Overviews of the phytogeography of Australian vegetation at the scale of the continent have included various descriptions of the major vegetation types and their distribution (see Specht et al. 1974, Carnahan 1976, Beard 1980, Beadle 1981, Groves 1981, 1994, Bridgewater 1987, AUSLIG 1990 and Specht et al. 1995). The majority of these have been based on classifications using structural classes of the vegetation, with secondary reliance on floristic information related to the overstorey, including eucalypt species assemblages (or associations). Others (for example, Beadle 1981) have placed more emphasis on biogeographic regions and floristic alliances with secondary emphasis on structural classes and habitat factors. Limited consideration, or none, has been given to the understorey in most continental-scale descriptions of Australian vegetation and its phytogeography. Brief summaries of three descriptions, from Beadle (1981), Groves (1981, 1994) and Bridgewater (1987), for eucalypt-dominated vegetation are included in Attachment 6.
The phytogeography of the genus Eucalyptus sensu lato has also been examined by Gill et al. (1985) using an explicit approach based on the Australian Map Grid 1:250,000 mapsheet series. Available herbarium records for the occurrence of eucalypt species were aggregated for each cell of the 1.0 º latitude x 1.5 º longitude mapgrid covering the entire continent. Quantitative pattern analysis techniques were used to identify and map patterns in the distribution of similar taxa using the grid cells. This approach allowed areas with similar species (defined as “zones”), areas with species that tend to co-occur and therefore have similar distribution (“taxon groups”), and areas with similar taxon density (number of taxa per grid cell) at the species and subgeneric level to be identified at the scale of the continent (Gill et al. 1985).
Eleven major zones each characterised by particular assemblages of eucalypt species were recognised by Gill et al. (1985). These occupied 95% of the area of distribution of Eucalyptus sensu lato in Australia. The largest zone (zone A) comprised the central arid regions of the continent. Other major zones were located around the northern, eastern and southern margins of the arid zone. The largest of these were the northern zone (zone C), the northeastern zone (zone D), two south-eastern zones including a northerly (zone O) and southerly zone (zone P), a southern zone extending across the continent (zone (I) and two larger south-western zones (zone K and zone L). The remaining zones occupied much smaller areas and were predominantly in the south-east and south-west of the continent (Gill et al. 1985, Figure 2).
Twenty-five taxon groups of co-occurring species were also identified. The taxon groups ranged in numbers of species from 101 (group a) to 1 (groups f, k, y). The taxon groups could be divided broadly into those with predominantly northern (group a), eastern (groups m, n), north-eastern (group d), central-eastern (groups e, f, l, u, v, w), south-eastern (groups b, c, o, p, q, r, s, t, x, y) and south-western (groups g, h, i, j, k) distributions. A detailed summary of the taxon groups is available in Gill et al. (1985, Figure 4 and Appendix 4).
Relationships between patterns in the distribution of eucalypt vegetation described by Beadle (1981), Groves (1981, 1994), Bridgewater (1987) and Gill et al. (1985) are summarised in Table 3. Within the context of the major biogeographical regions of the continent identified by Beadle (1981), a broad correspondence is evident between the patterns in eucalypt vegetation described in these different studies (see Table 3). At the broadest level, the general continental-scale pattern in eucalypt distribution corresponds to the tropical wetter areas in the north, the temperate wetter areas in the south, east and west, and the drier areas in the centre. At a finer scale, it is evident that major differences occur between the east and west regions in the south, separated by the drier centre of the continent. The far northeast region also has greater affinity to the east coast than to the wet-dry tropics region which extends across the top of the continent. The pattern of eucalypt distribution down the east coast can be differentiated into northern, central and southern components.
Within these broad biogeographic patterns of eucalypt distribution, an exceptionally wide diversity of eucalypt communities can be delineated, representing the extraordinary structural and floristic variation of this vegetation. For example, in their recent assessment of the conservation status of plant communities in Australia, Specht et al. (1995) identified a total of 297 major and minor communities for eucalypt open forest and woodland overstoreys and 62 major and minor communities for mallee open-scrub within 17 biogeographic regions across the continent (Specht et al. 1995, Table 3, p. 8).
It should be noted that the identification of pattern in vegetation, and therefore the identification of eucalypt communities within the vegetation, is study-dependent. The actual patterns identified depend upon a range of factors including scale of resolution, attributes used for description, availability of data, types of classification or other methods used, and subjective interpretation. For example, in comparison with Specht et al. (1995), Beadle (1981) identified 89 vegetation alliances containing a further 63 suballiances within the eight broad biogeographic groupings to describe eucalypt-dominated vegetation across the continent. In other studies, AUSLIG (1990) used 57 groups of eucalypt vegetation types within nine broad structural classes to describe and map eucalypt-dominated vegetation. Similarly, Bridgewater (1987) used 16 classes of eucalypts (based on Angophora and the informal subgenera within Eucalyptus sensu lato) within three broad biogeographic and structural categories for describing broader patterns amongst eucalypt-dominated vegetation.
The wide variation in eucalypt vegetation is particularly evident at regional and local scales and may involve patterns of considerable complexity at these scales. As discussed previously, the eucalypts show exceptional diversity in terms of their responses to environmental variation and this often results in both a high alpha diversity and a high beta diversity. For example, the open forests of the southeast of the continent can exhibit very high species diversity in some parts, with up to 10 eucalypt species per hectare recorded for some areas, and also with a very high species turnover across the landscape (see Wardell-Johnson et al. 1997). These changes in eucalypt species composition can occur over relatively short distances, depending on the steepness of environmental gradients. Marked structural changes over short distances also occur in response to changing environmental conditions. The effect of these can have a major influence on the structural and floristic composition of the eucalypt overstorey, resulting in highly-complex spatial mosaics of eucalypt communities.
Table 3 Continental-scale patterns in the distribution of eucalypt-dominated vegetation types
The understorey component of eucalypt-dominated vegetation also varies widely in response to the extraordinary geographic and environmental range of the eucalypts in Australia. Taller forests often have dense understoreys of one to several layers of shrubs and small trees and, in the wetter areas, often include large numbers of treeferns, ferns and bryophytes. Forests and woodlands of the drier areas and in the north of the continent tend to have simpler shrub understoreys, and sometimes have an understorey of grasses or herbs. Forests and woodlands on poorer soils have predominantly shrubby or heathy, herbaceous or grassy understoreys or sedge understoreys (ie Cyperaceae, Restionaceae). The vegetation typical of mallee understoreys also ranges widely from scleromorphic shrubs, to semi-succulent saltbush, to perennial grasses, including hummock or spinifex grasses.
Examples of major understorey types associated with eucalypt forest, woodland and mallee vegetation are listed in Table 4. These broad understorey types are based on a combination of structural, physiognomic and taxonomic information. They have been derived largely from the descriptions of Australian vegetation in AUSLIG (1990) based on the classification system of Carnahan (1976).
Table 4 Major types of understorey vegetation associated with eucalypt forest, woodland and mallee (derived from AUSLIG 1990)
Within these broad types, the understorey of eucalypt-dominated vegetation encompasses a wide range of communities. For example, Specht et al. (1995) described 110 types of major and minor communities within four broad structural classes for the understoreys of all Australian vegetation. These structural classes, together with the total number of communities recorded in each, were: sclerophyll (heathy) understoreys (21 communities), hummock grass understoreys (20 communities), savanna (grassy) understoreys (39 communities) and forested wetland vegetation (30 communities) (see Specht et al. 1995, Table 3, p. 8). A large proportion of the understorey communities described by Specht et al. (1995) occur within eucalypt-dominated vegetation.
The understorey component of eucalypt-dominated vegetation is also outstanding in terms of species diversity. With the exception of rainforest (tropical and cool-temperate), alpine and some swamp vegetation and arid-zone communities, the eucalypts dominate most other vegetation throughout the continent. Australia has a high species diversity in the global context with an estimated total of about 25,000 species of native vascular plants; a large proportion of these species occur within the understoreys of eucalypt-dominated communities (see Kirkpatrick 1997), including perhaps as much as 60-70% of the entire vascular flora of the continent.
Eucalypt-dominated vegetation has also been described as including some of the most species-rich understoreys in the world (Kirkpatrick 1997). Examples of species-rich eucalypt-dominated communities include: 93 species of vascular plants in a 128 m2 sample area in grassy woodlands in Victoria (Lunt (1990); 86 vascular plant species in a 1 x 10 m sample area in grassy woodlands in Tasmania (Kirkpatrick et al. 1988); 52 plant species including bryophytes in a 75 x 75 cm sample area in eucalypt-dominated vegetation at Wog Wog in New South Wales (M.P. Austin, CSIRO, personal communication). The understorey of eucalypt-dominated communities can also vary widely in species richness and some extremely species-poor understoreys have been recorded. In one example, as few as 3 species were recorded for sample areas of 100 m2 in mixed species callidendrous closed-forest with eucalypts emergent over Nothofagus cunninghamii in Tasmania (Kirkpatrick 1997).
The relationship between the overstorey and understorey components of eucalypt-dominated vegetation appears to be complex. In some instances, eucalypt species are associated with a wide variety of understorey communities. In other cases, an understorey community may also be associated with a wide range of overstorey dominants. Overall, the available evidence suggests that the extent of correlation between overstorey eucalypt species and understorey species varies between regions and, in many instances, the relationship is either weak or non-existent (Kirkpatrick 1997). In contrast, at the scale of the individual tree, there is often an association between eucalypts and understorey species, with patterns of variation in the understorey clearly dependent on position in relation to the canopy or the root zones of eucalypt trees.
In terms of diversity of understorey communities, eucalypt-dominated vegetation is outstanding compared with any other ecosystem in the world dominated by a single phylogenetic group of woody overstory taxa. The structural diversity of eucalypt understorey communities is also at least as diverse those of other comparable vegetation in the world.
A large part of the Australian fauna is taxonomically distinct from the fauna of other continents. This distinctiveness results both from the persistence of ancient elements in the fauna, and also from adaptive radiation and evolutionary change associated with the long period of isolation and environmental change that followed the separation of the continent from Gondwana (see Heatwole 1987). An outstanding example of this distinctiveness relates to the unique character of the mammal component of the Australian fauna. The mammals include the monotremes, of which two of the world’s three extant species occur in Australia. They also include the marsupials, a large and diverse group thought to have originated in North or South America and subsequently to have dispersed into Antarctica and Australia where they radiated into a great diversity of different forms rivalling that of placental mammals in other continents (see Heatwole 1987).
The vertebrate fauna of the Australian continent displays high levels of species diversity with 282 species of mammals, including 141 species of marsupials, over 770 species of birds, approximately 750 species of reptiles and about 200 species of amphibians including frogs (DEST 1994). The diversity of the invertebrate fauna of the continent is also high, with overall estimates in the vicinity of 225,000 species, half of which are undescribed and at least one third have yet to be discovered (E. Neilsen 1993 personal communication quoted in DEST 1994). Australian fauna groups with exceptionally high diversity on a global scale include the marsupials, reptiles, ants and cockroaches. For example, the latter two invertebrate groups have been estimated to include about 15 per cent and 12 per cent respectively of the world’s taxa for each group (Naumann et al. 1991, Groombridge 1992).
The Australian fauna also has very high levels of endemism. For example, overall endemism amongst the mammals is about 82 per cent, including the following levels for the terrestrial groups: monotremes (50%), marsupials (93%), bats (58%), rodents (88%). Levels of endemism amongst the other major vertebrate groups is also high, including: birds (more than 40%), reptiles (89%) and frogs (93%) (see DEST 1994).
A substantial proportion of the Australian fauna is found within eucalypt-dominated ecosystems. Woinarski et al. (1997) calculated relative percentage of mammals, birds and reptiles occurring in broad vegetation types in Australia. Eucalypt formations, together with estimates of the proportional representation of the main vertebrate groups in each, included: eucalypt open forest - mammals (20%), birds (19%), reptiles (7.5%); eucalypt woodland - mammals (35%), birds (32.5%), reptiles (29%); and mallee - mammals (2%), birds (2.5%), reptiles (6%) (Figure 13.7, p. 323). To a large extent, the fauna of eucalypt-dominated vegetation can be seen as broadly representative of the Australian fauna as a whole. This results, in part, because of the extraordinary domination by the eucalypts of the majority of woody vegetation communities throughout large parts of the Australian continent. Eucalypt-dominated ecosystems also provide the habitat for many taxa which extend widely in distribution beyond eucalypt vegetation, as well as those with strong association with the eucalypts.
Expansion of the eucalypts to dominate the continent is thought to have influenced the geographic extent of particular elements of the fauna, greatly increasing their opportunities to expand their distributions while restricting the opportunities of others. The elements of the fauna with strong associations with eucalypt-dominated vegetation have been described as “providing much of the ecological distinctiveness of the Australian biota” (Woinarski et al. 1997).
In terms of overall species diversity, the vertebrate fauna of eucalypt-dominated vegetation is broadly similar to other, non-eucalypt vegetation in Australia, and is also comparable to vegetation with similar structure to eucalypts on other continents. (Woinarski et al. 1997). Exceptions include the species diversity of nectarivorous birds which is higher in Australia compared to other continents and frugivores which are almost absent. Other differences include the absence of large mammals and a higher species diversity for nocturnal mammals in Australia. For some groups, particularly for the mammals and birds, there are generally fewer taxa overall in Australia compared to other continents. Woinarski et al. (1997) suggest that the reasons for this and for the relative homogeneity of the vertebrate fauna over large parts of Australia may be associated with the combination of low relief, shallow environmental gradients, and the dominance by the eucalypts of large parts of the continent, resulting in extensive areas of relatively similar environments compared with other countries.
Variation in vertebrate species diversity in eucalypt-dominated vegetation, measured on a regional scale (gamma diversity), is summarised by Woinarski et al. (1997, Table 13.1, p. 313). These data, derived from a limited number of the regions, show greatest species diversity on a regional scale for mammals in eastern Australia (113 species, including 53 in the southeast and 68 in the northeast) and northern Australia (59 species). High levels of gamma diversity for reptiles occur in northern Australia (106 species), eastern mallee areas (89 species), the northeast (85 species) and the jarrah forests of the southwest (45 species, see Nichols and Muir 1989). Regions with highest species diversity for frogs included the northeast (31 species), the north (24 species) and the southeast (21 species). Other data show that regional species diversity of birds is greater in the southeast than the southwest of the continent and also Tasmania (Woinarski et al. 1997).
The species diversity of the invertebrate fauna of eucalypt-dominated ecosystems at the regional scale is also high (Majer et al. 1997). For example, recent estimates indicate that the diversity of canopy arthropods in eucalypt forest is intermediate between the exceptionally high levels characteristic of rainforest and the much lower levels of temperate deciduous forest (Majer et al. 1997). As well, the invertebrate fauna of eucalypt vegetation is characterised by the combination of many rare species and only a limited number of species with high abundance (Recher et al. 1996).
There is also evidence that there are major differences in the taxonomic composition of the invertebrate fauna of eucalypt canopies compared to temperate forests in the northern hemisphere. In general, eucalypt canopies are dominated by sap-sucking and gall-forming psyllids and leaf-eating beetles, compared with the domination of northern hemisphere trees by sap-sucking aphids and leaf-eating caterpillars (Majer et al. 1997).
The distribution of fauna within eucalypt-dominated vegetation varies widely over a range of scales. At broader scales, environmental factors such as latitude, altitude, rainfall, and temperature are important determinants of fauna distribution. In a broad regional context, there is good correspondence between the major zoogeographic regions for vertebrates and the continental distribution of eucalypt forests and woodlands (Schodde and Calaby 1972). This correspondence is thought to reflect the broad influence of climate and also the evolutionary history of the eucalypts and the vertebrate fauna (Woinarski et al. 1997). Continental-scale regional patterns may also reflect the influence of isolation and extinction, leading to regional divergence amongst particular groups.
At finer regional and local scales, factors such as topography, disturbance regimes and particularly the fire regime, vegetation communities and structure, and soil characteristics are known to be important. For some groups of fauna, including birds and bats, there is a strong similarity in species composition across a range of eucalypt-dominated vegetation communities. For other groups, particularly arboreal and terrestrial mammals, a rapid species turnover across the landscape is evident in response to change in both the structure and floristic composition of the vegetation. In some cases, there is little similarity in the composition of these fauna groups between adjacent eucalypt communities with different canopy dominants (see Woinarski et al. 1997). There may also be marked discontinuities between vertebrate fauna associated with the canopy and fauna associated with the understorey and, in some instances, little or no relationship has been found between the distribution of understorey fauna and the distribution of eucalypt species in the canopy (Gullan and Robinson 1980). At the scale of individual trees, combinations of co-occurring eucalypt species may also play an important role in determining distributions of some fauna groups, particularly invertebrates.
Interactions between eucalypts and the fauna (both vertebrate and invertebrate) are important in influencing or regulating ecological processes in eucalypt-dominated ecosystems. These processes include predator population dynamics, pollination rates, effectiveness of seed dispersal, the success of seedling regeneration, and rates of plant growth, litter turnover, and nutrient cycling (see Woinarski et al. 1997, Majer et al. 1997).
The majority of studies of vertebrates in eucalypt-dominated vegetation have been concerned with birds and mammals (see Woinarski et al. 1997). Specialist vertebrate groups dependent on the eucalypts, for example as a food source, include arboreal mammals and foliage-gleaning and nectarivorous birds (e.g. Keast 1981). Studies in eastern Australia indicate that eucalypt-dominated communities of greatest structural complexity and floristic diversity tend to occur in response to high resource availability, particularly the combination of nutrient-rich soils and higher rainfall (Woinarski et al. 1997). These resource-rich sites display a particularly high diversity and species diversity of birds and arboreal mammals. In the case of birds, it has been postulated that abundance of invertebrates may also mediate a link between foliar nutrients and vertebrate abundance (Majer et al. 1992).
It should be noted that the relationship between nutrients and the diversity and abundance of fauna is not a simple one. For example, the vertebrate fauna of karri forest in Western Australia does not reflect the relatively higher nutrient status of its soils compared with surrounding laterite or sandy soils. Some nutrient poor sites also display exceptional diversities of vertebrate fauna; for example, associated with an abundance of hypogeal fungi (Johnson 1994), or nectar-rich plant species (Milewski 1986). Diversity of understorey plant species and recent disturbance history may also play important roles in determining diversity and abundance of fauna (Woinarski et al. 1997).
Disturbance regimes in eucalypt ecosystems may have a marked effect on the fauna, both directly through mortality, and indirectly through effects on resources, habitat cover and breeding habitat. Many components of the fauna of eucalypt communities show specific adaptations to recurrent disturbances such as fires, enabling them to exploit different stages in the post-disturbance regeneration of the community (Christensen et al. 1981, Bridgewater 1987).
The composition and dynamics of the invertebrate fauna of eucalypt-dominated ecosystems is known to vary considerably, both spatially, at a range of scales, and temporally. There is evidence that diversity and abundance of invertebrates in eucalypt communities may be influenced by nutrient levels of the foliage of host plants, as well as other factors such as canopy structure, and leaf biochemistry. For example, the most nutrient-rich sites support vegetation with high levels of foliar nutrients and also include the most diverse and abundant canopy arthropod communities (Majer et al. 1997). The eucalypt subgeneric group to which the host tree belongs is another important factor that may influence canopy arthropod composition at the tree scale. At a larger scale, there is also some evidence that invertebrate diversity may be largely independent of the species diversity of the vegetation (Majer et al. 1997).
Evolutionary dependencies and co-evolutionary relationships have been shown between the eucalypts and both vertebrate and invertebrate fauna (see Woinarski et al. 1997, Majer et al. 1997, Landsberg and Cork 1997). For example, the folivores are adapted to cope with the low levels of nitrogen typical of eucalypt leaves, and especially with well-developed chemical defences such as high tannin levels in eucalypt tissues to prevent attack by herbivores. The koala (Phascolarctos cinereus) is an outstanding example of a vertebrate species with an evolutionary dependence on eucalypts, adapted as it is to a diet that is almost exclusively confined to eucalypt foliage (Lee and Martin 1988). Common ringtail possums (Pseudocheirus peregrinus) and greater gliders (Petauroides volans) are also dependent on eucalypt foliage as their main food source (Kavanagh 1984, Pahl 1984, also see Landsberg and Cork 1997).
The mallee moths (Oecophoridae) provide an outstanding example of co-evolution between the eucalypts and the invertebrate fauna. The group as a whole is extremely well adapted to utilise eucalypt foliage as a food source, especially dry leaves which are normally very slow to break down on the forest floor. The mallee moths play an important ecological role in breaking down dry foliage, and thereby in facilitating the recycling of nutrients. For example, of 2000 species examined, 88 per cent were found to feed on eucalypt foliage, and 56 per cent of these on dry eucalypt foliage (Common 1990). Some taxa are further specialized in that they use eucalypt leaf remains in possum and koala faeces (Horak 1994).
Outstanding universal value: Wide diversity of eucalypt-dominated communities
Eucalypt-dominated vegetation encompasses an exceptionally wide diversity of vegetation and fauna communities associated with its distribution throughout the continent. This diversity, defined on the basis of overstorey species, is expressed broadly at the scale of the continent by patterns in eucalypt vegetation which correspond to the major biogeographic zones including tropical wetter areas in the north, temperate wetter areas in the south, east and west, and the drier areas in the centre. With the exception of the drier parts, these areas include the major centres of taxonomic, structural and ecological diversity of the eucalypts and therefore form the main expression of the sub-theme of eucalypt-dominated vegetation in Australia.
Finer scale patterns in the vegetation are evident within these continental-scale zones, dividing the west from the east, and the east into northern, central and southern parts. Further differentiation of eucalypt-dominated communities at finer regional and local scales is recognisable on the basis of structure, physiognomy and taxonomy of both overstorey and understorey vascular plant species. Exceptionally high rates of species change across the landscape, very high species diversity in some areas, marked structural gradients in the vegetation, complex mosaics of environmental factors and a variable relationship between understorey and overstorey all contribute to very high levels of diversity of eucalypt-dominated communities at regional and local scales.
The overall effect at landscape, regional and continental scales is an exceptional diversity of eucalypt-dominated communities that is globally-outstanding, encompassing many hundreds of communities and perhaps as much as 60-70% of the estimated 25,000 species that comprise the vascular flora of the continent.
Eucalypt-dominated vegetation also includes a large proportion of Australia’s fauna. The fauna of eucalypt vegetation is broadly representative of the continental fauna as a whole. The eucalypt fauna demonstrates exceptional levels of endemism characteristic of some Australian groups, particularly amongst the marsupials, birds and reptiles. It also displays high levels of diversity for faunal groups, including the marsupials, some of the reptiles, and parts of the invertebrate fauna.
The species diversity of the vertebrate fauna of eucalypt vegetation at regional scales (gamma diversity) is broadly comparable to other vegetation types in Australia, and also to vegetation types on other continents. The invertebrate fauna of eucalypt vegetation demonstrates levels of gamma diversity which are intermediate between the very high levels typical of rainforest vegetation and the relatively low levels that characterise temperate deciduous forests of the northern hemisphere. The invertebrate fauna of eucalypt-dominated vegetation differs markedly in its major functional groups or guilds compared with similar faunas in the northern hemisphere.
Regional differences in species diversity are evident for some vertebrate fauna groups in eucalypt vegetation, with higher gamma diversity for mammals and birds in the eastern regions of the continent and higher gamma diversity for reptiles in north and southeast and southwest.
Distribution of the fauna in eucalypt-dominated vegetation varies widely depending on environmental factors. At the broad, continental scale, there is good correspondence between the vegetation and the vertebrate fauna in terms of bioregional variation. At finer scales, there appears to be less regional variation amongst birds and bats compared with arboreal and terrestrial mammals. Local-scale variation and species turnover across the landscape can be high for the latter groups. Invertebrates also show high levels of distributional variation at fine scales, including marked differences between adjacent canopies involving different eucalypt taxa.
Many fauna groups, such as koalas and the possums, exhibit an evolutionary dependence on the eucalypt component of the vegetation. There is also evidence of substantial co-evolutionary relationships between the eucalypts and some faunal groups; for example, the mallee moths.