Field of Science

Island Life

"When I see these Islands in sight of each other, & possessed of but a scanty stock of animals, tenanted by these birds but slightly differing in structure & filling the same place in Nature, I must suspect they are only varieties. The only fact of a similar kind of which I am aware is the constant asserted difference between the wolf-like Fox of East & West Falkland Islds. If there is the slightest foundation for these remarks the zoology of Archipelagoes will be well worth examining; for such facts [would] undermine the stability of Species"
C. Darwin's zoology notes of the Beagle expedition in 1835.

Modern legend tells that Darwin's idea of natural selection was mainly influenced by his visit on the islands of the Galapagos archipelago; he later puzzled also about inhabitants of oceanic islands in a chapter of his "Origin of Species". Alfred Russel Wallace, who lived many years in the Malay Archipelago and there independently of Darwin's ongoing research developed an idea of variation and selection, dedicated even an entire book to the life found on islands ("Island Life", first edition in 1880).

Research on the fauna and flora of islands begun in the second hal
f of the 18th century whit the first expeditions sailing the Indian and Pacific Ocean and visiting the numerous islands distributed in these vast seas. For example the young German Johann Georg Adam Forster (1754-1794) acted with his father as naturalist during the second voyage of Captain James Cook in 1772-1775. They visited many islands in the Pacific, described 270 new species and in 1777 Forster published travel accounts that arouse great interest; he even impressed Humboldt so much that he decided to dedicate his life to the exploration of the world, and so many other naturalist later (Darwin himself was influenced by the adventures of Humboldt).
Even the first naturalists noted that islands display important peculiarities in the animals and plants found on them, but it was only with the formulation
of Darwins and Wallaces theory that these phenomeas could be explained.

Islands habitats show an impoverishment in species diversity compared to a similar habitat on the mainland. Darwin reassumed that

"The species of all kinds which inhabit oceanic i
slands are few in number compared with those on equal continental areas."
C. Darwin "On the Inhabitants of Oceanic Islands" in "The Origin of Species" (1859)

However islands encompass unique species and species assemblages - many species are endemic on specific islands, and also the assemblage and proportions of the fauna differs from that experienced on a continent.
Oceanic islands are dominated by birds and reptiles, mammals are r
are and amphibians can lack completely. It is clear that such proportions are the result of different dispersal ability.
Reptiles, like tortoises, survive saltwate
r in difference to amphibians; some mammals can fly, as most of the birds.
Once some individuals reach an island, they can establish a circumscribed population and evolve own lineages.

"Oceanic islands are sometimes deficient in animals of certain w
hole classes, and their places are occupied by other classes; thus in the Galapagos Islands reptiles, and in New Zealand gigantic wingless birds, take, or recently took, the place of mammals."
C. Darwin "On the Inhabitants of Oceanic Islands" in "The Origin of Species" (1859)

So flying animals, like insects and birds, once stranded on the isla
nd tend to reduce or even loss their wings and grow larger in size. Insular species tend to differ notably in size in comparison to related species on the mainland - this is often referred as island rule.
Most species of land snails and beetle species on Pacific islands for example are
unusually small, also the chameleons of Madagascar are known by their tiny size.
On the contrary the remote island of St
. Helena holds the biggest known ear wing (Labidura herculeana) and some species of (now extinct) birds of New Zealand and Madagascar grow to gigantic sizes. Also in mammals a general trend is observed, large species, like elephants, tend to become smaller on islands, and small species, like rodents, tend to became larger.

The description of a large marabou stork would surely have d
elighted both Darwin and Wallace, latter even knowing the place where this new discovery happened, the island of Flores in today's Indonesia.

Fig.1. Artist's impression of the size of Leptoptilos robustus sp. nov. (estimated at 1,8m) compared to Homo floresiensis (estimated at 1.0 m). Drawing by I. van Noortwijk, from MEIJER & DUE 2010.

The island of Flores got already great interest by palaeontologists and the media when in 2004 the discovery of extraordinarily small skeletons, attributed to a new h
ominid species, was published. The estimated 1m high Homo floresiensis would be a dwarf compared even to modern "pygmies" (1,5m high).
The new described marabou stork, Leptoptilos robustus, is the first species of fossil bird described from the island of Flores and was discovered in the sa
me sediments of the Liang Bua cave, site that held also the bones of H. floresiensis. This bird would be large even for our standards, estimated 1,8m in height.
The island of Flores lacked great carnivorous mammals during the Pleistocene and Holocene, so the stork could occupy the niche of a large predator and scavenger.
Also there were an abundant prey supply, very large rodents and juvenile Komodo dragons.

Fig.2. From the island of Timor, near Flores, Australian researchers in 2010 described the fossil remains of a new species of rodent of the genus Coryphomys, with an estimated body mass of 6 kg. The fossils resulted surprisingly young, dated between 1.000 to 2.000 years. Today the heaviest known murids belong to the genus Mallomys, endemic to Indonesia and reaching a mass of 2 kg.
The photo shows the ventral side of the broken fossil skull of Coryphomys (
left) compared to the skull of a modern common rat (Rattus rattus), after APLIN & HELGEN 2010.

From the thickness and dimensions of the recuperated bones it is also probably that this species was flightless, as so many large birds evolved in the isolation of islands.

Fig.3. The islands of Flores and Timor remained isolated even during the glacial periods, when the sea level was 180m lower than today forming the landmasses of Sunda and Sahul. It is part of the peculiar biogeographic zonation of Wallacea with highly endemic species.

Bibliography:

APLIN, K.P. & HELGEN, K.M. (2010): Quaternary Murid Rodents of Timor Part I: New Material of Coryphomys buehleri Schaub, 1937, and Description of a Second Species of the Genus. Bulletin of the American Museum of Natural History: 341: 1-80
MEIJER, H.J.M. & DUE, R.A. (2010): A new species of giant marabou stork (Aves: Ciconiiformes) from the Pleistocene of Liang Bua,
Flores (Indonesia). Zoological Journal of the Linnean Society 160: 707-724
QUAMMEN, D. (1987): Evolution and Extinction in the Galapagos and Beyond. In "The flight of the Iguana." (1988-1998): 161-175

No comments:

Post a Comment

Markup Key:
- <b>bold</b> = bold
- <i>italic</i> = italic
- <a href="http://www.fieldofscience.com/">FoS</a> = FoS

Post a Comment