Queensland Wet Tropics

James Peter Stanton^{111, 112}, Peter D. Bostock¹¹³, Keith R. Mcdonald¹¹⁴, Garry L. Werren¹¹⁵ and Atticus Fleming¹¹⁶

Biodiversity
Flagship Species
Threats
Conservation

At the base of Cape York Peninsula in northeastern Australia, guarded by the coral ramparts of the Great Barrier Reef, is an outpost of equatorial splendor in an otherwise vast, brown land. As its name implies, the Queensland Wet Tropics is located in the State of Queensland, abutting the coastline for over 400 km, between south latitudes 15°40' and 19°15', and varying in width from 20 to 80 km. This region of extraordinary diversity also includes a number of mountainous offshore islands, the largest of which is Hinchinbrook Island (399 km²). Altogether, the rainforests and associated forests and woodlands of the Queensland Wet Tropics form an assemblage of ecological communities spanning 18 487 km².

Today, the rainforests of Australia represent almost an alien presence in a land dominated by eucalypts and acacias. Occupying less than 0.2% of the land area of the continent, and about 1% of Queensland, rainforests are indeed a rarity, but this was not always the case. When flowering plants first appeared on Earth, Australia was part of the supercontinent of Gondwana. At the final breakup of this continent, about 50 million years ago, Australia took with it a significant component of the original flora of Gondwana. However, despite the fact that broadleaf rainforest originally covered much of the continent, from that time until the present – with many shortterm fluctuations–, Australia has become increasingly drier and today is a largely arid continent.

Out of the ancestral Gondwanan stock arose sclerophyllous (thick-leaved) plants adapted to drier climates, increasingly impoverished soils, and a landscape in which fire became a prominent evolutionary force. It is believed by some that these plants arose from rainforest progenitors under selection pressure from harsh new environments. It is more likely, however, that sclerophyllous and rainforest species co-existed from the earliest days of angiosperm history, their proportions being modified as the environment changed (White 1986). As the Australian Continent drifted northwards into warmer latitudes, compensating for a general global cooling, the tropical flora of Gondwana was preserved in Australia to a greater extent than anywhere else. Its long isolation as the island continent finally ended about 15 million years ago, when the Australian continental plate collided with that of Asia, and some interchange of flora and fauna that had been evolving separately for over 80 million years occurred.

Today, the Queensland Wet Tropics retains a unique record of these major events. Within its deep gorges and on its mountain tops survives an unparalleled collection of flowering plants with primitive characters unchanged since the beginnings of angiosperm evolution. As a result of this unique geological and climatic history, combined with a present-day environment of great physical diversity, the Queensland Wet Tropics harbors a vastly disproportionate share of the biodiversity of Australia, which itself is recognized as a megadiversity country (Mittermeier et al. 1997).

The Queensland Wet Tropics, as defined here, conforms to the Wet Tropics Bioregion (Goosem et al. 1999) and is part of a larger area identified by WWF as the Queensland Tropical Forests Ecoregion, which extends south of the Wet Tropics to incorporate the Central Queensland rainforests. However, the Central Queensland rainforests are separated by a gap of 200 km from the Wet Tropics and enjoy a significantly drier climate (a maximum of 2 000 mm per annum against a maximum of 4 000 mm per annum in the Wet Tropics). Accordingly, we have not included the Central Queensland rainforests in the Wet Tropics for the purposes of this analysis.

The features that set the Wet Tropics apart as a unique region in the Australian context are related to its physiography and high rainfall, and also its situation well within tropical climes. The physiography of the area is dominated by a north-south spine of high mountains cut by eight major water courses which have incised gorges or deep valleys. From north to south these include the rivers of Daintree, Mossman, Barron, Mulgrave, Russell, Johnstone, Tully, and Herbert. While other parts of eastern tropical Australia have a rainfall high enough to support well-developed rainforest, the potential of mountains to strip moisture from the onshore stream by orographic uplift reaches its maximum in the Wet Tropics. This process is one of the four rain-generating mechanisms that influence tropical regions, including convergence, convection, and cyclonic phenomena, and which here function synergistically as they do in few, if any other rainforested regions of the world. Its rainfall regime, therefore, can not be directly compared to any of them, occupying the extreme wet end of the spectrum (Bonell et al. 1991). Unlike that of equatorial regions, however, it is strongly seasonal, with a marked concentration of rainfall in the summer months, over 60% of annual rainfall falling between December and March. The region is noted for reporting some of the world's most intense rainfall events, as well as the heaviest individual events. Rainfalls of up to 1 140 mm in a 24-hour period have been reported.

The Wet Tropics Bioregion was originally defined “as the limits of rainforest in the wet tropics, the western boundary approximating the 1 500 mm rainfall isohyet” (Stanton and Morgan 1976). In spite of the emphasis on rainforest in the definition, only 8 816 km² of its 18 487 km² is actual rainforest (Wilson et al. 2002) and it is estimated that the total area of rainforest never exceeded 10 885 km². The relationship between the rainforest and the vegetation communities surrounding it is a dramatic one. Rainforest expands under suitable climatic and soil conditions into adjacent communities if its scattered invaders can consolidate to change the microclimate and ground cover before they are destroyed by fire. In the presence of regular fire its boundaries are sharp, and there are few more striking contrasts in the Australian environment than the change along a straight-line boundary from eucalypt forest to rainforest.

Apart from the frequency of fire, limits to the expansion of rainforest within the bioregion are largely set by the depth and drainage of the soil, as well as exposure on steep slopes. The biodiversity of the region resides in its complex interweaving of habitats as diverse as rainforest, sclerophyll shrubland, sclerophyll woodland, tall sclerophyll forest, and melaleuca-dominated woodlands and swamp forests. Indeed, and particularly on the coastal plain, where vegetation type is determined by small changes in soil depth down to a permanent water table, the long-term survival of a wide range of communities is dependent on preservation of the landscape and vegetation complexes in which they occur.

A traveler on the highway north from the dry tropics city of Townsville may see the southern margins of the Wet Tropics as a line of mountains to the west of the road, their flanks covered by eucalypt woodland, with pockets of rainforest in “fire shadows” created by rocky valleys. However, it is easy to overlook the low woodlands of melaleuca and eucalyptus that stand at the portals of one of the most biologically rich and scenically splendid parts of the continent.

Past this point, one enters the wide valley of the Herbert River. The ranges now sweep inland to a fardistant blue scarp rimming the floodplain. Woodlands that once filled the valley have been replaced by an endless sea of sugarcane. Gone, too, are most of the picturesque grassy woodlands, lily-covered lagoons and swamps, and riverine rainforest which, 120 years ago, graced this widest and most complex floodplain of all the region's rivers. Beyond the floodplain, the glory of the river remains, however, as it cuts a wide gorge through the region. Tributary streams that have cut their headwater course from the Herbert River now provide, at their knick points, some of the highest waterfalls in Australia.

The Herbert River works a 10-km-wide gap in the north-south distribution of continuous rainforest within the Wet Tropics. North of the river, rainforest continues unbroken to the northern end of the region, although shrinking to a tenuous, narrow band just north of the city of Cairns. South of the river, rainforest is confined to two distinct blocks separated by a gap of 30 km in which it retains only a foothold as scattered patches on steep slopes and in deep valleys protected from fire.

Between the Herbert River and the Tully River, the next major river as one heads north, is the largest surviving remnant of the sclerophyll communities of the coastal sand plain. It is a complex mix of paperbark (Melaleuca spp.) swamp forests and woodlands, eucalypt forests on dunes, and patches of rainforest dominated by the fan palm Licuala ramsayi. As one approaches the ancestral and current floodplain of the Tully River, rainfall doubles within a few kilometers and the “super-wet” belt, with annual average rainfalls of 4 000 mm or more, is reached. This “super-wet” belt continues to within 30 km of the city of Cairns, coinciding with an extremely sharp drop in annual rainfall. Until the 1960s, when massive clearing for a pastoral development scheme took place, the plains on either side of the lower Tully River supported a complex vegetation mosaic of grassland, sedge swamps, swamp forests of melaleuca and palm, and eucalypt forests and woodlands. These are now almost entirely gone and survive, elsewhere on the coastal plain, largely as dismembered fragments of the original mosaic.

The most complex and best-developed rainforests clothe the deeper soils of the coastal plain footslopes from the Tully River to the Mulgrave River and occupy basalt slopes and plateaus in a wide transect of the region following the catchment of the Johnstone River. More than half of these forests on basalts have been cleared, and the largest part of what remains has been selectively logged. However, an outstanding example of virgin complex mesophyll vine forest survives in the valley of the Russell River, while the rarest type of rainforest on basalt survives only as small, scattered remnants on the western side of Atherton Tablelands. Some of the most extensive remaining tall wet sclerophyll forests in the Wet Tropics, dominated by Eucalyptus grandis and E. resinifera, cover the slopes of the Great Dividing Range west of Atherton, and to the northeast of it in the Tinaroo Range.

To the east of Cairns, there are communities developed on wind-formed dunes that are unique to the Wet Tropics. These are covered with closed sclerophyll shrublands in which the myrtaceous plant Thryptomene oligandra is a common canopy species. In depressions amongst the dunes, extensive open sedge and melaleucadominated swamp communities are formed. The insectivorous pitcher plant (Nepenthes mirabilis) is common in some of these swamps, which along with Wyvuri Swamp and adjacent areas to the south, are the only localities within the region from which it is known.

To the north of Cairns, rainforest shrinks to a narrow band at the Black Mountain corridor before expanding northwards into the great wilderness of the mountains of the Mossman and Daintree river catchments. The headwaters of the Daintree River are lightly impacted by human activity. North of the river, Thornton Peak (1 374 m) dominates the scenery and forms, with the headwaters of the Bloomfield River, another area of rugged wilderness almost entirely dominated by rainforest. This northern wilderness outpost of the Wet Tropics provides refuge for many habitat types that elsewhere have been drastically impacted by humans. These include complex mesophyll vine forests and tall wet sclerophyll forests in the headwaters of the Daintree River. This is the only area left where mosaics of intact lowland forest habitats still have direct connection with communities of the high mountain peaks.

The area between Cooktown and Cardwell contains the only existing Australian Aboriginal rainforest culture. The oral prehistory of the surviving Aboriginal rainforest culture is the oldest known for any indigenous people without a written language (Bottoms 2000). Aboriginal occupation of the Wet Tropics of Queensland is thought to date back at least 40 000 years (Sluiter and Kershaw 1982), and the tribes of the area are considered to be among the oldest rainforest cultures in the world (Dasett 1987). Rainforest culture differs markedly from that of most other Australian Aboriginal tribes, with a heavy dependence on arboreal skills, everyday use of toxic plants, and unique weapons (Horsfall 1984).

Biodiversity

The Queensland Wet Tropics contains 3 181 vascular plant species in 224 families representing approximately 18% of Australia's vascular flora. Of this total, 576 species and 44 genera are endemic. There are also two endemic plant families (Austrobaileyaceae and Idiospermaceae), the former containing species with pollen features consistent with primitive flowering plants; both plant families are represented by single species, Austrobaileya scandens and Idiospermum australe. Of a total of 19 of these primitive rainforest angiosperm families in the world, 13 are found in the Wet Tropics.

Vertebrate diversity and endemism are also very high, with 107 mammal species found in the Wet Tropics, including 11 endemic species and two monotypic endemic genera, the musky rat-kangaroo (Hypsiprymnodon moschatus) and the lemuroid possum (Hemibelideus lemuroides). The mammal endemics include four ringtail possums, confined to altitudes above 300 m, along with a native rodent, the Thornton Peak melomys (Melomys hadrourus). Two endemic tree-kangaroo species, Bennett's (Dendrolagus bennettianus) and Lumholtz's (D. lumholtzi), are found at all altitudes, although the latter is rarely encountered in the lowlands. Several mammal species that extend into New Guinea reach their southern limits in the Wet Tropics, including the attractive striped possum (Dactylopsila trivirgata) and long-tailed pygmy possum (Cercartetus caudatus). At the same time, other species reach their northern limit in the Wet Tropics, especially in the open forests and woodlands. Two species of mammals, the arboreal yellowbellied glider (Petaurus australis) and swamp rat (Rattus lutreolus), are confined in the Wet Tropics to the tall open forests adjoining the western edge of the rainforests.

In terms of avifauna, there are 368 bird species, of which 11 species are endemic. The golden bowerbird (Prionodura newtoniana) is a monotypic endemic genus restricted to the higher-altitude rainforest. The Queensland Wet Tropics as we have defined it overlaps to a large degree with the Endemic Bird Area of the same name identified by BirdLife International.

Australia is renowned for its reptile diversity and this is evident in the Wet Tropics, which contains 113 reptile species of which 24 species are endemic. The small, brown, leaf litter skinks of the genera Saproscincus and Lampropholis and the slightly larger Glaphyromorphus are particularly well represented, with four, two, and two endemic species, respectively. There are three endemic reptile genera, and all are represented by single species: the chameleon gecko (Carphodactylus laevis) and prickly rainforest skink (Gnypetoscincus queenslandiae) are located in moist rainforest, mostly above 300 m, while the Mount Bartle Frere skink (Bartleia jigurru) is found only in boulders and wind-swept vegetation of the highest peak in the region, Mt. Bartle Frere.

The diversity of amphibians (51 species, including 22 endemic species) is also significant, with all Australian families represented. However, there are no genera restricted to the area. The family Microhylidae and the rainforest stream frogs of the family Hylidae such as the Australian lace-lid (Nyctimystes dayi, EN) and the waterfall frogs and mistfrogs (Litoria nannotis, EN), although having their greatest Australian diversity in the Wet Tropics, exhibit higher species diversity in New Guinea.

The freshwater fish fauna is characterized by 51 native species including seven endemics. Furthermore, there are two endemic genera, both monotypic, and restricted to clear, fast-flowing, perennial streams. The Cairns rainbowfish (Cairnsichthys rhombosomoides) has a narrow distribution from Cairns to Tully. There are no records of the species on the coastal plains, which have been developed for sugarcane. The recently discovered Bloomfield River cod (Guyu wujalwujalensis) has a limited distribution on the Bloomfield River.

Flagship Species

The southern cassowary (Casuarius casuarius, VU) is usually regarded as the region's flagship species, although it also ranges into northern Cape York and southern New Guinea. With an average weight of 60 kg, the southern cassowary is one of the world's largest birds and Australia's largest land animal; its eggs are the third largest of any bird species. The southern cassowary can live to 50 years of age, and is an important seed disperser, helping to spread the seeds of as many as 150 species of trees and shrubs. The southern cassowary is still regularly encountered throughout the Wet Tropics, although populations are only estimated to number between 1 200 and 1 500 individuals. Perhaps the most striking bird of the rainforests is the golden bowerbird, which is confined to altitudes above 800 m. This species engages in elaborate courtship displays, including the construction of bowers – dens of twigs and other plant matter that are sometimes decorated with material such as egg shells – which may be up to three meters high.

Of particular scientific interest is the musky rat-kangaroo. Standing only about 25 cm high, and the only member of its genus, this species is considered to represent an early stage of evolution of the kangaroos from an arboreal, possum-like stock. It is one of the few Australian mammals that is completely diurnal. Another curious creature is the mahogany glider (Petaurus gracilis, EN), which was believed extinct until its rediscovery in 1989. It now clings to a precarious existence in the lowland eucalyptus forests and woodlands of the Wet Tropics. Over 80% of its habitat has been cleared, mainly for sugarcane, and additional measures are urgently needed to ensure its survival (Maxwell et al. 1996). The northern bettong (Bettongia tropica, EN), one of Australia's rarest kangaroos, survives only in three small populations in the drier eucalyptus woodlands and open forests. Habitat change in these sclerophyll communities poses some urgent problems for its conservation.

Among carnivorous mammals, only the Atherton antechinus (Antechinus godmani) and rusty antechinus (A. adustus) are considered endemic to the region. The Atherton antechinus, weighing less than 100 g, feeds on a variety of insects, arachnids, frogs, and lizards. One notable feature of the biology of this species, and indeed of all members of the genus that have been studied, is that males die soon after the mating season, when they are probably only 11-12 months old (Strahan 1995).

The microhylid frog genus Cophixalus is a distinctive element of the Wet Tropics amphibian fauna, with most species confined to mountain tops and tablelands at altitudes greater than 300 m or in special habitats. The Black Mountain boulder frog (Cophixalus saxatilis, VU) is found only between 100 and 300 m altitude in 580 ha of the granite boulderfields of the Black Trevethan Range south of Cooktown. Females at night are recognized by their spectacular canary-yellow coloration. Two mountain-top nursery frogs, the Bellenden Ker nursery frog (C. neglectus, VU) and the Mt. Elliot nursery frog (C. mcdonaldi, VU), are restricted to the cloud forest at high altitudes of mountain tops, where they occur in leaf litter and lay their eggs on the ground where the young develop, hatching as fully developed froglets.

The chameleon gecko is an endemic encountered on the rainforest floor or facing head-down on small twigs and shrubs. It will readily cast its distinctive black-andwhite banded tail, which can be regenerated. The cast tail produces a squeaking sound – an attribute that has not been noted in any other Australian gecko. The other endemic gecko of the rainforest, the leaf-tailed gecko (Saltuarius cornutus), has a lichen-colored body with a spiny-edged, leaf-like tail and lime-green and brown eyes. The flattened tail and body results in the animal's having a very low profile as it clings to the lichen-covered tree trunks. It has the ability to remain active at ambient temperatures at which other reptile species become inactive.

Threats

The European settlement of northeastern Australia has been a very recent event. One hundred and thirty years of development have resulted in a thriving and prosperous region that has been carved out of a wilderness, and this has led to the almost total annihilation of an indigenous way of life that had survived for tens of thousands of years.

In those 130 years, 23% of all the vegetation of the area has been totally cleared, mostly in the lowlands and on the tablelands to the west of the main coastal range, for the growing of sugarcane and for pastures. Of the 14 242 km² remaining uncleared, an estimated 3 000 km² has been subject to selective logging activity and, although its essential features remain, can not be considered pristine. Some areas of woodland have also been subject to light grazing activity, although these ecosystems remain essentially intact. Looking at the region as a whole, we estimate that 58% of the Queensland Wet Tropics remains in pristine condition.

Although much of the region is officially protected as part of a World Heritage Site, clearing of forest for agriculture, pastoral activities, and urban infrastructure development continue outside the World Heritage Area; however, it is increasingly being regulated by legislation. The greatest threats to the area now arise from altered fire regimes, introduced weeds, feral animals, water extraction from streams and aquifers, and drainage of lowland areas.

Global warming poses serious threats to the region, which have yet to be clearly defined (Williams et al., 2003). A number of high-altitude species may find that they are unable to survive and reproduce in a warmer climate. A one-degree increase in temperature, considered a certainty, is predicted to decrease the range of endemic species to an average 63% of their current range size. A temperature variation of 3.5°C, considered a strong possibility within the next century by the Intergovernmental Panel on Climate Change, will reduce range sizes to an average of 11% of their current area (Williams et al. 2003).

In recent years, the appearance of chytrid fungus has devastated frog populations (Berger et al. 1998), while tree deaths in widely scattered patches of rainforest have been caused by the soil-borne disease Phytophthora cinnamomi (Gadek and Worboys 2003), such that there is now a growing awareness of the destructive potential of introduced diseases.

Relatively few invasive plant or animal species are able to survive in intact rainforest. The most harmful of these is the feral pig, which has occupied – at various population levels – all habitat types. Their impact on the ground cover can look severe, but has not been quantified. The cane toad (Bufo marinus) can invade more successfully along roads and trails of disturbed rainforests, and the toxic venom they exude is potentially lethal for mammalian and reptilian predators. In recent years, rusa deer (Cervus timorensis), escaped from deer farms, have begun spreading into disturbed and open forest habitats. A wide range of plant species has been recorded invading disturbed rainforest habitats and sclerophyll habitats. The most serious of these, which has the capacity to prevent the regeneration of many of the communities it invades, is the pond apple (Annona glabra), a small tree from southern parts of the United States which favors wetland environments.

With the removal of regular fire from many areas of sclerophyll forests and woodlands, particularly within the last 50 years, changes in the understory have created conditions due to which canopy species no longer regenerate. In particular, the removal of fire has allowed rainforest species to invade sclerophyll habitats, particularly those dominated by tall eucalypts. These habitats can not regenerate without the periodic exposure of the soil surface following the removal of ground cover and understory species by fire. In the absence of fire, the understory and canopy are progressively replaced by rainforest species, and the sclerophyll community is replaced by a rainforest one. Anywhere in the wet tropics where soils are fertile enough to support rainforest growth, the interface between sclerophyll and rainforest communities is maintained by fire. The loss of any sclerophyll habitat as a result of this process represents a significant loss of biodiversity and raises particularly serious questions for the future of species such as the yellow-bellied glider.

Conservation

A critical moment in the conservation of the Wet Tropics was the inscription in 1988 of 8 944 km² of the Wet Tropics, including most of the state-owned lands in the area, as the Wet Tropics World Heritage Area. Around that time, most of the selective logging of rainforest, which had proceeded at varying levels of intensity for more than 80 years, was halted by federal legislation. The cessation of logging activity created enormous controversy in north Queensland. The Wet Tropics became a major national political issue, with relevant environmental laws being unsuccessfully challenged in Australia's High Court.

Since World Heritage listing, the area of the Wet Tropics with national park status has increased to 3 762 km². Major national parks in the region include Barron Gorge (28 km²), Cedar Bay (56 km²), Daintree (760 km²), Edmund Kennedy (69 km²), Ella Bay (37 km²), Hinchinbrook Island (399 km²), Lumholtz (1 400 km²), Paluma Range (106 km²), and Wooroonooran (798 km²). A further 4 694 km² is in the process of transfer to national park tenure. Recent state and federal legislation provides additional protection, with groundbreaking new federal legislation (the Environment Protection and Biodiversity Conservation Act of 1999) stipulating fines of over $5 million for any actions adversely affecting World Heritage Site values. Notwithstanding, there are still areas of rainforest outside the Wet Tropics World Heritage Area and not under protection that are at risk of being cleared in the future.

An emerging strategy for conservation in the Wet Tropics, as it is in other parts of Australia, is the development and implementation of private sector (nongovernment) initiatives to protect habitat. The largest non-government conservation area in the Wet Tropics, the Mount Zero-Taravale Wildlife Sanctuary, is owned by the non-profit Australian Wildlife Conservancy (AWC). In addition to protecting critical habitat for several threatened species, AWC is working with government agencies to conduct management-focused research on key issues such as the ecological role of fire in maintaining wet sclerophyll communities. The role of NGOs and other “off-reserve” measures will become increasingly important in delivering landscape-scale conservation in the Wet Tropics.

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