Over millions of years the fauna and flora of North America have evolved in time with continental shifts. But these changes pale when compared with the devastating impact made by man, writes Tim Flannery.

North America came into being about 65 million years ago, as a result of the fusion of its eastern and western portions and the drying of a vast inland sea. At around 24 million square kilometres, the land thus spawned is the middle child among the world's continents - smaller than Eurasia (54 million square kilometres) and Africa (30 million square kilometres) - but larger than the rest. Despite its relative youth and its "middleness", North America has had an extraordinary evolutionary history. Since its formation, the continent has acted as a global crossroads, resulting in a rich mix of living organisms.

Only a few of the world's mammal groups originated in North America, most notably the camel, horse and dog families. This may seem strange, for the first European visitors to the continent encountered no horses or camels: they had become extinct just 13,000 years earlier, bringing a full stop to a continuous episode of habitation stretching back 40 million or more years. During most of their evolutionary history they were found nowhere else. Their few survivors have dispersed into South America and Eurasia during the past few million years.

When periods of contact allow for an extensive mixing of fauna and flora, the result, as far as the colonisation of species is concerned, is largely determined by the relative sizes of the connected land masses. At different times, North America has had direct land contact with Eurasia - via the currently submerged Bering land bridge, South America and Europe - which was an island archipelago, isolated from Asia, until 33 million years ago.

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When North and South America were joined by the Panamanian land bridge about 2.8 million years ago, North American species roamed south, giving South America, which is just three-quarters the size of North America, its deer, tapirs, llamas and bears. Indeed, 60 per cent of South America's mammal fauna was derived in this way, while just a few South American species, such as armadillos and opossums, migrated north. The same pattern of North American domination occurred 45 million years ago when global warming allowed extensive contact between North America and Europe.

But for North America, the long-term constant in terms of immigration history has been Eurasia, which, because of its much larger size, has acted as the major contributor to North America's fauna. From bison to mammoth and antelope to Columbus, Eurasian species have been finding their own frontier in the fertile habitats of North America for 65 million years. Each has affected its homeland, but the impacts are generally not detectable in the fossil record until the arrival of man.

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The first Americans arrived via Siberia and the Bering land bridge. There is still debate about the exact time and their impact, but evidence is growing that they were present by 13,200 years ago, and that they devastated North American fauna, destroying 75 per cent of the continent's larger (more than 45kg) mammal species. Mammoth, giant sloth, camel and American horse all vanished to be replaced by grizzly bear, moose and Eurasian bison, none of which had inhabited the continent before. By the time of Columbus this mega-fauna had changed North America's ecology.

The new ecosystem was characterised by the abundance of a few key species, the most notable being bison. By 1800, between 30 and 60 million of them roamed the continent, sometimes in herds a million strong, constituting the largest single-species assemblage, if measured by weight, ever recorded on a continent. The passenger pigeon was to the skies what the bison was to the plains - the most abundant bird ever. Observers reported that vast flocks blacked out the sun as they passed and that their droppings fell like snow. Some biologists calculate that four North American birds out of every ten were passenger pigeons before their decline in the wake of the European colonial invasion.

The bounty such creatures presented must have seemed limitless to the pioneers who killed without a thought for the future. Bison were shot ruthlessly until, in 1884, a rancher who rode 1,600km over the plains related that he was "never out of sight of a dead buffalo, and never in sight of a live one". It was a history that a leading American conservationist labelled "a disgrace to the American people (that will) cause succeeding generations to regard us as being possessed of the leading characteristics of the savage and the beast of prey - cruelty and greed".

The story was no different for the passenger pigeon. In one hunting competition, the winner had to tally 30,000 birds. By 1870, the flocks were on the wane and the last bird died in Cincinatti Zoo in 1914.

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Of all North America's resources, the most precious are its waterways and other freshwater reserves. When all the God-given well-watered and arable land in the United States had been occupied, the minds of the great capitalists whose fortunes had been made on the frontier - the railwaymen, the land speculators and the media magnates - turned to new ways to prolong the binge of spectacular profit-taking. In the West, where water had long dictated settlement patterns, a particularly destructive new avenue of exploitation was opened, for the speculators learned that water could be used, monopolised and exploited just like any other commodity. By 1982, the National Rivers Inventory found that only 2 per cent of the nation's streams or stream segments were free-flowing, undeveloped and with outstanding natural and cultural values.

The exploitation of the water frontier is arguably the greatest blow to the continent's biodiversity by the European Americans, blasting the oldest and most distinctive biological element on the continent. Many of the continent's aquatic denizens are of venerable ancestry, being some of the few North American survivors from the age of the dinosaurs.

The diversity of life that had built up in North America's waterways over the past 65 million years was astounding. In the late 19th century they were home to spectacular numbers of freshwater crayfish, caddis flies and myriad other aquatic insects. They sheltered more than 800 species of freshwater fish, including the ancient paddlefish and gars, as well as nurturing the world's greatest diversity of freshwater snails. No group, however, had diversified more wildly in North America's aquatic habitats than the freshwater mussels. An obscure stretch of the Tennessee River in northwestern Alabama holds the world record for mussel biodiversity. The record was established in the early 1920s when A. E. Ortmann sampled the shallows at Muscle Shoals. Ortmann found 63 species of mussels in that idyllic stretch of river, whose clear waters flowed through a valley dotted with wooded islands. Nowhere else on the planet even approached this diversity. When Ortmann returned there sometime later, he found a dam had been built. The islands, clear water and beautiful riffles were gone, along with more than half of the species of mussels.

By the 1930s, North America's soils as well as its water were in deep trouble. The frontier had carried farmers deep into marginal lands at a time of above average rainfall, but by 1933 the climatic pattern had reversed, and rainfall dropped. Mechanised farming and overgrazing had bared the soil and the "dustbowl" had begun to blow thick. As a result, 35 million hectares of arable land was destroyed and 50 million hectares severely depleted, while 14 million hectares were rendered marginal for agricultural use. In all, 750,000 Americans were made destitute.

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North America's economic pre-eminence has come about because the resources of a rich yet middle-sized continent have been mined to provide a capital base that is the envy of the world. In a biological sense, the over-exploitation of the frontier was akin to going out in a blaze of glory. By the 1950s, North Americans had destroyed about four-fifths of the continent's wildlife, cut more than half its timber, all but destroyed its native cultures, dammed most of its rivers, obliterated its most productive freshwater fisheries and depleted a good proportion of its soils. They had won a great victory in war and had created one of the most affluent and self-contented societies, yet still the pillage of their natural resources was not finished. By 1999, nearly 1,200 native North American species had been placed on the official endangered list. But worse, it has been estimated that as many as 16,000 species are in grave danger of extinction.

Tim Flannery is director of the South Australian Museum. The Eternal Frontier is published by William Heinemann on May 24, ?20.00.

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