Khwopa CollegeBSc, Environment ScienceZoology

Island Biogeography

Saroj Raj Gosai2014

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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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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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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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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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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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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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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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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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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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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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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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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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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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.

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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

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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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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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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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).

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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).

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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).

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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).

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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.

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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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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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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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