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were supreme opportunists, infiltrating sites more and more frequently disturbed. As they broadened their domain, entire biotas had to reorganize around the defining properties of Eucalyptus. Eucalypts were too effective as scavengers and as hoarders of scarce nutrients and water to ignore. They were aggressive competitors—and a vital focus for grazing by insects, mammals, and birds. They concentrated bionutrients into particular forms; their hard gum nuts, for example, were accessible to some species and not to others. They created special niches and coevolved unique associations with koalas, termites, possums, and parrots; while eucalypts covered 25 percent of the surface of Australia, they harbored some 50 percent of its avifauna. The patterns of eucalypt forests defined the structure of critical habitats; the processes of eucalypt life determined the flow of nutrients and water.

      If they wished to survive, other organisms had to seek out an accommodation with the Universal Australian. But the revolution did not end with the breakup of rainforest into scleroforest. The last 20,000 years—the epoch of the eucalypt revolution—have been marked by massive biotic realignments and extinctions. Each stress inspired others. Selective aridity encouraged fire, and fire fostered another suite of conditions, both abiotic and biotic. If they wished to survive, flora and fauna had to adapt not only to fire in the abstract but to the kinds of fire their scleromorphic neighbors supported. How their associates burned and reproduced determined in no small way the kind of fire they confronted. Amid fire the eucalypts flourished.⁶

       THE EUCALYPT AS PYROPHYTE

      The spread of Eucalyptus traced the spread of fire. Charcoal and eucalypt pollen march side by side in the geologic record of the late Pleistocene and Holocene. Fire proliferated across the spectrum of Old Australian biotas—in scleroforest of course, but also in the grasslands, the acacia-splattered savannas, the heaths; it rolled back the rainforest into sharply bounded sanctuaries. The environments were varied, and so, not surprisingly, were the responses even among the prolific eucalypts.

      Those inherited traits for contending with deteriorating soils and unreliable water preadapted the genus to survive fire. It knew how to cope with irregular nutrient fluxes, with an erratic tempo of too much and too little. Its quest for water already plunged roots safely out of the way of surface fires. Its weedy ancestry had groomed the eucalypt into an opportunist, ready to seize disturbed, opened sites. Eucalypts could capture nutrients released by fire, could store them until another release, could in emergencies live off internal caches in heartwood and lignotuber. Bark was thick, tough, and it shed as it burned like the ablation plate of a descending spacecraft. If branches were seared off, new ones could sprout from beneath the protected layer. If the bole burned, new trunks could spring from the buried lignotuber. A eucalypt could pour old nutrients into new growth, even as it scavenged liberated minerals from freshly burned ground. Fire could, for a couple of years, purge hostile microbes from the site; it might encourage better percolation of groundwater; it opened an area to sunlight, allowing the sun-worshiping eucalypt seedlings a chance to outgrow more shade-tolerant rivals. For most eucalypts, fire was not a destroyer but a liberator.

      There were differences between fire and other pressures toward sclerophylly. Fire acted on a scale of minutes or hours, not over decades or millennia or eons. It was also interdependent with life in ways that leaching and drying were not. Soils degraded regardless of vegetative cover. Droughts arrived and departed whether there was anything on the surface or not; rocks or rainforest, it mattered little, for while organisms could alter the surface concentrations of minerals and water, while they could modulate the force of climate, they could not prevent rain or drought from appearing. But fire could only thrive in the presence of organic fuels. The character of those fuels profoundly influenced the character of the fires that resulted. And those fires, in turn, shaped the kind of biotas on which the fires fed. Fire and flora entered into a process of mutual selection, of positive reinforcement, that was far more rapid, intimate, and compelling than any of the relationships that preceded it.

      Eucalyptus was excellent at extracting and hoarding precious nutrients; but so were most of the Australian flora. It was successful at persevering through dry seasons and episodic droughts; but so, again, were the other scleromorphs. Eucalypts, in fact, tended to occupy the relatively better sites of Old Australia—shunning the driest, the worst waterlogged, the most nutrient-degraded. In none of these attributes was there anything to account for its extraordinary ubiquity or its supremacy within the scleroforest. What made the eucalypt special was its extraordinary opportunism, a relationship reinforced by fire. Eucalypts accepted wretched soils and tolerated drought, but they thrived amid fire.

      A eucalypt forest became a fire forest. The alliance between Eucalyptus and fire compelled other organisms to respond likewise to fire—and not just to any and every fire, but to fires occurring at certain seasons and across a specified range of frequencies and intensities, a fire ensemble to a considerable extent dictated by the burning properties of the eucalypt and its scleromorphic associates. No organism could survive in Quaternary Australia because of its fire-hardiness alone; but it became equally, increasingly true that generic sclerophyllous traits were by themselves inadequate. Fire was too sudden, too powerful. Fire could even allow the eucalypt, within limits, to defy climatic oscillations, to preserve a relatively dry environment against pressures to restore elements of rainforest or araucarian forest. This apparently explains the otherwise anomalous persistence of Eucalyptus and scleroforest pollen at Lake George on the Atherton Tableland during the wet cycle of the last glaciation. The growing prevalence of fire revolutionized the internal relations among the scleromorphs.⁷

      As Pleistocene inflected into Holocene, Eucalyptus was primed for a biological explosion. Once torched, the burning bush resembled a spiral nebula, its fuels and fires like paired arms locked into an accelerating vortex. Anything that altered the bush altered the regime of fire. Any change in fire behavior, timing, or frequency rippled throughout the entire biota. One encouraged the other. Unlike many organisms—Acacia, for example—Eucalyptus did not mold microenvironments unfavorable to fire, or shape fuel complexes unlikely to burn routinely, or inhibit those environmental parameters that supported free-burning fire. It burned readily, greedily, and gratefully. A fire weed had discovered a fire continent.

      The Universal Australian became the archetypal fire colonizer of Australia. Granted a certain abundance of water, its range was limited by fire, and fire, by the prevalence of ignition. The Pleistocene revolution dramatically expanded those sources of ignition. With fire the genus Eucalyptus and the genus Homo had common cause and shared a common future.

       THE EUCALYPT AS EMIGRANT

      Eucalyptus is almost, but not quite, confined to Australia, and the exceptions are revealing. A few eucalypts have crossed the Torres Strait (or its land bridge, the Sahul) northward, and an extraordinary quantity of eucalypts have, through human efforts, become established throughout the world. The contrast between the natural and the anthropogenic—the extra-Australian eucalypts and the emigrant eucalypts—is expressive.⁸

      Some ten species of eucalypts have infiltrated northward, half of which belong to the Corymbia subgenus, and half to the Symphyomyrtus—the later branches of the grand Eucalyptus alliance to emerge. They represent, that is, Australian indigenes that have attempted to occupy extra-Australian sites. Some probably crossed the Sahul, the shallow plains that, from time to time, have joined northern Australia to Papua New Guinea. Others may have colonized afterward, a product of catastrophic windstorms that biologically bridged the strait. Of the ten, four are still found in Australia, and only two exist outside the provenance of Gondwanic Australia before it sutured with the Sunda arc.

      The New Guinea eucalypts claim drier sites, outliers of rainforest. In effect, they have rediscovered more ancient niches, not unlike those that species of Eucalyptus occupied during the early tremors of the Great Upheaval. They are minor, marginal constituents of New Guinean rainforest. All but E. deglupta live in monsoonal climates where fire is seasonally important. Two species, however, continued their move away from Australia. E. urophylla entered Timor, the Lesser Sunda Islands, and New Caledonia. It claims seasonally dry or disturbed sites, but it shows few of the typical eucalypt traits, and it competes poorly with other scleromorphs of the Myrtaceae family such as Melaleuca.

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