biogeography of the australian monsoon tropics

SPECIALPAPER

Biogeography of the Australian monsoontropics

D. M. J. S. Bowman1*, G. K. Brown2,3, M. F. Braby4,5, J. R. Brown6,

L. G. Cook7, M. D. Crisp5, F. Ford8, S. Haberle9, J. Hughes10, Y. Isagi11,

L. Joseph8, J. McBride12, G. Nelson2 and P. Y. Ladiges2

1School of Plant Sciences, University of

Tasmania, Hobart, Tas. 7000, Australia,2School of Botany, The University of

Melbourne, Vic. 3010, Australia, 3National

Herbarium of Victoria, Royal Botanic Gardens

Melbourne, Vic. 3141, Australia, 4Museum

and Art Gallery Northern Territory,

Department of Natural Resources,

Environment, the Arts and Sport, GPO Box

4646, Darwin, NT 0801, Australia, 5School of

Botany and Zoology, The Australian National

University, Canberra, ACT 0200, Australia,6School of Geography and Environmental

Science, Monash University, Clayton, Vic.

3800, Australia, 7School of Integrative Biology,

The University of Queensland, St. Lucia,

Brisbane, Qld 4072, Australia, 8Australian

National Wildlife Collection, CSIRO

Sustainable Ecosystems, GPO Box 284,

Canberra, ACT 2601, Australia, 9Department

of Archaeology and Natural History, College of

Asia and the Pacific, Australian National

University, Canberra, ACT 0200, Australia,10Griffith School of Environment, Griffith

University, Nathan, Qld 4111, Australia,11Graduate School of Agriculture, Kyoto

University, Sakyo-ku, Kyoto 606-8502, Japan,12Centre for Australian Weather and Climate

Research, Melbourne, Vic. 3001, Australia

*Correspondence: David Bowman, School of

Plant Sciences, University of Tasmania, Hobart,

Tas. 7000, Australia.

E-mail: [email protected]

ABSTRACT

Aim This paper reviews the biogeography of the Australian monsoon tropical

biome to highlight general patterns in the distribution of a range of organisms

and their environmental correlates and evolutionary history, as well as to identify

knowledge gaps.

Location Northern Australia, Australian Monsoon Tropics (AMT). The AMT

is defined by areas that receive more than 85% of rainfall between November

and April.

Methods Literature is summarized, including the origin of the monsoon

climate, present-day environment, biota and habitat types, and phylogenetic and

geographical relationships of selected organisms.

Results Some species are widespread throughout the AMT while others are

narrow-range endemics. Such contrasting distributions correspond to present-

day climates, hydrologies (particularly floodplains), geological features (such as

sandstone plateaux), fire regimes, and vegetation types (ranging from rain forest

to savanna). Biogeographical and phylogenetic studies of terrestrial plants

(e.g. eucalypts) and animals (vertebrates and invertebrates) suggest that distinct

bioregions within the AMT reflect the aggregated effects of landscape and

environmental history, although more research is required to determine and

refine the boundaries of biogeographical zones within the AMT. Phylogenetic

analyses of aquatic organisms (fishes and prawns) suggest histories of associations

with drainage systems, dispersal barriers, links to New Guinea, and the existence

of Lake Carpentaria, now submerged by the Gulf of Carpentaria. Complex

adaptations to the landscape and climate in the AMT are illustrated by a number

of species.

Main conclusions The Australian monsoon is a component of a single global

climate system, characterized by a dominant equator-spanning Hadley cell.

Evidence of hot, seasonally moist climates dates back to the Late Eocene, implying

that certain endemic elements of the AMT biota have a long history. Vicariant

differentiation is inferred to have separated the Kimberley and Arnhem Land

bioregions from Cape York Peninsula/northern Queensland. Such older patterns

are overlaid by younger events, including dispersal from Southeast Asia, and

range expansions and contractions. Future palaeoecological and phylogenetic

investigations will illuminate the evolution of the AMT biome. Understanding the

biogeography of the AMT is essential to provide a framework for ecological

studies and the sustainable development of the region.

Keywords

Biogeography, fire, geological history, monsoon climate, northern Australia,

phylogeny.

Journal of Biogeography (J. Biogeogr.) (2009)

ª 2009 Blackwell Publishing Ltd www.blackwellpublishing.com/jbi 1doi:10.1111/j.1365-2699.2009.02210.x

INTRODUCTION

The many animal and plant taxa endemic to Australia have

made the island continent of prime interest to biogeographers

(e.g. Burbidge, 1960; Barlow, 1981; Keast, 1981). Yet, unrav-

elling the causes of this diverse endemism remains a major

research challenge encumbered by a fossil record that is biased

to the humid eastern and southern edges of the continent.

Overlaying the continent’s biogeography are the impacts of

two distinct waves of human dispersal: (1) colonization some

50,000 years ago by fire-wielding hunter-gatherers, which gave

rise to diverse Aboriginal cultures that established across the

continent, and (2) settlement over the past two centuries by

Euro-Asian colonists with industrial technologies. Compared

with southern and eastern Australia, the biogeography of the

Australian Monsoon Tropics (AMT) remains little studied.

The AMT has low human population density, mostly

concentrated in a few isolated towns. The primary reason for

the limited development in the region is the harshness of the

environment (Ridpath et al., 1991; Woinarski et al., 2007).

Soils are generally infertile, and the climate is characterized by

a seasonal oscillation between tropical wet and dry seasons.

Broad plains flood in the wet season, and stony hills and

sandstone and laterite plateaux have sparse surface waters in

the dry season. Hot, windy conditions and desiccated biomass,

particularly of tall tropical grasses, make fire an outstanding

characteristic of the dry season (Woinarski et al., 2007).

The AMT includes one of the least developed savanna zones

on Earth; globally the area supports about 25% of the

remaining tropical savanna in good ecological condition

(Woinarski et al., 2007). Thus in the AMT it is possible to

study a great expanse of minimally developed and biologically

diverse tropical and semi-arid landscapes at a continental scale,

to document how hunter-gatherers interacted with their

environment, and to chart the ecological changes associated

with changed fire regimes and invasive plants and animals.

This intact environment also invites analysis of distributional

patterns to ask broader questions relating to the evolutionary

origins and adaptive radiations of the monsoonal biota. Which

taxa are relicts from Gondwana that have persisted in

environmental refugia? Which autochthonous elements have

radiated adaptively in the region? Which species have arrived

in the recent geological past, by crossing Wallace’s Line? Do

current distributional patterns of a cross-section of organisms

recapitulate the region’s biogeographical history? Has the

seasonal climate stimulated novel adaptive responses among a

suite of organisms? There has been a long tradition of raising

such questions about northern Australia in the belief that it is

the gateway for biotic exchange with the rest of the Old World,

but only recently have there been sufficient distributional data

and robust taxonomic research to tackle these problems

meaningfully.

Frontiers are dynamic places, where social and environ-

mental change is the norm. The AMT provides an opportunity

to chart the ecological changes associated with humans. In

the AMT there is mounting pressure to exploit the region’s

abundant water resources and establish broad-scale agricul-

ture, driven by the recent and sustained drying trend in the

southern Australian agricultural belt. Thus the window of

opportunity to study the biogeography of this largely ecolog-

ically intact region is quickly closing. Such biogeographical

knowledge is not only of fundamental scientific interest, but is

critical for informed natural resource management and

conservation planning. The purpose of this review is to take

stock of what is known about the historical biogeography of

the AMT, to identify knowledge gaps, and to consider how this

knowledge can be applied to the challenge of conserving

biodiversity of the region.

GEOLOGICAL FEATURES

Northern Australia includes stable, deeply weathered land-

forms of great antiquity (Precambrian basement rock) through

to recent dynamic sediments (Quaternary alluvials, dunes and

estaurine muds). A striking feature is the Proterozoic sand-

stone, which forms blocks with abrupt escarpments. Three

major sandstone regions – Kimberley, Arnhem Land plateau

and the ranges of Cape York Peninsula – are separated by

topographic barriers associated with Cretaceous sea floors

(Fig. 1). The Cape York ranges are isolated from the rest of the

sandstone in the AMT by clay plains that form the hinterland

of the Gulf of Carpentaria, often referred to as the Carpen-

tarian Gap (MacDonald, 1969); the clay pans abut the

Borroloola block in the western Gulf region, which is

essentially a south-eastern extension of the Arnhem Land

plateau. The Kimberley and the Arnhem Land plateau are

separated by the narrower Ord Arid Intrusion and to some

extent the plains of the Daly River region of the Bonaparte

Gulf. Being near-coastal uplands, these ranges provide moister

habitats than elsewhere in the monsoon region by generating

some orographic rainfall and providing shady gorges

and slopes.

TODAY’S MONSOON CLIMATE

The major meteorological influence on the AMT is the

Australian summer monsoon. The term monsoon is used to

describe tropical regions with a dominant annual cycle in

rainfall and seasonal reversal in winds (Fein & Stephens, 1987).

In the Australian context, the word has been used since the

time of the early European explorers (e.g. Flinders, 1814).

However, it was not studied quantitatively until the seminal

paper of Troup (1961), who noted that the Australian

monsoon is characterized by periodic episodes of low-level

westerly monsoon winds accompanied by widespread rainfall.

These monsoon bursts (or wet-westerly spells) occur only

through austral summer months, and their accumulation

constitutes the monsoon season.

The Australian monsoon is a component of the large-scale

Asian–Australian monsoon system (e.g. McBride, 1998; Web-

ster et al., 1998). The monsoon is associated with the seasonal

migration of the Inter-Tropical Convergence Zone (ITCZ), the

D. M. J. S. Bowman et al.

2 Journal of Biogeographyª 2009 Blackwell Publishing Ltd

zone where the surface trade winds of the northern and

southern Hadley cells converge and drive rising air and

convection. The ITCZ is located 10–15� north of the equator in

austral winter, and migrates southward over the north of the

Australian continent during austral summer, driven by the

development of intense heat lows over north-west West

Australia and north-west Queensland (Sturman & Tapper,

2006). The north Australian region experiences dry low-level

south-easterly trade winds in winter, which undergo a reversal

to dominant moist north-westerly flow in summer. While

monsoon onset typically occurs between late November and

early January, the monsoon season includes ‘break’ periods

when south-easterly flow brings temporarily dry conditions.

These transient components or intraseasonal oscillations are a

fundamental component of the monsoon system (e.g. Wheeler

& McBride, 2005; Hoyos & Webster, 2007).

In terms of defining the AMT as a biogeographical region,

the southern limit can be defined by areas that receive more

than 85% of the rainfall between November and April (Fig. 2).

These areas are tropical (high temperature) and the major

rainfall events are associated with disturbances in the monsoon

trough or ITCZ (McBride & Keenan, 1982). Despite the fact

that the underlying mechanisms are monsoonal by any

definition, for northern Australia this does not necessarily

mean high annual rainfall. A characteristic of the AMT is its

strong latitudinal gradient of rainfall, with large regions along

the northern and eastern coastlines having annual median

rainfalls exceeding 1500 mm, but with much of the inland

region (delineated by the 85% and 90% contours in Fig. 2)

experiencing annual medians in the range 300–600 mm.

Besides the dominant seasonal cycle, the transient component

of ‘active’ and ‘break’ phases of the monsoon (wet and dry

periods during the season) is of fundamental importance.

Ongoing research aims to identify the extent to which both

interannual and intraseasonal rainfall variability play a role in

determining vegetation type and distribution of taxa.

El Nino–Southern Oscillation

We assume that the above-defined monsoon circulation is a

key factor determining the current distributional patterns of

the biota and ecosystems in the AMT. There are, however,

other underlying climatic influences. One of these is the El

Nino–Southern Oscillation (ENSO), which leads to interan-

nual variability of monsoon rainfall due to an influence on the

timing of monsoon onset and duration (e.g. Nicholls et al.,

1982; McBride & Nicholls, 1983; Lo et al., 2007). ENSO also

influences the occurrence and location of tropical cyclones

in the region, which in turn contribute to monsoon rainfall

and may trigger monsoon onset. ENSO also influences

Figure 1 Location map showing monsoonal

regions and major biogeographical barriers.

Monsoonal regions: (1) The Kimberley,

(2) The Top End, including Arnhem Land,

(3) Cape York Peninsula, (4) Trans-Fly

Plains, southern New Guinea. Elevated areas,

which include scattered sandstone blocks, are

in light shading. Low sea-level stand

()150 m) coastline indicated by dark shad-

ing. Biogeographical barriers: (A) Gulf of

Carpentaria, (B) Carpentarian Gap,

(C) Ord Arid Intrusion.

120E

50 55 60 65 70 75 80 85 90 95 100

140E

(%)

40S

30S

20S

10S

Figure 2 November–April rainfall as a percentage of annual

rainfall (1957–2005) in Australia, showing northerly distribution

of summer rain.

Biogeography of the Australian monsoon tropics

Journal of Biogeography 3ª 2009 Blackwell Publishing Ltd

landscape fire activity through its impact on seasonal rainfall

and therefore fuel loading.

ORIGIN AND EVOLUTION OF THE MONSOON

CLIMATE

Limited palaeontological evidence from sites in central and

northern Australia suggests that a monsoonal climate origi-

nated between the Late Eocene and Early Oligocene (40–

30 Ma) (Greenwood, 1996; Pole & Bowman, 1996; Macphail &

Stone, 2004). Other evidence for a monsoonal climate comes

from silcrete fossil soils, which formed in a major event from

40 to 30 Ma in the Lake Eyre basin (Alley, 1998; Frakes, 1999;

Alexandre et al., 2004). Both lines of evidence suggest that

strong rainfall seasonality had commenced by the time

Australia separated from Antarctica. Lake deposits found

across central Australia suggest that relatively high rainfall

persisted from c. 30 to 15 Ma, but with the slow drift of

Australia northward, drying of the Australian continent

became widespread (Truswell, 1993; McGowran et al., 2004).

Uplift of the Tibetan plateau and steepening of the

temperature gradient between the equator and South Pole

c. 15–8 Ma (Bowler, 1982; McGowran et al., 2004) provided

conditions conducive to development of the fundamental

aspects of Australian Monsoon climate pattern: an intensified

anticyclonic high-pressure cell and compression of circulation

belts, with consequent general development of arid conditions

from south to north. Progressive aridification of central

Australia intensified between 4 and 2 Ma (Fujioka et al.,

2005). Continent-wide conditions that first approximated

those of the present occurred after the uplift of the Panama

isthmus and the establishment of modern circulation patterns

in the Pacific by c. 3–2.5 Ma (Haug & Tiedemann, 1998).

Enhanced climatic oscillations began with the onset of

Quaternary glacial cycles, with associated fluctuations in

temperatures, precipitation and major shifts in sea level

(Fig. 3). Monsoon rainfall varied enormously over interglacial

to glacial time-scales.

The origin of the monsoon and ENSO reflects the complex

interactions between continental drift, ocean currents and

global climate associated with orbital forcing and biogeochem-

ical feedbacks. Circumstantial evidence associated with biota,

fossils, phylogenies and landforms, such as laterite, point to the

great antiquity of hot seasonal climates, which would be

consistent with a palaeo-monsoon (summarized in Fig. 3).

However, it is impossible to determine with current evidence if

these past climates were strictly monsoonal in the sense of

being part of Hadley cell circulations. Given the limited

knowledge base of the palaeo-monsoon, further understanding

of the biogeography of the Australian monsoon is important to

provide insights into evolutionary processes that enable

organisms to cope with seasonally dry hot climates, which

characterize the AMT.

FIRE AND THE ARRIVAL OF PEOPLE

The AMT is the most fire-prone region in Australia. The high

diversity of fire-adapted organisms indicates a long history of

Figure 3 A summary of the history of changes and influences on the monsoon in Australia (see ‘Origin and evolution of the monsoon

climate’ for detail on source of information for these events). The diagram incorporates key developments in time (note logarithmic scale on

the x-axis) of the fundamental aspects of the Australian Monsoon, which have been outlined by Bowler (1982), McGowran et al. (2004),

Miller et al. (2007) and Byrne et al. (2008). The natural climate drivers associated primarily with the Milankovich cycles of solar insolation

are represented schematically in the upper curve (not to scale) and are likely to reflect fluctuations in monsoon rainfall (more than

temperature) over interglacial to glacial time-scales in the tropics. This depicts the progressive desiccation over the past 20 million years

across Australia culminating in extreme aridity during glacial cycles (Byrne et al., 2008).



exposure to fire – a potent selective agent. Fire relates to the

annual cycle of fuel build-up and abundance of flammable

grasses during the wet season, and is triggered by electrical

storms. In contrast to the humid tropics, above-average annual

rainfalls in the monsoon tropics associated with the ENSO lead

to increases in herbaceous biomass production (e.g. Scanlon

et al., 2005), which in turn increases fire frequency and severity

in the subsequent dry season (Felderhof & Gillieson, 2006).

Fire activity declines with decreasing mean annual rainfall and

reduced herbaceous biomass production (Spessa et al., 2005;

Felderhof & Gillieson, 2006).

The palaeoecological record suggests that fire frequencies

have changed dramatically through time. Fire may have been

infrequent or even absent, for example during the Last Glacial

Maximum, with weak monsoon and cooler temperatures.

Prior to the arrival of people in the Australian–New Guinea

region, the fire regime was most probably characterized by

infrequent high-intensity fires (Bowman, 2002). Thus pre-

human landscapes are likely to have been different from today,

with fire-adapted species less abundant, the savanna (mixtures

of tropical grass and trees) more geographically restricted, and

evergreen dry forests and rain forests more widespread.

With the arrival of people, the fire regime shifted towards

more frequent, and eventually to less intense and smaller-sized

fires, which is likely to have changed the balance between fire-

adapted and fire-sensitive taxa, possibly causing some extinc-

tions or contractions to fire-protected refugia (Bowman,

2000). For example, in eastern Australia, Dacrydium sp.

(Podocarpaceae) disappears around 25 ka (and no earlier

than 40 ka) at Lynch’s Crater (Queensland) and Caledonia Fen

(Victoria), possibly due to greater fire activity (Bohte &

Kershaw, 1999; Kershaw et al., 2007). Such a process may

explain the very restricted occurrence of the conifer Podocarpus

grayae in a few canyons in western Arnhem Land (Russell-

Smith et al., 1993). In a pollen record from lowland Papua

New Guinea, there was a rapid and dramatic shift from wet

swamp forest to open savanna around 4 ka – clear evidence

that people caused the shifting of vegetation boundaries

(Haberle, 2007). Miller et al. (2007) speculate that the strength

of the monsoon was reduced by destruction of woody habitats

by the anthropogenic fire regime: this may be coincidental

rather than causal, or any effect may have been only on local

weather patterns.

Since the arrival of Europeans more than 200 years ago, the

fire regime has changed to one of more frequent and more

intense fires. Species that are pre-adapted to Aboriginal

burning regimes, including the frill-necked lizard (Chlamydo-

saurus kingii) (Griffiths & Christian, 1996), northern brown

bandicoot (Isoodon macrourus) (Pardon et al., 2003), partridge

pigeon (Geophaps smithii) (Fraser et al., 2003) and the

northern cypress pine (Callitris intratropica) (Bowman et al.,

2001), have been disadvantaged. Another consequence is the

recent expansion of grassy understorey, which in turn has

amplified the grass–fire cycle, driven by the desire of land

managers to burn more frequently as grasses become more

dominant (Bowman et al., 2007).

BIOTIC ELEMENTS OF THE AMT

A suite of organisms are limited to the tropical monsoon

region of northern Australia and provide a distinctive signa-

ture of biological coherence of the AMT biome (Crisp et al.,

2004). Some species are relatively common and widespread.

Notable examples with broad distribution patterns include the

Darwin stringybark (Eucalyptus tetrodonta) and woollybutt

(Eucalyptus miniata–Eucalyptus chartaboma group); northern

cypress pine (Callitris intratropica); broad-leaf paperbarks

(Melaleuca leucadendra group); ironwood (Erythrophleum

chlorostachys); the common rock rat (Zyzomys argurus); bush

coconuts (galls induced by the scale insect Cystococcus); the

grass-skipper butterflies (Neohesperilla); and the impressive

cathedral termite mounds constructed by Nasutitermes triodiae

and magnetic termite mounds built by Amitermes spp.

Termites play a key ecological functional role (Andersen et al.,

2005), and they have greater diversity and abundance in

this region than elsewhere in Australia, with more than 100

species recorded from northern Australia (Watson & Abbey,

1993).

Distinct units or bioregions can be recognized within the

AMT, reflecting the aggregated effects of climate, vegetation

structure, ecophysiology and shared history. Thackway &

Cresswell (1995) identified about 18 such bioregions, although

more research is required to determine biogeographical

zonation within the AMT.

Vegetation and habitat types

The AMT supports a range of habitats and vegetation types,

including savanna, rain forest, heath and shrublands, man-

grove, salt-marsh flats and freshwater (Woinarski et al., 2007).

The most extensive vegetation type is savanna, in which the

canopy is usually dominated by eucalypt trees and the

understorey comprises a layer of tall, dense C4 grass and

scattered broadleaf shrubs. Savanna on oligotrophic sandy soils

has an understorey with a higher proportion of sclerophyll

shrubs and the grass layer is dominated by Triodia (spinifex).

In contrast, on richer clay substrates, eucalypts are less

common and are replaced by other trees (e.g. Melaleuca,

Bauhinia and Vachellia), or may be absent, as is the case in the

extensive Mitchell grass (Astrebla) plains of the Barkly

Tableland. On seasonally flooded savannas, shrubs are replaced

by herbs (e.g. Heliotropium), and grasses are replaced by

sedges. These richer-soil communities are more typical of

savannas world-wide, such as those in Africa. All of the AMT

savanna vegetation types are highly adapted to torrential rains,

periodic flooding and fire, having rapid growth during the wet

season and low growth or dormancy during the seasonal

aridity.

Rain forest occurs patchily in areas where soils are more

fertile, where the landscape is protected from fire, and where

water is available during the dry season along perennial rivers

or springs associated with underground aquifers. The highest-

density rain forest occurs in western Arnhem Land and is



dominated by the regional endemic Allosyncarpia ternata

(Myrtaceae). Coastal sites typically support littoral rain forest

or semi-deciduous monsoon vine-thicket, while gorges and

escarpments house gallery forest or evergreen forest.

Tropical heathlands persist on the most infertile, acidic soils

with high water drainage, and occur in limited areas on Cape

York Peninsula and the rocky plateaux of Arnhem Land and

the Kimberley. Mangrove and salt-marsh flats occur along the

northern coastline of Australia. In the Top End and Kimberley,

mangrove closed-forests support animals more typically asso-

ciated with rain forest communities on Cape York Peninsula.

A complex network of seasonal rivers and streams carry

freshwater to the sea. During the wet season, some of the

largest rivers feed into extensive floodplains in the near-coastal

lowlands, forming significant habitats, such as the wetlands of

Kakadu and the south coast of the Gulf of Carpentaria.

The sandstone regions form habitat islands in the sur-

rounding matrix of the ‘savanna sea’. They have the highest

level of endemism in the AMT (Woinarski et al., 2006),

although they are less species-rich than their counterparts in

southern Australia. Some endemics have very narrow ranges,

restricted to very specific habitats, including deep gorges, steep

escarpments and cliffs. Overall species diversity is greater in

Arnhem Land than in the Kimberley, which may reflect greater

extinction in the latter, especially during glacial arid cycles.

While the extensive lowland sandy and lateritic plains that

surround the sandstone blocks share taxa in common, such as

chestnut mice (Pseudomys spp.), Triodia and eucalypts, they

have lower endemism, being less isolated and providing fewer

refugia from landscape fire or past climate change.

Evergreens in a seasonal environment

On a global scale, the AMT is exceptional in that evergreen

trees are dominant despite the extreme seasonality of rainfall

(Bowman & Prior, 2005). It is well established that deciduous

species are disadvantaged by the highly variable onset of

growing season and by infertile soils, because leaves are

‘expensive’ to construct and maintain (Chabot & Hicks, 1982;

Sobrado, 1991). Evergreens are predicted where there is large

inter-annual variation in the onset of the wet season. Further,

the deeply weathered, infertile soils select for prolonged

lifespan of leaves because scarce nutrients are costly to obtain

(Givnish, 2002; Orians & Milewski, 2007). The deep regoliths

are very effective in storing wet-season rains, and this stored

soil water is tapped by the deep root systems of the evergreen

trees, enabling them to continue to grow, and often flower and

set seed, during the dry season. Fire is a constraint on

evergreens through loss of all or part of the canopy. This

disturbance may explain why only a few groups of evergreen

trees, notably eucalypts, dominate the savanna landscapes.

IN SEARCH OF BIOGEOGRAPHICAL PATTERNS

The isolation of Australia following rifting from Antarctica

(c. 45–32 Ma) has undoubtedly shaped its biota, but the

contributions of in situ adaptation and dispersal to the

assembly of Australian monsoon communities is uncertain.

Furthermore, these processes may differ among biomes,

regions and taxa.

The AMT differs from other areas of Australia in being the

closest to Southeast Asia, thus minimizing the distance for

dispersal. Major sea-level shifts occurred during Quaternary

glacial cycles. Compared with today, sea level ranged from

)120 to )140 m during glacial maxima to +5 to +8 m during

the warmest interglacials (Hope et al., 2004). These shifts

caused alternating appearance and disappearance of wide

continental shelves, including land bridges between mainland

Australia and New Guinea, and the formation of a major

shallow lake, Lake Carpentaria (Chivas et al., 2001), with brief

periods of freshwater alternating with brackish or saline

conditions during low sea level.

Taxa that dispersed into the AMT may already have been

pre-adapted to strongly seasonal patterns, or they may have

become established in Australia in another biome before

moving into the region. Taxa that originated in situ could

have adapted to the monsoonal climate from ancestors that

were adapted to pre-existing climatic regions. Such diver-

gences between AMT taxa and their sisters in other biomes

could have occurred any time since about 40 Ma, which is

the earliest of the estimates for the origin of the monsoon

climate in Australia. Examples of taxa originating in situ

include eucalypts, ‘egg and bacon’ peas (Fabaceae: Mir-

belieae), Melaleuca, Syzygium, Casuarina, Allocasuarina,

freshwater fishes, and most passerines and marsupials.

Possible examples of immigrant taxa include rodents (Aplin,

2006); genera of peas (Fabaceae: Faboideae) such as

Crotalaria, Tephrosia, Indigofera, Cullen and Kennedia/

Vandasina, whose crown groups are all estimated to be

< 25 Myr old (Lavin et al., 2005); Cycas (Hill, 1998); boab

(Adansonia; Baum et al., 1998); Bambusa (Franklin, 2008),

Bauhinia and Erythrophleum (pantropical genera with one or

two Australian species) and Bombax ceiba (widespread in

south Asia and Australia). Examples of Australian exports

include Pittosporaceae (Chandler et al., 2007), Scaevola

(Howarth et al., 2003), eucalypts and Melaleuca (Ladiges

et al., 2003).

Given the climatic and geological history of the AMT, and

the heterogeneity within the region, it is widely assumed that

there are strongly congruent biogeographical patterns exhib-

ited by taxa in disjunct habitats (such as the sandstone

blocks). If the isolation of disjunct endemics has a common

cause, such as climatic or sea-level changes, or migrations

such as hypothesized by Schodde (1989), then congruence

would be predicted in the patterns and timing of divergences

across different lineages. Further, taxa of the savannas should

exhibit high levels of connectivity and wider distributions

compared with endemics of the rocky country or drainage

systems. Generalist savanna species may present varied

histories compared with the more narrowly distributed

endemics of the sandstone blocks (Ford & Blair, 2005).

Congruence may be expected in host-specific interactions,



where the distribution of parasites should match that of

their hosts.

Cracraft (1991) hypothesized area relationships within

Australia based on a broad sample of vertebrates and

parsimony analysis of endemicity. Although his approach has

been criticized (e.g. Nelson & Ladiges, 1991) his conclusions

are a basis for further testing (see Crisp et al., 1995). He found

that the Kimberley (K) and Arnhem Land (L) are sister areas,

and that these two areas together are related to those to the

east, including Cape York (Y), Atherton (A, sister areas),

eastern Queensland (Q) and south-east Australia (S). This

pattern, (K + L)(Y + A, Q + S), suggests a major vicariant

event that separated the Kimberley and Arnhem Land region

from the east – the Carpentarian Gap.

The following case studies illustrate how biogeographical,

phylogenetic and ecological studies shed light on the evolution

of the AMT biota.

Eucalypts

The eucalypts are a lineage of seven genera of capsular fruited

Myrtaceae that has origins in the Late Cretaceous, with major

lineages differentiated by the Palaeogene (Ladiges et al., 2003).

Three genera are relictual taxa in northern rain forest. The

monotypic Allosyncarpia (Allosyncarpia ternata) dominates

monsoonal rain forest on the Arnhem Land sandstone plateau

(Russell-Smith et al., 1993). It is related to Stockwellia and its

sister taxon Eucalyptopsis (Fig. 4a). Monotypic Stockwellia

(Stockwellia quadrifida) is endemic to the Wet Tropics of

north-east Queensland (Carr et al., 2002). Eucalyptopsis (two

species) occurs in New Guinea, Woodlark Island and the

Moluccan Archipelago (White, 1951; Craven, 1990). The

divergence between Allosyncarpia and the other two genera is

congruent with Cracraft’s area cladogram, and probably dates

back to at least the Oligocene (30–25 Ma, Ladiges et al., 2003;

37–35 Ma, Crisp et al., 2004). Rain forest is assumed to have

been extensive throughout the Palaeogene, with its contraction

and replacement by open forest and woodland occurring with

subsequent increasing seasonality of rainfall and associated fire

(Archer et al., 1989; Truswell, 1990; Bowman, 2000). Russell-

Smith et al. (1993) and Prior et al. (2007) concluded that

Allosyncarpia survives today in valleys and gorges by its

tolerance of seasonally dry substrates but is restricted in

distribution by its sensitivity to fire.

Within the sclerophyllous eucalypts, the genera Corymbia

and Eucalyptus have geographically overlapping clades in the

AMT. Northern endemic red bloodwoods occur in the

Kimberley, Arnhem Land, Cape York–New Guinea, north-

eastern Queensland and Burdekin region. Among the paper-

fruited bloodwoods, Corymbia section Blakearia, series

Confertiflorae occurs in the Kimberley, across the Top End

and in Papua New Guinea (Fig. 4b). There is a replacement

sequence of species, including the narrow-range endemics

Corymbia dendromerinx (south-west Kimberley), Corymbia

karelgica (central Kimberley), Corymbia pauciseta (Arnhem

Land), and two non-overlapping widespread species Corymbia

confertiflora and Corymbia disjuncta (Hill & Johnson, 1995).

Series Confertiflorae is related to series Tessellares (e.g.

Corymbia tessellaris), which occurs in Queensland and New

Guinea.

Eucalyptus subgenus Eudesmia is another example of the

nested biogeographical patterns of nine taxa within the AMT.

The widespread distribution of species such as Eucalyptus

tetrodonta, which extends from the Kimberley to eastern Cape

York Peninsula, may be explained by an ability to grow not

only on oligotrophic soils derived from sandstone and laterite,

but also on well drained clay soils, such as on the plains of the

Carpentarian Gap. In contrast, other species are endemic to

particular regions. For example, Eucalyptus lirata (Kimberley)

is sister to Eucalyptus similis of eastern Queensland (Gibbs

et al., 2009). This pattern overlaps that of the tropical boxes,

Eucalyptus subgenus Minutifructus (Brooker, 2000), with

Eucalyptus brachyandra in the Kimberley and north-west

Northern Territory related to Eucalyptus howittiana (Queens-

land Burdekin region), Eucalyptus raveretiana (Queensland)

and Eucalyptus deglupta (which extends from New Britain and

northern New Guinea to Sulawesi and Mindanao in the

Philippines).

Future combined-area cladistic analysis of these various

eucalypt groups has the potential to test congruent patterns,

vicariant events, and range expansions and contractions that

are of various ages, some probably deep and relatively old,

some relatively recent.

Peas

The egg and bacon peas of genus Jacksonia (Fabaceae:

Mirbelieae) are restricted to oligotrophic sandy and lateritic

soils, with disjunct narrow-range endemics across the mon-

soon tropics. Jacksonia (Fig. 4c) includes 20 species in the

region, which is less species-rich compared with south-west

Western Australia with 53 species (Chappill et al., 2008). The

unpublished phylogeny by M. D. Crisp, L. G. Cook and D. C.

Morris is from maximum-likelihood analysis of cpDNA

sequences (ndhF and trnL-trnF). The two main clades show

a deep divergence between a lineage in the north-western

monsoon tropics (Jacksonia aculeata, Jacksonia forrestii,

Jacksonia ramosissima and Jacksonia thesioides) and a lineage

exending along the east coast (Jacksonia scoparia and Jacksonia

chappilliae).

Butterflies

The biogeography of butterflies in the AMT was analysed

recently by Braby (2008) in terms of patterns of species

richness, endemism and area relationships. It was hypothesized

that the Carpentarian Gap is a biogeographical filter, func-

tioning as a barrier for some species but as a corridor for

others, and that divergence among taxa between Cape York

Peninsula and the Top End–Kimberley has occurred relatively

recently, probably through vicariance during the Quaternary.

Available data indicate that the region supports a relatively rich



(a)

(b)

(c)

(d)

(e)

(f)

(g)

(h)

(i)



fauna, comprising 265 species (c. 62% of the total Australian

fauna), but endemism is low (14 species or 6%).

No genus is endemic to the AMT, but two genera

(Neohesperilla and Nesolycaena) are characteristic components.

Of 67 species restricted to the AMT, 15 (nearly all associated

with savanna) are endemic to the region, while 52 (mostly

associated with rain forest or monsoon forest) are non-

endemic, occurring also in Southeast Asia and/or mainland

New Guinea. A pronounced attenuation in species richness

from Cape York Peninsula across the Gulf of Carpentaria to

the Top End and Kimberley is evident. Cape York Peninsula

stands out as an area of exceptional biodiversity, with 95% of

the butterflies (251 species, seven endemics) recorded from the

entire monsoon tropics, compared with the Top End (123

species, three endemics) and Kimberley (85 species, one

endemic). Differences in species richness between Cape York

Peninsula and the Top End–Kimberley probably reflect

differences in the extent of tropical forest refugia and

proximity to mainland New Guinea (Kikkawa et al., 1981;

Kitching, 1981) and in levels of extinction during the Last

Glacial Maximum. A sister-area relationship between Cape

York Peninsula and the Top End is evident in one clade

(Acrodipsas hirtipes + Acrodipsas decima), with a pairwise

divergence of about 1% based on mitochondrial DNA

(Eastwood & Hughes, 2003; Braby, 2008), and is suspected

in another (Nesolycaena; see below). A further five species

show similar sister-area relationships across the Carpentarian

Gap, but at the level of subspecies or geographical form.

Nesolycaena exemplifies the biogeographical patterns of the

sandstone blocks in the AMT. The genus contains four species,

all of which are allopatric and have narrow-range, patchy

distributions (Braby, 1996). Nesolycaena caesia is restricted to

the Kimberley; Nesolycaena urumelia occurs more widely in the

Top End and western Gulf Country, including Arnhem Land;

Nesolycaena medicea is confined to the White Mountains

in northern Queensland; while Nesolycaena albosericea is

restricted to south-eastern Queensland (Fig. 4d). The larvae

of these butterflies specialize on species of Boronia section

Valvatae as their food plants, which likewise include narrow-

range endemics on sandstone and granite sands (Duretto &

Ladiges, 1999). Nesolycaena caesia and N. urumelia are sister

taxa, and this clade is sister either to N. medicea or to

Figure 4 Geographical distribution and phylogenetic hypotheses. (a) Distributions, relationships and vicariance of three relictual rain

forest genera of eucalypts, including Allosyncarpia endemic to the AMT (Arnhem Land). (b) A clade of five species of paper-fruited

bloodwood eucalypts, Corymbia series Confertiflorae, shows a replacement pattern in the AMT and parts of southern Papua New Guinea.

All species are facultatively deciduous in the dry season (map after Hill & Johnson, 1995, Figure 107). Corymbia dendromerinx (triangle),

C. karelgica (solid circle), C. disjuncta (open circle), C. pauciseta (square), C. confertiflora (cross, drier regions to the south). Molecular

phylogenetic analysis of all species is lacking, but it is hypothesized that C. dendromerinx (south-west Kimberly) is related to C. karelgica

(central Kimberley); and C. pauciseta is related to C. disjuncta (both of open savanna, wetter north coast). Inset photo (P. Y. Ladiges):

C. dendromerinx, Kimberley. (c) Jacksonia (egg and bacon peas; Fabaceae: Mirbelieae) includes two main clades based on chloroplast DNA

sequences, which show a deep divergence between a lineage in the monsoon tropics (J. aculeata, J. forrestii, J. ramosissima and J. thesioides)

and a lineage extending along the east coast (J. scoparia and Jacksonia chappilliae). Inset photo (M. Crisp): J. thesioides. (d) Butterfly

Nesolycaena (Lepidoptera: Lycaenidae) is associated with sandstone plateaux: N. caesia (Kimberley, Western Australia); N. urumelia (Top

End, North Territory, and western Gulf Country, Queensland); N. medicea (White Mountains, northern Queensland); N. albosericea (south-

eastern Queensland). Phylogeny based on morphological characters of the four species (Braby, 1996). Inset photo (M. F. Braby):

N. urumelia. (e) Haplotype network for the gall-inducing scale insect Cystococcus, based on 18S rDNA. Galls (bush coconuts, bloodwood

apples) are restricted to eucalypts of the red-bloodwood group (Corymbia section Rufaria) in the AMT. Photo insert (L. G. Cook): a

cylindrical, legless and wingless adult female of Cystococcus pomiformis seen in the centre of the gall. (f) Phylogenetic relationships among the

Australian grass finches: black-throated finch Poephila cincta of north-eastern Australia and two closely related species of long-tailed finch,

P. acuticauda (Kimberley) and P. hecki (Top End) (Jennings & Edwards, 2005). One individual of each of the three populations was

sequenced for the same 30 anonymous nuclear loci. Coalescence-based methods were used to analyse multi-locus data for estimation of

divergence times among populations and ancestral effective population sizes. Bayesian 95% credibility intervals of divergences between

P. acuticauda and P. hecki (300,000 years ago) and between P. cincta and the others (600,000 years ago) are dated as within the Pleistocene.

Effective population sizes for basal ancestor and long-tailed finch ancestors were 521,000 and 384,000, respectively. (g) Four of five rock rats

(Zyzomys) occur in monsoon Australia. The common rock rat (A: Z. argurus; orange) has a broad monsoonal distribution, while the larger

bodied Kimberley rock rat (B: Z. woodwardi; pink), Kakadu rock rat (C: Z. maini; blue) and Carpentarian rock rat (D: Z. palatalis; green) are

restricted to moister sandstone escarpments. Population studies of isozymes suggest that the common rock rat shows little genetic divergence

across its range, and the only phylogeny to include all species suggests that the larger-bodied species may not be sister taxa. Inset photo (Jiri

Lochman): a large rock rat. (h) Fishes (Leiopotherapon). Below, major freshwater drainages (3–16) of northern Australia, based on Table 1

and Figure 6 in Unmack (2001). Upper left, relationships of freshwater drainages (3–16) based on parsimony analysis of endemism of 88

species occurring in more than one drainage throughout Australia, showing Gulf drainages (5–13) related to those of the Kimberley (3–4)

more closely than to other drainages farther south, based on Table 1 and Figure 12 in Unmack (2001). This pattern also reflects relationships

among genera of grunter perches (Teraponidae), Pingalla (Gulf), Syncomistes (Kimberley), Scortum (south) (Figure 9 in Vari, 1978;

distributions from Allen et al., 2002). Upper right: spangled perch, Leiopotherapon unicolor (Teraponidae), the most widespread freshwater

fish in Australia (http://www.mdbc.gov.au/subs/fish-info/native_info/). (i) The giant freshwater prawn (Macrobrachium rosenbergii) occurs

in coastal river systems across northern Australia and in New Guinea. The network is based on mitochondrial COI sequence data from 457

individuals collected from across the range. Figure modified from de Bruyn et al. (2004). Inset photo (M. de Bruyn): M. rosenbergii.



N. medicea + N. albosericea. Isolation of populations over

time presumably facilitated speciation of these butterflies,

and a phylogenetic analysis of the group with estimates of

divergence times would be extremely rewarding. Phylogenetic

relationships of Boronia (Duretto & Ladiges, 1999) do not

provide evidence of co-speciation of the two lineages,

suggesting that the plants and butterflies have independent

evolutionary histories in the AMT.

Gall-inducing scale insects

Australia has a diverse range of gall-inducing scale insects

(Cook & Gullan, 2004; Gullan et al., 2005) that are represented

in the AMT by several examples. Apiomorpha hilli is restricted

to inducing galls on Darwin woolybutt (Eucalyptus miniata)

(Gullan, 1984; L. G. Cook, unpublished data). The host-

specific females of Cystococcus induce galls (called bush

coconuts) on branchlets of red-bloodwood eucalypts (Corym-

bia section Rufaria) (L. G. Cook and P. J. Gullan, unpublished

data). The distribution of this endemic genus matches closely

the climatic envelope circumscribing the AMT. A haplotype

network for Cystococcus, based on 18S rDNA (Fig. 4e), suggests

geographical structuring among Cape York, Northern Terri-

tory and Kimberley populations of Cystococcus pomiformis

(L. G. Cook, unpublished data).

Birds

Avian diversity across the AMT has been described quantita-

tively, most importantly by Ford (1978), Johnstone & Burbidge

(1991) and Schodde (2006). Basic patterns that emerge and

mirror patterns in other taxonomic groups include: (1) ende-

mism present in Arnhem Land and Kimberley sandstone blocks,

including some endemism in the south-eastern extension of the

Arnhem Land plateau block; (2) attenuation in species diversity

west from Cape York Peninsula to the drier Kimberley monsoon

rain forests; (3) high diversity of granivores and nectarivores in

the savannas; and (4) other instances of endemism, such as the

purple-crowned fairy-wren, Malurus coronatus, which is re-

stricted to riparian Pandanus vegetation in the Kimberley and

Gulf country. Notably, the AMT has the highest bird diversity in

mangroves in Australia (Ford, 1982; Johnstone, 1990).

Historical processes responsible for this avian diversity

appear to include vicariance, adaptation to prevailing ecology

and tracking of historical changes in the landscape – e.g.

exposure of the Arafura Platform and impact of changes about

the Carpentarian Gap. Further studies are needed to build on

work already conducted (Keast, 1961; MacDonald, 1969) to

explore the relative importance of these processes, whether the

Carpentarian Gap has been a barrier or corridor, or both. At

the species level, Jennings & Edwards (2005) estimated

divergence times among different populations of the black-

throated finch Poephila cincta complex (Fig. 4f). Divergence

across the Carpentarian Gap between P. cincta to its east and

P. acuticauda + P. hecki to its west was estimated at

600,000 years ago. Divergence between populations of

P. acuticauda + P. hecki on either side of the Ord Arid

Intrusion, which divides the Kimberley and Arnhem Land

sandstone blocks and through which these populations inter-

grade (Schodde & Mason, 1999), was estimated at

300,000 years ago. A later study by Lee & Edwards (2008)

estimated the divergence across the Carpentarian Gap within

another species, red-backed fairy-wren (Malurus melanoceph-

alus), as having occurred c. 270,000 years ago. This is about

half as recent as the species-level divergence recorded by

Jennings & Edwards (2005) across the same gap.

Parrot–moth–termite association

There is a tripartite association between parrots, termite mounds

and oecophorid moths, suggesting the possibility of co-speci-

ation. A clade of parrots, Psephotus subgenus Psephotellus, is

associated with the mounds of Amitermes and Nasutitermes

termites. The parrots excavate nesting hollows in mounds

during the short breeding season. The rare hooded parrot,

Psephotellus dissimilis, from the Top End, is associated with large

often bulbous mounds of Nasutitermes triodiae (Garnett &

Crowley, 1999; Forshaw & Cooper, 2002). This species is related

to the endangered golden-shouldered parrot, Psephotellus chry-

sopterygius, from Cape York Peninsula (Christidis & Norman,

1996), which nests mainly in old conical mounds of Amitermes

scopulus or the magnetic mounds of Amitermes laurensis

(Weaver, 1987). Nests are closely associated with a species of

moth of the genus Trisyntopa: golden-shouldered parrot with

Trisyntopa scatophaga and hooded parrot with Trisyntopa

neossophila (Edwards et al., 2007; Zborowski & Edwards, 2007;

Cooney et al., 2009). The two species of moth from the AMT

have not been found outside the range of their respective parrots

and are not known from nesting hollows of other birds.

The ecological nature of the parrot–moth association (e.g.

mutualism or commensalism) is not well understood. Larvae

of Trisyntopa feed on the faeces of the parrot nestlings and

keep the nesting hollow relatively clean; however, available

data suggest that the parrots benefit little, if at all, from

the association in terms of increased reproductive success

(S. J. N. Cooney, pers. comm.). This is an example of a highly

unusual complex relationship, unique to northern Australia,

which deserves closer study.

Rats

Rodents colonized Australia across Wallace’s Line from Asia,

around 5 Ma (Aplin, 2006), apparently following the mid-

Miocene ‘collision’ between the continents. Five species of

rodent found in the Top End are shared with New Guinea: the

mangrove-dwelling Xeromys myoides, stream-dwelling Hydro-

mys chrysogaster, and grassland/savanna-dwelling Melomys

burtoni (syn. lutillus?), Pseudomys delicatulus and Conilurus

albipes (Flannery, 1995; Breed & Ford, 2007), emphasizing the

importance of recent contact between faunas of these regions.

However, the areas of the northern Top End and north

Kimberley are the centre of endemic Australian rodent



diversity (Breed & Ford, 2007). The presence of the northern

hopping mouse, Notomys aquilo, also suggests an important

influence from periodic expansion of the arid zone during

glacial phases, because all members of this genus are highly

desert-adapted and the arid zone is the centre of their diversity

(Breed & Ford, 2007).

Part of the reason for the endemic rodent diversity of the Top

End–Kimberley is the presence of species tied to small pockets of

habitat nested within the distributions of more widespread

relatives. For example, three rock rats in the genus Zyzomys are

confined to sandstone gorges containing monsoonal vine

thickets (Fig. 4g). One species occurs in the north Kimberley,

the second in the west Arnhem Land escarpments, and the third,

the endangered Carpentarian rock rat, in only four sandstone

gorges near the head of the Gulf of Carpentaria. The common

rock rat, however, typifies the widespread nature of much of the

monsoonal fauna, being found across Cape York, the Top End,

Kimberley and also in the Pilbara. It shows very little genetic

divergence across this range (Baverstock et al., 1977).

Such nested distributions are typical of the entire

fauna (Ford & Blair, 2005), with the notable exception of the

pebble-mound mice. These mice have a unique requirement

for pebbly substrate, and accordingly, a unique biogeography

(Ford & Johnson, 2007). Pebble-mound mice have an irregular

distribution across the AMT, extending southwards into the

summer rainfall-dominated parts of the arid zone and into the

Pilbara. Pebble-mound mice and the common rock rat may

have maintained past connections of populations across the

Carpentarian Gap by exploiting temporary rocky connections

provided by low eroding lateritic mesas in the Gulf hinterlands

(Ford & Johnson, 2007). Such habitats, however, would have

been unsuitable for many species of wetter monsoonal habitats

to the extent that the Gulf of Carpentaria would have

comprised a barrier to dispersal.

DRAINAGE PATTERNS AND AQUATIC

ORGANISMS

Aquatic organisms in the AMT are limited to rivers; therefore

connectivity and isolation of populations depend on drainage

patterns. While many drainages are isolated today, they were

not in the past, because of drainage rearrangements, river

capture and lower sea levels, which resulted in rivers coalescing

before they reached the sea (Hurwood & Hughes, 1998; Waters

et al., 2001).

Freshwater fishes

Of the 10 larger families of Australian freshwater fishes, eight,

with 119 species, occur mainly in the north. Most fishes are

endemic to one or a few adjacent rivers; however, a few species

are widespread. Clusters of related species centre on larger

regions, such as the Gulf of Carpentaria, Arnhem Land, the

Kimberley and drier areas to the south. These patterns may

reflect historically widespread distributions, which have been

fragmented and reduced because of increasing aridity. Some

perhaps harbour cryptic diversity and some reflect recent or

ongoing connections, possibly due to higher dispersive ability.

For example, the most widespread fish in Australia is the

spangled perch, Leiopotherapon unicolour, which is widely

distributed in the north (Unmack, 2001). It is a hardy species

that inhabits forest streams to desert bores under variable

regimes of salinity (pure fresh to seawater), pH (4.0–8.6)

and temperature (5–44 �C). It may be capable of surviving

droughts by aestivating in wet mud or under moist litter on the

bottom of ephemeral waterholes (2007, www.fishbase.org). A

recent genetic study of the species shows very little genetic

divergence across its range, suggesting that populations have

been connected until relatively recently, probably until the

most recent aridification (Bostock et al., 2006).

What do the distributions of freshwater fishes tell us about

the boundary of the AMT and relationships of regions?

Unmack (2001) recently summarized the distributions of all

Australian freshwater fishes, including 167 species. These are

distributed among 31 river drainages. Most of nine analyses

agree in showing an association of 14 major northern

drainages, extending from the western Kimberley to the

Burdekin in the east, comprising the AMT. His parsimony

analysis of endemicity provides a hypothesis of the historical

relationships of these drainages (Fig. 4h).

The basic division is between three areas to the east (south-

eastern Cape York Peninsula, north-eastern Queensland and

Burdekin River) and those to the north and west. The

drainages of the north and west divide into western and

eastern Kimberley and a group of nine drainages surrounding

the Gulf of Carpentaria. These nine drainages divide into a

western group of six (Victoria-Ord, Daly, Arnhem Land,

Western Gulf of Carpentaria, Nicholson and Southern Gulf of

Carpentaria) and an eastern group of three (Eastern Gulf of

Carpentaria, Archer River and Cape York Peninsula).

Earlier phylogenetic analysis of three genera in the grunter

family of fishes (Vari, 1978) also showed the relationship of

taxa around the Gulf region (Pingalla) to taxa of the Kimberley

(Syncomistes) and the relationship of these to taxa farther

south and east (Scortum). These fishes may date at least from

the Eocene based on a fossil otolith attributed to the family

(Henstridge & Missen, 1982; Turner, 1982).

Freshwater fishes are also informative of a relationship

between the northern Gulf area and New Guinea, e.g. the gulf

saratoga (Scleropages jardinii), the black bream (Hephaestus

fuliginosus) and Lorentz’s grunter (Pingalla lorentzi), all three

of which occur in both areas (also see McGuigan et al., 2000).

Australian saratogas (two species) relate to the arowanas

(Scleropages species complex) of Southeast Asia and the two

arowanas (Osteoglossum) of the Amazon Basin. These inter-

continental relationships suggest a Gondwanan pattern (Nel-

son & Ladiges, 2001).

Lake Carpentaria, rainbowfishes and prawns

As recently as 10,000 years ago, Lake Carpentaria existed as a

lake into which a number of rivers drained from the Gulf of



Carpentaria basin and New Guinea (Chivas et al., 2001).

Although it is thought to have been fresh for only a short time

(between 14,000 and 12,000 years ago; Reeves et al., 2008), it

could have allowed mixing of freshwater taxa from New

Guinea and the Gulf drainage basin. Molecular data on

rainbowfishes support this hypothesis, with a group of closely

related haplotypes shared between northern Australia and

southern New Guinea (McGuigan et al., 2000).

Evidence of Lake Carpentaria as a biogeographical conduit

also comes from studies of the giant freshwater prawn

(Macrobrachium rosenbergii; de Bruyn et al., 2004). Popula-

tions from currently isolated drainages flowing into the Gulf

of Carpentaria form a clade of closely related haplotypes, with

populations estimated to have been connected during the

Late Pleistocene, coinciding approximately with the time the

lake was fresh (Fig. 4i). The prawns also show a divide

between the eastern and western rivers of the gulf, which

matches patterns found in terrestrial organisms (de Bruyn &

Mather, 2007).

SUMMARY

The emerging picture of the AMT is of a biogeographically

coherent region defined by life forms and distributions of taxa

shaped by the physical environment and geological history.

The diverse AMT biotas are adapted to the challenges of a

hot, seasonally dry, flammable environment, which is also

exposed to flooding and high humidity. The origins of the

AMT biota are diverse, with a complex mixture of taxa:

comparatively geologically old (e.g. saratogas, eucalypts,

placental mammals; Archer et al., 1994) and young (e.g.

rats). Nested within the AMT are centres of endemism and

concentrations of species with restricted ranges. Restricted

species are often associated with distinct landforms, most

notably the insularity of the sandstone plateaux. Recent

molecular phylogenetic analyses among a diverse range of

lineages lend support for recurrent vicariant speciation events

in response to barriers, such as the Carpentarian Gap, that

divide the longitudinal range of the AMT. However, the

paucity of fossils and palaeoecological studies in the

region makes understanding these biogeographical patterns

individually and collectively problematic.

Our few case studies demonstrate the potential of biogeo-

graphical analyses and phylogenetic studies in disclosing both

geographical patterns and shared histories. The challenge is to

synthesize these patterns, disentangling different layers of

history – old and young – and discovering underlying causes.

This demands more coordinated research in this region, which

would make substantial contribution to both pure and applied

research programmes. Of prime importance is increased

systematic collecting and analysis to delineate centres of

diversity and refugia, and timing of fragmentation of taxa

and populations. Collectively, biogeographical and phyloge-

netic research will provide an evolutionary framework to

understand the ecology of the monsoon tropics and inform

conservation policy and planning.

ACKNOWLEDGEMENTS

This paper was an output from the Australian Research

Council Environmental Futures Network meeting program on

the Biogeography of Australia coordinated by Margaret Byrne

and David Yeates.

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BIOSKETCH

David Bowman is the Professor of Forest Ecology in the

School of Plant Science at the University of Tasmania. He uses

a range of tools, including remote sensing and geographical

information analysis, stable isotopes, ecophysiological analysis,

mathematical modelling, biological survey and molecular

analyses to understand how Australian landscapes have

responded to climatic change, varying fire regimes, the

introduction of large vertebrate herbivores, and the impacts

of contemporary and prehistoric management.

Author contributions: This paper is the result of a workshop

conceived by D.M.J.S.B. All authors were involved in the

collection of data and writing; D.M.J.S.B., P.Y.L. and M.D.C.

led the writing; G.K.B. led the editorial process, assisted by

M.F.B.; and S.H. led the production of figures.

Editor: Eileen O’Brien



biogeography of the australian monsoon tropics

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