Zoogeography
and
Wallace

Zoogeography and the line

Animal and plant species naturally occur where they are able to survive and reproduce. Animals require suitable habitats that supply the food, cover, water and other resources that they need.

The distribution of a species is an indication of its ecological and physiological requirements, under the influence of historical factors. These factors include the pattern of land surfaces and water barriers in the past, and climatic changes which will influence the prevalence of vegetation communities and biomes over time.

The edge of a species range shows the presence of a barrier; in the Malay Archipelago most of these barriers are stretches of salt water, however volcanos have also influenced patterns of distribution.

Where there is a barrier or barriers that are common to most species of fauna, the regions on either side will become centres of speciation and dispersal for unique (endemic) animals.

In 1858 Sclater divided the world into zoogeographic regions on the basis of their faunas. Their delineation remains basically the same today.

The Wallace Line is an attempt to provide a dividing line between the faunas of two zoogeographic regions - the Oriental and the Australian.

Where particular areas remain isolated for long periods of time, the local fauna develops independently and this may result in the formation of new species.

Endemic species often develop on islands, as for example in the Banda Arc islands of Indonesia, on Madagascar off the coast of Africa, on the Galapagos islands, and Hawaii.

The Banda Arc islands emerged from the sea in the past 10 million years and various Oriental species were able to colonise from the Sunda shelf. There has been an interchange of organisms over many millions of years, and in several separate waves.

The exchange of animals and plants has to a large extent been dependent on their natural ability to disperse.

Indonesia

The island nation of Indonesia is a republic formed when the Dutch East Indies obtained independence in 1950. Its territories straddle the equator, stretching over 4,000 kilometres from Sumatra in the west to the island of Aru lying off New Guinea in the east.

It embraces the large islands of Sumatra, Java and Sulawesi, the major portion of Borneo (Kalimantan), and the western half of New Guinea (West Papua or Irian Jaya). There are also hundreds of islands in the Lesser Sundas (or Nusa Tenggara) and the Moluccas.

Wallace described this part of the world as follows:

If we look at a globe or a map of the Eastern hemisphere, we shall perceive between Asia and Australia a number of large and small islands, forming a connected group distinct from those great masses of land, and having little connexion with either of them. Situated upon the Equator, and bathed by the tepid water of the great tropical oceans, this region enjoys a climate more uniformly hot and moist than almost any other part of the globe, and teems with natural productions which are elsewhere unknown.

(The Malay Archipelago, p. 1)

Read about the geographical extent and islands of this region on page 4 of The Malay Archipelago. Continue to read on as much as you like.

Examine a globe, obtain an Atlas, or check out Google Maps or similar to locate Indonesia, the Malay Peninsula, Australia and New Guinea, and think about the geography of the region.

This would also be a good opportunity to familiarise yourself with the names of some of the islands of the archipelago. As well, look at the 'Physical Map of the Malay Archipelago' between pages 14 and 15 of The Malay Archipelago.

The Sunda Shelf is a continental shelf that incorporates Borneo, Java and Sumatra, and is joined to peninsular Malaysia and the mainland nations of Vietnam and Thailand.

Bali is joined to the shelf but Lombok is separated from the shelf by the comparatively deep Lombok Strait. The Sahul Shelf in the east joins Australia and New Guinea.

Between the Sahul Shelf and the Sunda Shelf is a constellation of islands and seas known as Wallacea.

The term Malesia is applied to the province stretching from Sumatra and the Malay Peninsula across Indonesia to New Guinea and the Bismarck Archipelago.

The Banda Arc is a geological term for the double arc of islands from Flores east and curving up to Ceram. The Inner Banda Arc includes Flores, Alor, Wetar, Damar and Banda, whereas the Outer Banda Arc is composed of Rote, Timor, Tanimbar, the Kai islands, Ceram and Buru. The Aru islands and Halmahera are linked to mainland New Guinea.

The Wallace Line runs between Bali and Lombok, curves up between Borneo and Sulawesi (through the Makassar Strait), and then separates the Phillipines from the islands that lie to the north-west of New Guinea. Separating the fauna of the Oriental realm from that of Australia and New Guinea, the Lombok Strait between Bali and Lombok is only 25 kilometres wide.

Wallace recognised the differences in the composition of species either side of the Line over 150 years ago. This was long before we had a detailed understanding of changes in the configuration of the earth's surface due to plate tectonics, or the advantage of being able to view the arrangement of land and sea by using Google Earth.

Travelling in uncharted territory and island hopping down the archipelago, Wallace explored and collected as he went, discovering species of beetles, butterflies and birds unknown to scientists in the northern hemisphere.

There continues to be controversy over the placement of the line, and it may be more fruitful to view the region of Wallacea as a gradient of species from east to west, with varying affinity to the Oriental or Australasian faunas.

The line between Bali and Lombok marks the end of a continental avifauna and the beginning of an oceanic or island one. There is a gradual reduction in the number of Oriental species from west to east.

each strait of the sea to be crossed to reach another island offers an obstacle and thus a smaller number get over to the next island ..

(The Malay Archipelago, p. 320)

As noted by Wallace in his book Island Life, wide expanses of ocean are a barrier to dispersal for most land animals, and also for birds. Colonisation of the islands of the Malay archipelago has occurred in both directions, with the sea barriers acting as filters to dispersal. The ability to disperse varies widely among animal groups, so that land mammals generally disperse poorly, whereas bats are able to fly and thus often colonise islands.

Reptiles are thought to be able to survive on rafts of vegetation, whereas frogs are rapidly killed by sea water (they are unable to effectively osmoregulate and become desiccated) and rarely cross such barriers. Similarly, some plants (such as mangroves) have floating fruits that disperse their seeds across the ocean, whereas certain land plants do not disperse far at all and may germinate under the canopy of the parent plant.

Along the chain of islands of Nusa Tenggara the climatic conditions gradually change from west to east. Islands in the east tend to have a more seasonal climate with a distinct dry season, and support a patchwork of habitats including drier areas and some rainforest. In comparison, Sumatra and Borneo lie on the Equator, receive rainfall all year round, and support large areas of tropical rainforest.

Some islands have generated their own species and there are high rates of endemism in the archipelago. Examples of endemic birds from Nusa Tenggara include pigeons (e.g. Sumba Green Pigeon, Timor Imperial Pigeon), parrots (e.g. Salmon-crested Cockatoo, Wallace's Hanging-parrot), kingfishers (e.g. Cinnamon-banded Kingfisher), honeyeaters (e.g. Yellow-eared Honeyeater, Red-rumped Myzomela) and white-eyes (Zosterops spp.).

Distribution patterns are complicated because different groups of organisms living in a particular area will have responded differently to historical factors; these include geographical, climatic and vegetational changes over time. Volcanic eruptions on many of the islands would have decimated populations, thus introducing a further element of chance. In discussing the mammals the patterns are further complicated by the fact that many species (including cuscus and pigs) have been transported between islands by people.

In summary we can say that Wallacea is an intermediate zone between major continental zoogeographic realms, with some representatives of the fauna from each, and with a host of endemic species.

The following is a sampling of some of the more obvious, unique or characteristic examples of animals from these areas:

Birds

Shorebirds or waders (Charadriidae and Scolopacidae) are long-range migratory birds that travel from the northern hemisphere to the southern hemisphere, spending their year in perpetual summer. Birds breed in Siberia and then travel south through Asia and Indonesia (via the East Asian - Australasian Flyway), arriving on the north coast of Australia in September and October in time for the austral summer. Thus they can occur anywhere where there is suitable foraging habitat of mudflats exposed at low tide.

See the Australasian Wader Studies Group website for more on shorebirds and the Flyway.

Migrants to Australia

The Dollarbird Eurystomus orientalis is the only species of roller (family Coraciidae) that reaches Australia; most species in this group are from Africa. The Dollarbird occurs in Indonesia and New Guinea, and arrives in Australia in summer where they breed. Other species that migrate to Australia from Wallacea and New Guinea include Rainbow Bee-eater Merops ornatus, Channel-billed Cuckoo Scythrops novaehollandiae and Spangled Drongo Dicrurus bracteatus.

Oriental groups

The woodpeckers (Picidae) are found worldwide and are diverse in Asia, but are absent from Australia. The endemic treecreepers (Climacteridae) of Australia are an approximate ecological equivalent, also foraging on tree trunks and actively searching under bark. Thailand has 36 species of woodpeckers, Malaysia has 24 species, and 15 occur in Java, but none occur east of Sulawesi. Other groups that have not reached Australia include the trogons, barbets and bulbuls.

Hornbills (family Bucerotidae) are found in tropical Africa, India, and South-east Asia. Hornbills are large frugivorous birds with a huge bill, reminiscent of the South American toucans. The wings in flight make a characteristic and loud 'whooshing' sound. There are 13 species in Indonesia including three species in Java and the endemic Sumba Hornbill of the Lesser Sundas, however there are none in Timor. One species reaches New Guinea; Blyth's (Papuan) Hornbill Rhyticeros plicatus.

Intermediate with major speciation

Kingfishers are diverse throughout the Melanesian region, and it is a major centre of diversity for the group and possibly their site of origin. Kingfishers were able to colonise oceanic islands and various subspecies and species are now recognised. Kingfishers also occur in Australia and New Guinea where aberrant forms such as the kookaburras evolved. The Tanysiptera group with their long tails are confined to the Moluccas, New Guinea and tropical northern Australia.

Pittas are plump, colourful birds of the rainforest floor, foraging amongst the leaf litter for insects, snails and worms. The centre of distribution is in southern Asia where there are many species, three species occur in New Guinea, and one in Timor. There are three species in rainforests of northern Australia; two in north Queensland which are shared with New Guinea, and one in the Northern Territory and Kimberley.

The Rainbow Pitta Pitta iris can be seen in the monsoon forest at East Point, Darwin. The Blue-winged Pitta Pitta moluccensis is a vagrant to north-western Australia, having been recorded several times in the Kimberley. Another vagrant from Asia, the Javan Pond Heron Ardeola speciosa, arrived in the Darwin area in 2007 following Severe Tropical Cyclone George.

Australia / Gondwanan Origin

The mound-nesting megapodes (22 species) including the brush-turkeys, malleefowl, and scrubfowl (Megapodius spp.), the Talegalla of New Guinea and the Maleo (Macrocephalon) of Sulawesi which uses heat from geothermal sources, have spread as far as the Phillipines.

The parrots (order Psittaciformes) are a southern or Gondwanan group, with numerous species in Australia, New Guinea, South America and southern Africa, and three extant species in New Zealand. Parrots in Indonesia occur mostly in the east although the hanging parrots (Loriculus spp.) have representatives in Sulawesi, Java and the Phillipines. There are a number of endemic species in the Moluccas and the Bismarck Archipelago, but only two species in Java.

In Australia there are a variety of cockatoos, lorikeets and long-tailed parrots. These include species such as the enigmatic Night Parrot.

The honeyeaters (family Meliphagidae) have undergone a major adaptive radiation in Australia. There are 67 species from 20 genera, and in any habitat in Australia you are likely to find five or more species from three or more genera. Honeyeaters vary from the larger friarbirds and wattlebirds down to the slender spinebills. Major genera within Australia are Lichenostomus and Melithreptus. The endemic tui and the bellbird of New Zealand are also members of the honeyeater group, having most likely arrived there by trans-Tasman dispersal.

In Wallacea the genera Myzomela and Meliphaga are well represented. Honeyeaters reach Timor (six species) and Lombok (three species). On Bali there is a single species the Brown Honeyeater Lichmera indistincta, which does not occur on Java, and no honeyeaters have reached mainland Asia.

Honeyeaters are just one example of a group of endemic passerine (perching) birds that have evolved into a variety of forms in Australia, having been derived from Gondwanan ancestors. Other examples are the families Climacteridae (treecreepers), Maluridae (fairy-wrens, emu-wrens & grasswrens), Acanthizidae (thornbills, gerygones & scrubwrens) and Corvidae (crows and allies).

Molecular evidence is further substantiating the hypothesis that a major clade of birds, the Corvida, evolved in Australia and spread from there to other parts of the world. This is in contrast to the classical interpretation which was that birds had colonised Australia from the north and then radiated.

Oriental species

Tigers were once much more widespread than they are now. They have been hunted and poisoned out of existence and habitat loss has also taken its toll. It is said that there are more tigers in California than there are living in the wild.

Several subspecies of tiger were distributed in the islands of the archipelago. The Bali subspecies Panthera tigris balica is now extinct. There have been rare sightings the Javan subspecies Panthera tigris sondaca since the 1970s, but it is possibly also extinct. In Sumatra there are still some tigers in the wild.

Up until the 19th century some of the tigers were put to death in cruel ways. One way was the tiger-buffalo fight, in which the Central Javanese princes put a tiger and an Asian water buffalo Bubalus bubalis to fight against each other until the death. Usually the buffalo won.

The tropics of South-east Asia is well known for its monkeys. In Indonesia these include the macaques (genus Macaca), the nocturnal wide-eyed tarsier, the langurs, and gibbons (Hylobates). The orang-utan (Indonesian 'orang hutan' literally 'forest man') is a great ape that naturally occurs in the forests of Sumatra and Borneo.

Read Wallace's account of the mammals of the region on pp. 218-220 of The Malay Archipelago starting with the sentence 'The number of Mammalia known to inhabit the Indo-Malay region is very considerable .. '

The single-horned Javan Rhino Rhinoceros sondaicus occurs only in western Java, although it was once more widely distributed in southern Asia. It inhabits dense rainforest where there are mud wallows and water. The two-horned Sumatra Rhinoceros Dicerorhinus sumatrensis is the smallest extant species. It is now limited to small, scattered populations in Borneo and Sumatra.

Fruit bats (megachiroptera), microbats and rodents occur in South-east Asia, New Guinea and Australia. These groups are thought to be relatively recent arrivals (5 million years; recent in geological terms) to Australian shores. They have radiated into various forms including many endemics.

Despite popular belief, bats and rodents which are placental mammals (i.e. not marsupials), make up approximately 50% of the native mammal fauna of Australia. This includes the hopping mice (Notomys spp.), native mice (Pseudomys spp.) and the rock rats (Zyzomys spp.).

Occurrence in south-east Asia reflects the influence of continental Asia as the main source region and the importance of areas of local speciation on islands of the Sunda Shelf.

Australian species

The radiation of marsupial species in Australia includes many endemic genera and numerous species. It includes such characteristic forms as the wombats, koala, bandicoots, bettongs and quolls, and the macropods (kangaroos, wallabies, rock-wallabies and allies).

The marsupials of New Guinea were derived from Australian immigrants, but a range of endemic species have evolved in tropical forest environments that favoured arboreal species. Only a few species occur further afield in Wallacea, including two species of cuscus (a type of possum) on Sulawesi, the grey cuscus on Timor, and the Seram bandicoot.

For further investigation, try to obtain some information about the tree-kangaroos (Dendrolagus spp.) and how they evolved from terrestrial macropods.
Do you think they evolved in New Guinea or in Australia?
What kinds of conditions and environments favoured the evolution of arboreal kangaroos?

Goannas (varanid lizards or monitor lizards)

The oldest varanid lizard fossils are from central Asia and are about 45 million years old. This lizard group subsequently moved into Africa and Australia. The oldest varanid fossils from Australia are about 24 million years old. Thus, the varanids arrived in Australia relatively recently in the history of the group. Overwater dispersal may have been possible on more than one occasion, and there has been more than one invasion of varanids into Australia.

An early immigration and establishment of goannas in Australia resulted in an extensive adaptive radiation. There were two main branches to this radiation - one consisting of the species that remained large (like their Asian ancestors), and a second group of pygmy monitors.

A second invasion brought the Mangrove Monitor Varanus indicus, which live in mangrove habitats along the north coast of Australia, New Guinea and Wallacea and V. prasinus of New Guinea which reaches the Cape York Peninsula of northern Queensland. These are much more recent additions to Australia's varanid lizards.

The adaptive radiation of varanids in Australia has been spectacularly successful. Of the 53 known species in the world, 24 are found only in Australia, and Australia also has the two recent Asian immigrants mentioned above. This evolutionary radiation of varanid lizards is thought to be related to the complex patterns of aridification that have occurred on the continent over the past 24 million years.

Plants

The mangroves are a group of trees, shrubs, ferns, mistletoes and palms from several different plant families that occupy mudflats and estuaries on low energy coasts in the tropics and subtropics. In northern Australia mangroves are mainly from the family Rhizophoraceae, with a smattering of species from the families Verbenaceae, Euphorbiaceae, Combretaceae, Myrtaceae, Meliaceae, Bombacaceae and Sonneratiaceae.

There are particularly well-developed mangrove forests (mangals) in intertidal areas in parts of South-east Asia, New Guinea and northern Australia. The mangroves have peculiar adaptations to saline conditions and low oxygen availability of the substrate (e.g. pneumatophores). Many of the mangrove species are widespread because they are able to disperse between islands, having buoyant propagules that are resistant to salt water.

Examples of types of distributions of mangrove plants in northern Australia and South-east Asia:

Nypa Palm Nypa fruticans - widespread throughout Asia, only just reaches Melville Island and Cobourg Peninsula in the NT.

Grey Mangrove Avicennia marina - a species with an exceptionally wide distribution, extending into southern Australia and as far south as New Zealand.

Cannonball Mangrove Xylocarpus moluccensis - South-east Asia to New Guinea and northern Australia.

Kapok Mangrove Camptostemon schultzii - Wallacea, southern New Guinea and northern Australia.

To learn more about mangroves, take a look at MangroveWatch

Many of the plants of beach forests are also shared between Indonesia and northern Australia. These low monsoon forests develop on coastal beaches, dunes and cliff tops, in many cases at the landward margin of mangals.

Examples of species that occur around the coasts of Indonesia and that have spread to the Top End of Australia include Beach Hibiscus Hibiscus tiliaceus, Beauty Leaf Calophyllum inophyllum, Scaevola taccada, Coastal Sheoak Casuarina equisetifolia, Beach Barringtonia Barringtonia asiatica, and Millettia (Pongamia) pinnata.

These plants are able to survive in generally sandy, partially saline conditions, and some are able to resprout following severe weather such as cyclones. The fruits of most species can withstand salt water for weeks (or longer) and are dispersed by ocean currents, winds and tides.

Example: Barringtonia asiatica has large floating capsules (similar to a coconut) that disperse the seeds.

Plants have much greater facilities for passing across arms of the sea than animals. The lighter seeds are easily carried by the winds and many of them are specially adapted to be so carried. Others can float a long time unhurt in the water and are drifted by winds and currents to different shores .. It thus happens that plants which grow on shores and lowlands have a wide distribution ..

(Malay Archipelago, p. 217)

The eucalypts or gum trees (Eucalyptus and Corymbia; Family Myrtaceae) are an example of an endemic (autochthonous) evolutionary radiation in continental Australia. There are over 800 recognised species, most of which are endemic to Australia. About 12 species have colonised the south of New Guinea where they dominate the savanna woodlands.

There are three species on Timor and the Rainbow Gum Eucalyptus deglupta reaches Celebes and the southern Philipines. Two species (E. alba and E. urophylla) are known from as far west as Flores in the Lesser Sundas, and parts of Timor are vegetated with eucalypt woodlands. The myrtaceous paperbarks Melaleuca spp. are another Australian group that have a small number of species that are shared by northern Australia and New Guinea.

The plant family Proteaceae is of Gondwanan origin and thus most representatives occur in South America, southern Africa and Australia. There are 73 species of Banksia (Proteaceae) in Australia, only one of which (Banksia dentata) reaches Papua New Guinea, West Papua and the Aru islands. The genus Grevillea includes over 300 species with the greatest diversity in the south-west of Western Australia and along the Great Dividing Range.

Three species occur in New Guinea and one species, Grevillea elbertii, is endemic to Sulawesi. A third genus of Proteaceae, Helicia, has spread as far as southern Asia and includes a range of species in New Guinea, Borneo and China. A single species, Helicia australasica, occurs in perenially wet rainforest patches in the Top End.

The palm (Family Arecaceae) genus Livistona has representatives through South-east Asia and Australia. Eighteen of the 34 species are restricted to Australia and southern New Guinea (two Australian species are shared with New Guinea). Several of the species in Australia have relict distributions, being restricted to moist locations within the otherwise dry arid zone. Examples include Palm Valley in central Australia (Central Australian Cabbage Palm Livistona mariae) and Millstream in the Pilbara region of Western Australia (Millstream Fan-palm Livistona alfredii).

In tropical northern Australia and Asia most of the Livistona palms occur in wet areas such as rainforests, but the Sand Palm Livistona humilis of monsoonal northern Australia is a savanna species. Recent studies suggest that a single Livistona ancestor colonised Australia from the north 10-17 million years ago and diversified within Australia.

The dominant trees in Indomalayan lowland rainforests are frequently members of the family Dipterocarpaceae. Among the dipterocarps are emergent species that top the forest canopy, some being up to 50 metres high. The greatest diversity of dipterocarps is in Borneo where there are 267 species.

Dipterocarps are characterised by their winged fruit, however they are poor dispersers and the seeds often end up in close proximity to the mother tree. As a consequence they show a high degree of endemism. The pattern within Malesia is of strong concentration in the Sundaland rainforests with limited extension east of Wallace's line. Although three genera (Anisoptera, Hopea and Vatica) reach New Guinea, no species of dipterocarp occur naturally in Australia.

Plants - an overall perspective

The Malesian region has four zones of contact with adjacent floral regions. These are with Asia in the Malay Peninsula and in the Phillipines, with the Pacific islands in the Bismarck Archipelago and Solomon islands, and with Australia via New Guinea.

Van Steenis (1950) analysed the distribution of plant (phanerogam or seed plants) genera in Malesia. The number of endemic genera is highest on the large islands of Borneo and New Guinea.

These islands are tropical and mountainous and have a wide range of habitats, thus providing opportunities for speciation.

The lowest number of endemic genera is in the islands of Nusa Tenggara and the Moluccas. The number of eastern-centred genera, i.e. genera derived from Australia or the Pacific, decreases from east to west, with the highest number of genera in New Guinea, and the lowest in the Malay Peninsula and Sumatra.