Chapter 12: ISLANDSThe Dodo,
one of the first examples of extinction caused by human disturbance on islands |
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Islands Island Biogeography: the Species-Area Curve Krakatau
Adaptive radiation on Islands
SPECIAL VULNERABILITY OF ISLAND ANIMALS AND PLANTS Role
of Introduced Species in Island Extinction |
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| Registered UCI students: View the slide show for this chapter, or download it: http://darwin.bio.uci.edu:80/~sustain/protected/chap12slides.ppt | |
Oceanic islands provide many spectacular examples of evolution. Study of their biogeography and the adaptive radiations of their flora and fauna provided inspiration to both Charles Darwin and Alfred Russel Wallace, the two co-discoverers of evolution by natural selection. Study of species distribution patterns on islands led to a much better understanding of what controls the number of species that can be supported in a habitat. Archeological and historical studies provide many examples of resource over-exploitation leading to extinction and the collapse of civilization. Most documented species extinctions have been on islands, and contemporary ecological studies of islands show very clearly the kinds of threats faced by both island and continental ecosystems.
Fascinating, highly readable account of the importance of island studies in the history of biology, and of contemporary threats to island ecosystems: The Song of the Dodo : Island Biogeography in an Age of Extinctions by David Quammen (1997). Touchstone Books, 704 pages.
Archipelago : Islands of Indonesia by Gavan Daws and Marty Fujita (Hardcover - November 1999)
Larger islands have more species. This unsurprising statement has led to some very elaborate mathematical modeling of island colonization and extinction, initially in a book by Mac Arthur and Wilson entitled "The Theory of Island Biogeography". Essentially, their thesis was that the number of species on an island was the result of two opposing forces: immigration of species from other landmasses which increases species number, and local extinction (if the species still survives elsewhere, the proper term is extirpation) which decreases it. When a new island is formed (as they often have been, usually by volcanic activity), it will have no species, and the number will gradually climb. Eventually some of the colonists will be driven to extinction by competition, predation or other factors and after a long time the rate of increase by immigration will equal the rate of loss by extinction, so an equilibrium number will be reached. Evolutionary production of new species by local speciation will also increase species number, but at a very low rate compared to immigration. Larger islands have a greater shoreline and area available for colonization, so the rate of increase will be higher. They also support larger populations, so the rate of extinction is lower; therefore the larger islands will have more species. Other factors such as habitat diversity and distance from other landmasses will, of course, also affect the species number.
Studies of species number in relation to island area show surprising agreement with the theoretical model. In most cases, only the steady-state "equilibrium" state can be observed. However, in one case, the volcanic island of Krakatau, it has been possible to observe the gradual build-up of species starting from a sterile beginning.
Krakatau has erupted several times in recorded history, but the last great eruption was in August 1883. The explosion, which was the largest in recorded history, blew off about 3/4 of the volcano and created a Tsunami (wave) about 120 feet high that destroyed 295 coastal towns and villages and drowned over 36,000 people. Ash and stone rained down over 300,000 square miles of territory and on all of the nearby islands day was turned into night.
All living things were destroyed on the remaining fragment of the island of Krakatau. It was subsequently recolonized by animals and plants in an evolutionary process that has been watched quite carefully. The species number is still increasing, so it has not yet been possible to verify the idea that immigration and extinction rates will eventually come into equilibrium.
Two of the most important thinkers in biology were Charles Darwin and Alfred Russel Wallace. In the nineteenth century they both went on long expeditions to remote corners of the world, and they both made meticulous observations of the distribution and characteristics of the animals and plants they saw. They both ended up with the same flash of inspiration - that biodiversity could be explained by evolution through natural selection. But they both also made numerous other important contributions to biogeography, ecology and taxonomy, and especially to island ecology and evolution.
Both Darwin and Wallace got much of their inspiration by studying island life. They both examined groups of islands and made striking observations about the geographic distribution of related forms, and about the relation between the form and behavior of animals and their adaptation to their environments. One of the key observations was that islands tended to host very distinct but limited sets or species - on a given island a spectacular collection of one related group of species would be present and thriving, but many entire families or even higher taxa would be completely absent. This is sometimes called an unbalanced fauna. This was difficult to explain in the prevailing intellectual climate, which attributed all nature to special creation. The explanation, which now seems almost obvious, is that these islands were colonized by chance, on very rare occasions, but that the few successful colonists had given rise to many new species. This is now called adaptive radiation. It also happens on continents, but there the situation is much more complex.
Charles Darwin made many of his crucial observations on the Galapagos Islands located 600 miles off the coast of Ecuador. He noted similarities and differences among organisms and compared them with mainland species. One of his most extensive studies was on the group of small birds that are now called Darwin's finches. Darwin realized that all of these species were closely related to one another, but that each species had a set of adaptations that suited it for a particular environment, and especially to a certain kind of food. The form and size of the bill was clearly suited to specific purposes.
One of the groups of animals that fascinated Alfred Russel Wallace was the Birds of Paradise. There are 42 species of these spectacular birds on Papua New Guinea (PNG), Irian Jaya, and in northeastern Australia. They have long, brightly colored feathers on the tail, wings or head, which are used in courtship. The natives of PNG hunted these beautiful birds for centuries for their feathers. After naturalists including Wallace sent information, skins and feathers back to Europe the feathers were in great demand for ladies' hats. Between 1904 and 1908, 155,000 skins were sold just in London. However, the Royal Society for the Protection of Birds (in England) and the Audubon Society (in the U.S.), as well as the PNG government, soon realized that this excessive trade was threatening these species, and in 1908 all hunting of the birds for export from PNG was banned. Later England banned the import of these feathers. In 1922 PNG banned the hunting of Birds of Paradise, but skins were still exported from Irian Jaya via Indonesia. Finally in 1990 the Indonesian government passed a law banning trade in the skins in that country, finally putting an end to trading, at least the legal kind. Fortunately, none of the species was driven to extinction by this trade, and none of them are considered to be seriously endangered. This is partly because their habitats are at high elevation and have been very inaccessible (as Wallace discovered to his frustration!). However, deforestation in PNG may turn out to be a more serious threat in the coming years.
Recently the World Bank, in response to pressure from Non Government Organizations, has insisted on forest reform as a condition of its support to the PNG government. In 1999 the Bank required a moratorium on new logging concessions as a condition for providing loans. This was to provide some time for environmental review before decisions were made on 30 new logging concessions that were waiting to be granted. However, the World Bank may now be preparing to release part of the Loan with no review of existing logging operations.
Alfred Russel Wallace made many of his observations on the plants and animals of the islands of Southeast Asia, then called the Malay Archipelago. He made some of the most important early observations on biogeography, leading to an understanding of the relationships between geographical changes and biological changes as they have occurred in past ages.
A phenomenon which fascinated Wallace, and has occupied generations of biogeographers ever since, is the sudden change in flora and fauna that occurs in the twenty-mile wide channel between the small islands of Bali and Lombok. On Bali the animals, especially the birds and mammals, were clearly related to those of the larger islands to the west - Java and Sumatra - and mainland Malaysia. But on Lombok the fauna was almost completely different, more closely related to that of New Guinea and Australia. The difference in the animal types present was as great as you would find between Europe and North America. Wallace identified the narrow channel between Bali and Lombok as the boundary (now known as Wallace's Line) between two great zoogeographic regions, the Oriental and Australian. The boundary extends northward between Borneo and Sulawesi, and there is still some discussion about exactly where its northern part should be drawn.
Quotation from a letter in which Wallace identified this biogeographic boundary:
"In this archipelago there are two distinct faunas rigidly circumscribed which differ as much as do those of Africa and South America and more than those of Europe and North America; yet there is nothing on the map or on the face of the islands to mark their limits. The boundary line passes between islands closer together than others belonging to the same group. I believe the western part to be a separated portion of continental Asia while the eastern part is a fragmentary prolongation of a former west Pacific continent. In mammalia and birds, the distinction is marked by genera, families, and even orders confined to one region; insects by a number of genera and little groups of peculiar species, the families of insects having generally a very wide or universal distribution."
Now plate tectonics provides the explanation for the biological discontinuity that Wallace found. It turns out that 60 million years ago Bali and Lombok were separated by thousands of miles and that they have since moved closer together.
Coral Reefs, by Charles Darwin.
Wallace and Darwin are appropriately remembered most (or, at least, Darwin is; Wallace tends to be forgotten) for providing the framework for understanding evolution.
Darwin returned to England from his travels and spent more than twenty years studying the specimens and his field notes. He did not feel ready to publish his ideas until, in 1858, he received a letter from Alfred Russel Wallace and was shocked to find that Wallace had also come up with the idea of evolution by natural selection. An agreement was negotiated and both authors presented their ideas at a special meeting in London. (Read: A Delicate Arrangement: The Untold Story of the Darwinian Conspiracy and Cover-Up, by Arnold C. Brackman).
Alfred Russel Wallace had made his observations in the Amazon River basin in Brazil and in the island chain then called the Malay Archipelago, now in Indonesia. He only needed three years to analyze his data and propose the theory of evolution by natural selection. In 1855 he published his first general statement of his belief that species evolved from pre-existing species ("Every species has come into existence coincident both in space and time with a pre-existing closely allied species").
Wallace and Darwin had come up with the same idea completely independently. Their ideas met with huge resistance and ridicule from the religious establishment, but the significance of their ideas was immediately appreciated in the scientific community.
Here is how each of them summarized the theory:
The
life of wild animals is a struggle for existence. The full exertion of all
their faculties and all their energies is required to preserve their own existence
and provide for that of their infant offspring. The possibility of procuring
food during the least favorable seasons and of escaping the attacks of their
most dangerous enemies are the primary conditions which determine the existence
both of individuals and of entire species.
The numbers that
die annually must be immense; and as the individual existence of each animal
depends upon itself, those that die must be the weakest—the very young, the
aged, and the diseased—while those that prolong their existence can only be the
most perfect in health and vigor, those who are best able to obtain food
regularly and avoid their numerous enemies. It is "a struggle for
existence," in which the weakest and least perfectly organized must always
succumb.
Most
or perhaps all the variations from the typical form of a species must have some
definite effect, however slight, on the habits or capacities of the
individuals. Even a change of color might, by rendering them more or less
distinguishable, affect their safety; a greater or less development of hair
might modify their habits. More important changes, such as an increase in the
power or dimensions of the limbs or any of the external organs, would more or
less affect their mode of procuring food or the range of country which they
could inhabit. It is also evident that most changes would affect, either
favorably or adversely, the powers of prolonging existence. An antelope with
shorter or weaker legs must necessarily suffer more from the attacks of the
feline carnivora; the passenger pigeon with less powerful wings would sooner or
later be affected in its powers of procuring a regular supply of food; and in
both cases the result must necessarily be a diminution of the population of the
modified species.
If, on the other
hand, any species should produce a variety having slightly increased powers of
preserving existence, that variety must inevitably in time acquire a
superiority in numbers.
The
hypothesis of Lamarck—that progressive changes in species have been produced by
the attempts of animals to increase the development of their own organs and
thus modify their structure and habits—has been repeatedly and easily refuted
by all writers on the subject of varieties and species.
The giraffe did
not acquire its long neck by desiring to reach the foliage of the more lofty
shrubs and constantly stretching its neck for the purpose, but because any
varieties which occurred among its ancestors with a longer neck than usual at
once secured a fresh range of pasture over the same ground as their
shorter-necked companions, and on the first scarcity of food were thereby
enabled to outlive them.
Journal of the
Proceedings of the Linnean Society, August 1858, London
When
on board H.M.S. Beagle, as naturalist, I was much struck with certain facts in
the distribution of the inhabitants of South America, and in the geological
relations of the present to the past inhabitants of that continent. These facts
seemed to me to throw some light on the origin of species—that mystery of
mysteries, as it has been called by one of our greatest philosophers. On my
return home, it occurred to me, in 1837, that something might perhaps be made
out on this question by patiently accumulating and reflecting on all sorts of
facts which could possibly have any bearing on it. After five years work I
allowed myself to speculate on the subject, and drew up some short notes; these
I enlarged in 1844 into a sketch of the conclusions, which then seemed to me
probable; from that period to the present day I have steadily pursued the same
object. I hope that I may be excused for entering on these personal details, as
I give them to show that I have not been hasty in coming to a decision.
My work is now
nearly finished; but as it will take me two or three more years to complete it,
and as my health is far from strong, I have been urged to publish this
Abstract. I have more especially been induced to do this, as Mr. Wallace, who
is now studying the natural history of the Malay archipelago, has arrived at
almost exactly the same general conclusions that I have on the origin of
species. Last year he sent to me a memoir on this subject, with a request that
I would forward it to Sir Charles Lyell, who sent it to the Linnean Society,
and it is published in the third volume of the Journal of that Society. Sir C.
Lyell and Dr. Hooker, who both knew of my work—the latter having read my sketch
of 1844—honoured me by thinking it advisable to publish, with Mr. Wallace's excellent
memoir, some brief extracts from my manuscripts.
In considering
the Origin of Species, it is quite conceivable that a naturalist, reflecting on
the mutual affinities of organic beings, on their embryological relations,
their geographical distribution, geological succession, and other such facts,
might come to the conclusion that each species had not been independently
created, but had descended, like varieties, from other species. Nevertheless,
such a conclusion, even if well founded, would be unsatisfactory, until it
could be shown how the innumerable species inhabiting this world have been
modified, so as to acquire that perfection of structure and coadaptation which
most justly excites our admiration. Naturalists continually refer to external
conditions, such as climate, food, etc., as the only possible cause of
variation. In one very limited sense, as we shall hereafter see, this may be
true; but it is preposterous to attribute to mere external conditions, the
structure, for instance, of the woodpecker, with its feet, tail, beak, and
tongue, so admirable adapted to catch insects under the bark of trees. In the
case of the misseltoe, which draws its nourishment from certain trees, which
has seeds that must be transported by certain birds, and which has flowers with
separate sexes absolutely requiring the agency of certain insects to bring
pollen from one flower to the other, it is equally preposterous to account for
the structure of this parasite, with its relations to several distinct organic
beings, by the effects of external conditions, or of habit, or of the volition
of the plant itself.
The author of the
'Vestiges of Creation' would, I presume, say that, after a certain unknown
number of generations, some bird had given birth to a woodpecker, and some
plant to the misseltoe, and that these had been produced perfect as we now see
them; but this assumption seems to me to be no explanation, for it leaves the
case of the coadaptations of organic beings to each other and to their physical
condition of life, untouched and unexplained.
It is, therefore,
of the highest importance to gain a clear insight into the means of
modification and coadaptation. At the commencement of my observations it seemed
to me probable that a careful study of domesticated animals and of cultivated
plants would offer the best chance of making out this obscure problem. Nor have
I been disappointed; in this and in all other perplexing cases I have
invariable found that our knowledge, imperfect though it be, of variation under
domestication, afforded the best and safest clue. I may venture to express my
conviction of the high value of such studies, although they have been very
commonly neglected by naturalists.
No one ought to
feel surprise at much remaining as yet unexplained in regard to the origin of
species and varieties, if he makes due allowance for our profound ignorance in
regard to the mutual relations of all the beings which live around us. Who can
explain why one species ranges widely and is very numerous, and why another
allied species has a narrow range and is rare? Yet these relations are of the
highest importance, for they determine the present welfare, and, as I believe,
the future success and modification of every inhabitant of this world. Still
less do we know of the mutual relations of the innumerable inhabitants of the
world during the many past geological epochs in its history. Although much
remains obscure, and will long remain obscure, I can entertain no doubt, after
the most deliberate study and dispassionate judgment of which I am capable,
that the view which most naturalists entertain, and which I formerly
entertained—namely, that each species has been independently created—is
erroneous. I am fully convinced that species are not immutable; but that those
belonging to what are called the same genera are lineal descendants of some
other and generally extinct species, in the same manner as the acknowledged
varieties of any one species are the descendants of that species. Furthermore,
I am convinced that Natural Selection has been the main but not exclusive means
of modification.
On the Origin of
Species by Means of Natural Selection. London. 1859.
For reasons that nobody has ever been able to explain, island forms often evolve a huge body size (gigantism) compared to mainland relatives. The tortoises, marine iguanas and land iguanas on the Galapagos Islands, studied by Charles Darwin, provide some of the most striking examples.
Scientific American: Feature Article: The Komodo Dragon: March 1999.
This enormous lizard weighs up to 365 pounds; measures up to ten feet long; and feeds on deer. Recent estimates of the population are that about 3,500 dragons live within Komodo Island National Park, which consists of the islands of Komodo (1,700 individuals), Rinca (1,300), Gili Motang (100) and Padar (none since the late 1970s), and some smaller islands in Indonesia. About 2,000 live on the island of Flores. The Komodo is now listed as a "vulnerable" species by the World Conservation Union, and is a CITES-listed species. It disappeared from the island of Padar in the late 1970's, probably because poaching depleted its main prey animal which was a deer.
Although nowhere near as common as gigantism, dwarfism also occurs on islands. The slide shows an example of a minute chameleon from Madagascar.
Another
common finding on islands is the evolution of flightlessness - usually
accompanied by loss or reduction of wings. It has been seen in huge numbers of
both insects and birds on most island groups. The slides show examples. There
is also one example of the evolution of leglessness.
A large fraction of documented extinctions have involved species living on islands. This is for several reasons, but their vulnerability to introduced species is one of the most important ones. Islands, especially isolated oceanic islands, have evolved in isolation for millions of years, and their animals and plants have had to compete with only a limited range of species. For this reason they provide a ready home for many species of exotic animals and plants, and their flora and fauna is especially vulnerable to extinction after the arrival of man and the exotic animals and plants that always accompany him to those islands. Among island birds, flightlessness made them especially vulnerable to introduced predators.
An ecological catastrophe, comparable or even worse than what is happening in tropical rain forests, has long been underway on many islands. The extinction of bird species as well as the less well-documented extinctions of other vertebrates, land snails, insects and plants that must have also occurred, has been described as one of the swiftest and most profound biological catastrophes in the history of the earth.
Before colonization,
New Zealand had 11 species of large flightless birds called Moas, including the
10 feet tall Dinornis maximus. Polynesians (later to be called Maoris) reached
New Zealand about 1200 A.D., and by the time that Europeans arrived in the
nineteenth century, all species of moas plus about 20 other bird species
had become extinct. Archaeologists have found numerous roasting pits containing
the remains of thousands of moas indicating that these were exterminated by
hunting. Out of 94 species of native land birds, 35 were exterminated
prehistorically and 8 more have become extinct in the historical period; thus
46% of the original bird fauna is now extinct. In addition, Hoplodactylus
delcourti, the world's largest gecko at over 1 ft long, became extinct during
this period.
In New Zealand the nocturnal
flightless parrot, the kakapo,
is extremely vulnerable because it nests on the ground and the female leaves
the nest unattended at night for long periods of feeding. It was originally
distributed throughout the main islands of New Zealand, but was severely
depleted by Polynesians because of direct hunting as well as predation on eggs
and chicks by introduced rats. Europeans added two more species of rat, cats,
pigs, dogs, goats, weasels and ferrets. At present the only remaining breeding
population consists of only about 60
individuals on Stewart Island where its survival is threatened by cats and
three species of rat. Attempts are being made to establish breeding populations of
kakapo on off-shore islands which have been cleared of these predators.
New
Zealand (where there were no native mammals except bats) has been inundated
with many other introduced mammals including several kinds of deer, rabbits,
cattle, sheep, and many kinds of birds as well (chart). (Thanks to Mike
Dickison for some unpublished information on New Zealand)
Hot
Spots and Mantle Plumes
(See Nat Geog. Sep 95). Just like
in New Zealand, in the Hawaiian Islands a wave of extinction occurred between
the time the Polynesians arrived (in this case, about 300 A.D.) and the time of
the first European contact in 1778. The extinctions included four species of
large flightless ducks, two or more flightless ibises, nine flightless rails,
owls, an eagle-hawk, a petrel and many other small birds. A total of 50
species, which is 51% of the total number (98) of native land birds, went
extinct.
The reasons for the prehistoric
extinctions are similar to those in New Zealand. Not only were the flightless
birds probably easy prey for humans, but the dense human population also
cleared large areas of forest and introduced pigs, chickens and dogs. The chickens
may have introduced avian diseases to the native birds. The Polynesians also
accidentally introduced rats, which may have fed on the eggs and chicks of
ground-nesting birds.
The European explorers added
cats, two new species of rats, the barn owl, the Indian mongoose and several
avian diseases. In the next two centuries, habitat degradation, disease and
predation caused the loss of 17 more endemic bird species, so in total 69% of
the land birds have become extinct because of human causes. A group of endemic
forest birds called honeycreepers has been decimated. The most recent case, a
honeycreeper called the o'o, is thought to have gone extinct in the last few
years. Up to 1940, 94 kinds of foreign birds had been introduced into Hawaii,
and 53 had established populations there.
Hawaii probably had 1,765 species
of native plants when the Polynesians first arrived, and over 90% of them were
found nowhere else. The Polynesians introduced about 30 exotic species of
plants, but more recent immigrants have brought many more. There are now more
than 4,600 species of introduced vascular plants growing in Hawaii, and at
least 700 of them have established reproducing populations in the wild. At the
same time, more than 200 endemic species have gone extinct, and another 800 are
endangered. Most sites below 500m elevation are entirely dominated by exotic
species.
Large areas of Hawaii and other
Pacific islands are dominated by the introduced guava plant, an aggressive weed
that has displaced large areas of native vegetation on many islands. Pigs,
introduced into Hawaii, eat the fruit and thereby disperse the seeds of guava.
Similarly, the Indian Mynah bird spread lantana seeds all over the Hawaiian
islands. These introduced species block the germination and growth of many
indigenous plants.
With more endangered and
threatened species per square kilometer than anywhere else on earth, Hawaii is
aptly called the "Endangered Species Capital of the World." Bishop
Museum's Hawaii Biological Survey (HBS) gathers and disseminates information on
its fauna and flora to the public. As part of this effort, HBS has created the "Hawaii's
Endangered and Threatened Species" web site. ACTION ALERT Protect the NW
Hawaiian Islands
A 1997 study released
by the World Wildlife Fund states that even though 97 percent of the Galapagos
Islands' land area has National Park status, human population growth,
introduced species, and commercial fishing threaten to destroy the fragile
ecological balance there. The population of the Galapagos islands has more than
doubled in the last 10 years, mainly due to emigration from the Ecuadorian
mainland. Tourism is the main economic activity and 68 percent of the islands'
active working population is involved in this sector. No guidelines have been
set for maximum numbers of tourists, however. The number of tourists is
increasing and adds to the human impact on the island ecology. Along with human
migration to the islands comes the inevitable introduction of exotic species.
The number of foreign
plant and animal species has grown from about 77 in 1971 to more than 260
today. Wild goats and pigs threaten the food supply of the magnificent
Galapagos tortoises, and rats eat the eggs of birds and reptiles that have
evolved without natural predators. As in Hawaii, one of the most invasive and
damaging exotics is the guava plant, which covers more than 12% of the farm
area of the biggest island in the group, Isabel Island. The Galapagos Islands
are also suffering from the introduction of 21 species of vertebrates including
goats, pigs, rats and cats, and 219 species of invertebrates including beetles
and cockroaches. The population of giant land tortoises has declined from about
250,000 several decades ago to only 15,000.
The number of fishermen in the
Galapagos has dramatically increased from less than 200 in 1971 to more than
600 today. They fish primarily for sea cucumbers and shark to satisfy the
demands of the Asian market for these delicacies. The populations of these
species have been seriously reduced as a result. In 1995, a ban on sea cucumber
harvesting was instituted but illegal fishing continues, sometimes with dire
consequences. In May of 1997, an Ecuadorian park ranger was shot while
investigating illegal sea cucumber fishing.
Source: World Wildlife Fund;
Passport Books, "Galapagos".
The President of Ecuador, Dr.
Fabian Alarcon, recently declared the islands a "national priority"
and has instituted temporary bans on migration to the islands, new personal
motor vehicles, and additional fishing vessels while permanent regulations and
conservation measures are formulated. Among those suggested in the WWF report
are:
 | A ban on all commercial fishing. |
| Increased protection for the waters around the Galapagos by designating them a protected area. |
| Strict limits on migration to the islands. |
| Creation of a system of inspection and quarantine to control the number of introduced species. |
| Establishment of appropriate limits on tourist numbers and dedication of a greater percentage of tourism revenue toward conserving the islands' wild animal and plant species. |
Biodiversity
in the Galápagos | Human
Insurrection on Galápagos | Sea
Cucumbers | The
Fishing of Sea Cucumbers | Tortoises
and Poaching in the Galápagos | Giant
Tortoises | Giant Tortoise
| Marine
Iguana | Land
Iguana
The Brown
Tree Snake, a venomous predator that grows up to eight feet long has spread
through the island of Guam since arriving on U.S. military cargo ships from the
Solomon Islands in the 1940's. Their density is up to 12,000 per square mile in
some places. This introduced snake has caused the extinction
on Guam of 12 of 14 forest bird species, two of three bat species, and at
least six lizard species. Several Brown Tree Snakes have been intercepted on
Hawaii, which has no native snakes and where the indigenous bird fauna could be
devastated if the Brown Tree Snake were to gain a foothold (foothold? I don't
think so!).
Similar catastrophes have
occurred on many other oceanic islands following man's arrival of humans. In
fact, it has occurred on all Pacific islands that have been investigated
archeologically including New Zealand, Hawaii, Tonga, the Marquesas, Society,
and Cook Islands.
Dwarf elephant,
hippopotamus and deer disappeared from Mediterranean Islands around the time of
human colonization.
Madagascar is the world's 4th
largest island, just a bit larger than California and situated just off the
east coast of Africa. It has been separated from Africa for about 65 million
years, and during that time has evolved a very distinctive flora and
fauna. Biologically it is one of the richest areas on earth, with about
10,000 endemic species of flowering plants, including 8 entire families found
nowhere else. It is one of ten recognized "hotspots" of biological
diversity on the earth. 97% of its butterflies and moths, 90% of its primates,
reptiles and frogs, and 75% of its flowering plants live nowhere else. Slides
show baobab trees, chameleons, lemur and
comet-tailed moths. The rain forests of eastern Madagascar have a very high
species richness per unit area. After Indonesians colonized Madagascar 1500
years ago all species of large animals became extinct, including the 12 largest
species of lemurs, (including one of gorilla size), the pygmy hippopotamus, an
aardvark, the giant flightless elephant birds (which were the largest birds
that ever lived) and giant land tortoises.
The tropical rain forests of
Madagascar before human colonization are thought to have covered most of the
eastern coastal plains that run along the length of the island. Now most
of the forest has been cleared by people, cattle and fire, leaving forest
covering less than 15% of the land, mostly on steep slopes and rugged
terrain. Only about 2% of the total area is in parks or reserves.
The rate of destruction slowed from 2.5% per year between 1950 and 1973 to 0.8%
per year between 1973 and 1985. This appeared to be a result of reduction in
the amount of forest that is accessible because of the elimination of most of
the forests on all but the steepest slopes. The satellite images also showed
extensive deforestation even in areas that had been established as Nature
Reserves.
Madagascar also has a tropical
dry forest, which is even more endangered than the rain forest.
Lemurs
called key to forest survival, ENN Daily News -- 8-2-99
Europeans first visited Mauritius in the early 1500's and released pigs and monkeys on the island. In the next 300 years, 20 species of birds and eight species of reptiles were lost. These included the most famous example of human-caused extinction on islands -the disappearance, in the 1680's, of the Dodo (picture at top of page) a 50lb giant flightless pigeon.
The Channel Islands National Marine Sanctuary includes Anacapa, Santa Cruz, San Miguel, Santa Rosa and Santa Barbara Islands and the surrounding waters off the coast of Southern California, a total area of 1500 square miles. It is located at the junction of warm and cold ocean currents, a situation that provides an unusual habitat and breeding area for many species, including more than 20 that are threatened or endangered.
In October 2002 the California Fish and Game Commission voted to ban fishing within 175 square miles of the Channel Islands National Marine Sanctuary, creating the largest system of marine reserves off the U.S. west coast. The action is an attempt to reverse the alarming declines over the past decade of previously plentiful species including red snapper, angel sharks and abalone.
Catalina Island Plants | Catalina: Catalina Island Conservancy | Channel Islands National Marine Sanctuary
Why were so many
endemic species lost from islands? The main problems arise from exotic
(introduced, non-native) plants, predators and herbivores. They have
caused island extinctions in the following ways:
Introduced predators
such as rats, cats, dogs, foxes and mongooses are thought to have been
responsible for about half of island bird extinctions. Indigenous animals on
islands are well known for their tame behavior, which makes them extremely
vulnerable to predation by man and introduced predators.
Rats. Introduced predators
responsible for the largest number of extinctions are the Polynesian rat Rattus
exulans carried across the Pacific by Polynesians and Micronesians and two
other rat species, Rattus rattus and Rattus norvegicus, carried around the
world by Europeans in the last 5 centuries. These rats often feed on eggs and
chicks of ground-nesting birds.
Introduced rats have exterminated
bird species on 26 islands including catastrophic extinction waves that
eliminated a large fraction of the native bird species within a few years on
Hawaii, Midway Island, Lord Howe and the South Cape Island.
On the Galapagos Islands, black
rats have reduced populations of the giant tortoise and the dark-rumped petrel
by preying on eggs and have wiped out some rodent species on the islands.
Some islands have received
introduced rats without consequent extinction waves. These include Fiji, Tonga
and Samoa. These are islands that have native rats and/or land crabs. Land
crabs are nocturnal scavengers that climb trees, enter holes and are the
invertebrate ecological equivalent of rats. Presumably, species that have
evolved along with land crabs are well adapted to defend themselves against
rats.
Cats. The flightless Stephen
Island wren, one of only a few species of flightless songbird ever to have
lived, was driven to extinction in 1894 either by the lighthouse keeper's cat
or by the lighthouse keeper and other collectors in response to museum demand.
Introduction of goats and other
herbivores on islands has often been devastating. Many plant species and half
the endemic bird species on Guadalupe Island off Baja California have been lost
due to destruction of most of the island's vegetation by introduced goats.
Goats have also caused great damage on the California Channel Islands and on
some of the Galapagos Islands. The goats are slowly being removed from some of
these islands, but they breed very quickly and it is difficult to remove every
one. Introduced rabbits removed vegetation from Laysan Island, leading to the
extinction of an endemic rail and two of the island's three songbirds.
When the introduced
species has a similar diet to the indigenous species, food competition can be
an important factor in the decline of the indigenous species. The decline of a
rare forest bird in New Zealand, the kokako, is attributed at least in part to
food competition since introduced herbivores, especially opossums, feed on the
same parts of the same plants eaten by the kokako, for example the fruit.
Competition for space
is an important factor leading to decline of indigenous animals and
plants. Many exotic plants crowd out or cover native species. Competition
for nesting sites has been shown to be an important factor in the decline of
the Puerto Rican parrot. The introduced honeybee uses the same kind of tree
cavities and has reduced the availability of nest sites for the parrots.
Diseases introduced
with exotic birds such as domestic chickens. Following the introduction of
numerous types of birds from Asia into Hawaii in the 1920's, avian malaria
spread throughout many species of native forest birds and sharply reduced their
abundance and range. It was spread by mosquitoes, all of which are introduced
species, Hawaii having no native ones.
Easter
Island is the world's most isolated inhabited island, in the South Pacific
2400 miles from Chile, with an area of only 64 square miles. The soil is sandy
soil and parched grass is the only vegetation except for a few shrubs and two
species of trees. There are almost no land animals larger than insects.
Easter Island is not only the most
remote, but also the most mysterious of oceanic islands. Over 200 huge stone
statues, up to 65 feet tall and weighing up to 270 tons, line almost the entire
coastline. All of them were knocked down hundreds of years ago but some of them
have been restored to a standing position. Archaeological evidence shows that
Polynesians colonized the island in about 400 AD. The islanders carved and
erected the stone statues, produced many other arts and crafts and developed
Rongorongo, the only written language in Oceania. They harvested fish and
dolphins, hunted seabirds and land birds, grew many crops and raised chickens.
The population peaked at about 10,000, which appears to have been far more than
the island's ecosystem could support. Archeologists have found evidence showing
that the island was densely forested and supported numerous animal and bird
species when it was first colonized. But the colonists gradually burned and cut
down the forests and consumed the animals. Eventually they no longer had
enough wood to build canoes, so they could not fish in the open ocean. Many islanders were captured and used as
slaves by visiting whalers and Peruvian sailors, and these visitors also
introduced new diseases including smallpox, which wiped out many of the
islanders in an epidemic.
By 1877 Easter Island's
population had decreased to approximately 111. It was annexed by Chile in 1888.
Thereafter, the population increased again to more than 2,000.
The plight of the Easter islanders was exacerbated by the extreme isolation of the island, but what happened there could easily happen on other islands or even on continents. Other civilizations - the Khmer, Maya, Anasazi have also disappeared and resource over-exploitation is being suggested as a possible contributor in many cases. Easter Island 's ecological disaster is a clear warning to humankind of what happens as a result of overpopulation and overexploitation of resources.
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Copyright © 2002 Peter
J. Bryant (pjbryant@uci.edu),
School of Biological Sciences, University of California, Irvine, Irvine, CA 92697, USA. Phone (949) 824-4714 Fax (949) 824-3571 A Project of the Interdisciplinary Minor in Global Sustainability |
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