The fossil record provides evidence of major expansions in the eucalypt component of the vegetation during the Quaternary Period (2.2 million years BP to present) associated with environmental change, particularly drier climates. Expansion of the eucalypts is thought to have been largely at the expense of drier rainforest and Casuarinaceae-dominated vegetation and to have been associated with a general transition to sclerophyll forest and woodland of increasingly open structure (Kershaw et al. 1994). The charcoal content of the fossil record is consistent with an increased incidence of fires accompanying the expansion of the eucalypts during this time.
Repeated glacial cycles during the Quaternary resulted in large, relatively rapid shifts in climatic conditions, especially temperature and rainfall. The eucalypts are believed to have persisted in sheltered sites through the cold, dry extremes of the glacial periods, and to have expanded their distribution again during the warmer interglacial periods. Hope (1994) has suggested that increased climatic variability associated with the glacial cycles is likely to have favoured generalists such as the eucalypts over species requiring greater environmental stability.
Repeated expansion and contraction of eucalypt distribution associated with climatic cycles is likely to have contributed to disjunctions in the distributions of eucalypt taxa and the creation of relictual populations, such as those observed amongst the eucalypts today (Wardell-Johnson 1997). Alteration of natural fire regimes by Aborigines is believed to have occurred during the last 50,000 years or more, but the implications of this for past and present composition and structure of Australia's vegetation are unknown (see Hope 1994). It has been suggested that Aboriginal burning may have resulted in an increased incidence of fires which may have favoured the expansion of the eucalypts (Singh et al. 1981, Kershaw 1986). The Holocene (the last 10,000 years), which comprises the present interglacial period, has seen the continuing geographic expansion and taxonomic radiation of the eucalypts to dominate most of the woody vegetation of the continent.
At the present time, eucalypts dominate the majority of Australia’s woody vegetation, except in the wettest and the driest areas of the continent. Analyses by Gill et al. (1985) of herbarium records associated with each of 541 (1:250,000) mapsheets for the Australian continent have demonstrated the presence of eucalypts in 502 mapsheets, or about 93% of the total. The inclusion of eucalypt records made subsequent to the analyses of Gill et al. (1985) would, in all probability, confirm the presence of eucalypts associated with every mapsheet for the entire continent (A.M. Gill, M.I.H. Brooker, CSIRO, personal communication). Although herbarium data may amount to only very few records for some of the map sheets, particularly those associated with the arid centre of the continent, the analyses of Gill et al. (1985) confirm the present, ubiquitous distribution of the eucalypts throughout the entire Australian continent.
When considered at the continental scale, the distribution of the eucalyptswithin Australia encompasses an exceptionally broad range of latitudinal, altitudinal and rainfall environments. For example, eucalypts occur throughout both the tropical and the temperate zones, extending across all latitudes and longitudes covered by the continent. Their distribution extends along the rainfall gradient from the high rainfall area associated with the mountain and coastal regions of the continent to its arid interior. Eucalypts also occur along the complete altitudinal sequence from sea-level into the alpine zone (see AUSLIG 1990).
At the wetter margins of the continent, eucalypts dominate most of the woody vegetation with the exception of tropical rainforest and cool-temperate rainforest in the wettest areas. In areas where mean annual rainfall falls below 200 mm, and particularly towards the arid centre of the continent, other woody dominants such as Acacia or non-woody taxa such as the hummock grasses or the tussock grasses replace the eucalypts as the dominants in the vegetation. Within the broad environmental range between these extremes, eucalypts are the dominant taxon, spanning an exceptionally-wide diversity of habitats (see Doing 1981, Figure 1.1, pp. 10-11, AUSLIG 1990, p. 9 and p. 14).
Flexibility of response to environmental variation is a feature exhibited by some species of eucalypts (Wardell-Johnson et al. 1997). Clinal variation has been shown for a range of phenotypic and genetically-linked characters and across a number of species in relation to environmental gradients such as altitude (see Potts and Wiltshire 1997) and is regarded as relatively common amongst eucalypt species (see Pryor 1976). Genetic variation and adaptation associated with environmental gradients such as topography, soils and climate has also been documented for eucalypt species, and marked genetic differentiation has been shown, even over short distances in response to steep environmental gradients (see Potts and Wiltshire (1997, p. 74).
It should be noted that most eucalypt species are unable to dominate environments characterised by extremes of aridity, low temperature, low light, soil infertility, high salinity or waterlogging (Wardell-Johnson et al. 1997). For example, in some of the drier parts of the arid interior of the continent the eucalypts are only a relatively minor component of the vegetation or may be absent. Similarly, while eucalypts dominate at the margins of rainforest vegetation, they are uncommon in the heavily shaded environments under closed rainforest canopies (Kirkpatrick 1997). As well, eucalypts only rarely regenerate within rainforest vegetation except in response to broad-scale disturbance. Eucalypts are also found in vegetation types dominated by non-eucalypt taxa. For example, eucalypts occasionally occur as canopy emergents in rainforest communities, as infrequent shrubs in some vegetation types, and even as prostrate, creeping shrubs in alpine communities (e.g. E. vernicosa) and at the seaward margins of some coastal communities.
The capacity of the eucalypts to dominate such a wide range of environments and habitats is not demonstrated by any other taxon in the world, even amongst closely-related ecosystems. For example, sclerophyllous vegetation types comparable to eucalypt-dominated vegetation are found in the Mediterranean regions of Europe, in southern Africa, and on the west coast of the American continents, including California and Chile. Together with regions of eucalypt-dominated vegetation in Australia, these have been classified by Udvardy (1975, 1984) in the “evergreen sclerophyllous forests, scrubs or woodlands biome”. This is one of fourteen principal biomes that, together, represent the major types of terrestrial ecosystems on earth. In contrast to eucalypt-dominated vegetation, other vegetation types characteristic of this biome are mostly confined in their distributions to the western or southern margins of continents (Udvardy 1975, Busby 1992). Eucalypt-dominated vegetation is exceptional, even amongst closely-related ecosystems in its wide distribution across an entire continent. There are no other cases in the world of the domination of most of the woody vegetation of an entire continent by a single phylogenetic lineage.
Outstanding universal value: Domination of an entire continent
The geographic expansion and taxonomic radiation of the eucalypts to become an increasingly dominant component of Australia’s vegetation has occurred during the past 2 million years or more. The geographic expansion of the eucalypts is thought to have been facilitated by climate changes affecting the Australian continent, particularly the transition to drier climates. Other factors likely to have favoured this increasing dominance of the eucalypts include the widespread soil infertility resulting from previous wetter periods, an increased incidence of fires associated with drier conditions, and climatic instability associated with the glacial cycles during the Quaternary. In very recent times, fire regimes may also have been influenced by Aboriginal use of fire.
Eucalypts are a characteristic feature of most present-day Australian landscapes. They dominate most of the woody vegetation on the continent. Their distribution extends throughout the tropical and temperate zones, from the wetter fringes of the margins of the continent to its arid interior, and from sea-level to the alpine zone. Within this broad range of environments the eucalypts also span an unusually wide diversity of habitats associated with their exceptional flexibility in coping with environmental change and habitat variation.
Domination of the majority of the environments of an entire continent by a single phylogenetic group, as exemplified by the eucalypts in Australia, is unique in a global context and the eucalypts have become universally associated with Australia and its landscapes. The differentiation amongst eucalypts that underpins this dominance by a single taxon is also exceptional, and is probably unparalleled amongst other dominant woody plant taxa in the world.