twenty-nine. Between the explosion and
1934, thirty-four species actually became established,
but five of them went extinct.
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20.  By 1951-52 thirty-three species were present, and by 1984-85,
thirty-five species. During this half century (1934-1985), a further
fourteen species had become established, and eight had become
extinct.
 As the theory predicted, the rate of increase declined as more and
more species colonized the island.
 In addition, as equilibrium was approached there was some turnover.
 The number in the cast remained roughly the same while the actors
gradually changed.
 The theory predicts other things, too. For instance, everything else
being equal, distant islands will have lower immigration rates than
those close to a mainland, and equilibrium will occur with fewer
species on distant islands. Close islands will have high immigration
rates and support more species. By similar reasoning, large islands,
with their lower extinction rates, will have more species than small
ones -- again everything else being equal (which it frequently is not,
for larger islands often have a greater variety of habitats and more
species for that reason). 20

21.  Island biogeographic theory has been applied to many
kinds of problems, including forecasting faunal changes
caused by fragmenting previously continuous habitat.
 For instance, in most of the eastern United States only
patches of the once-great deciduous forest remain, and
many species of songbirds are disappearing from those
patches.
 One reason for the decline in birds, according to the
theory, is that fragmentation leads to both lower
immigration rates (gaps between fragments are not
crossed easily) and higher extinction rates (less area
supports fewer species).
21

22.  Island biogeographic theory has been applied to many
kinds of problems, including forecasting faunal changes
caused by fragmenting previously continuous habitat.
 For instance, in most of the eastern United States only
patches of the once-great deciduous forest remain, and
many species of songbirds are disappearing from those
patches.
 One reason for the decline in birds, according to the
theory, is that fragmentation leads to both lower
immigration rates (gaps between fragments are not
crossed easily) and higher extinction rates (less area
supports fewer species).
22

23.  Long-term studies of a bird community in an oak wood in
Surrey, England, also support the view that isolation can
influence the avifauna of habitat islands.
 A rough equilibrium number of 32 breeding species was
found in that community, with a turnover of three
additions and three extinctions annually.
 It was projected that if the wood were as thoroughly
isolated as an oceanic island, it would maintain only five
species over an extended period -- two species of tits
(same genus as titmice), a wren, and two thrushes (the
English Robin and Blackbird).
23

24. Species-Area Relationships
 The biological question is why does area affect species
numbers? There are two schools of thought:
 1) Area determines the total population size of the
collection of species living there. Area is the direct
determinant of diversity, since the multiplicity of factors
which determine relative abundance and species diversity
are prescribed, and independent of the specific island of
area being studied.
 2) The alternative school suggests that area is of only
indirect importance. Area fits because area is a good
indicator of the amount of habitat diversity present on an
island. It is really the 'number of niches' that determines
the number of species, but there is no established method
for counting, or even estimating the number of niches in an
environment. Instead, physical variables are usually
measured. 24

25. British Isles
 The British Isles consist of two main islands off the
coast of Europe
 Britain about 89,000 sq. miles and Ireland about 32,000
sq. miles in extent
 Climate is of cold temperate type and the islands are
characterised by low mountains and low lands
 When compared with the adjacent portions of the
continent Great Britain is known for poor representation
of mammals, reptiles and amphibians
 The paucity of the above mentioned forms has been due
to the successive glaciation to which the island seems to
have been subjected
 There are shrews, mole, hedgehogs, rabbits and hares,
squirrels, mice, roedeer, reddeer and a few bats
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26. British Isles
 Many European mammals for which the climate seems to
be suitable are absent
 Lemmings, wolves, beavers which were once typical of the
islands are no longer found
 Reptiles and amphibians are very poorly represented
 Among the fishes, perches, pikes, carps and loaches seem
to predominate
 The modern fauna of England is sparse when compared
with Europe
 There is only one endemic species of land vertebrates
namely the Red Grouse Lagopus scoticus
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27. Borneo and Java
 These islands lie close to south eastern corner of Asia
 Borneo is about 2,90,000 sq. miles while Java is about
49,000 sq. miles
 The islands are tropical, ecologically rich and diverse
 A number of Malayan animals are found in Borneo but not
in java
 Few examples are the elephant, Malayan bear and tapir
 The Oriental mammalian fauna seems to extent into
these islands
 Fresh water fishes, amphibians, reptiles and birds are all
well represented in these islands
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