Humans the most successful invasive species

The magnitude of the invasive species problem in agricultural and natural ecosystems prompted U. S. President Bill Clinton to organize the heads of eight federal agencies into the National Invasive Species Council in 1999. Actually, humans rank among the most successful invasive mammal species. Humans are believed to have spread from Africa to Europe and Asia over 100,000 years ago, and reached the island continent of Australia between 40,000 and 60,000 years ago. But humans are apparently relative newcomers to the Americas, having arrived between 15,000 and 20,000 years ago. The human invasion did not reach many Pacific Ocean islands until 1,000 to 2,000 years ago. A small human presence on the continent of Antarctica is a twentieth-century phenomenon.

Between 20 and 40 bird species have become extinct in North America over the past 11,000 years, a period when the presence of humans is well documented and not controversial. Many of these bird species likely disappeared because they had narrow ecological niches dependent upon now extinct large mammals like mammoths, mastodons, horses, tapirs, camels, and ground sloths. Human hunting likely played a role in the extinction of large mammals like the mammoth. But the magnitude of the prehistoric human role in extinctions involves conjecture and is still being debated.

Several species of long-legged, flightless moas and an eagle, Harpagornis moorei, are among the birds possibly hunted to extinction by New Zealand's first human inhabitants, the Maori. The loss of 62 endemic bird species in the Hawaiian Islands is associated with the arrival of the first human inhabitants from Polynesia. In North America, more recent European immigrants hunted the passenger pigeon to death at the end of the nineteenth century.

Rhinoceros species were hunted to the brink of extinction for their horns in Africa by the end of the twentieth century. Thanks to a new ecological consciousness sweeping the planet, small rhino populations still exist in protected reserves at the start of the twenty-first century. Humans have also extinguished species via habitat loss. This is among the problems addressed in the United States by legislation like the Endangered Species Act of 1973.

Invasive mammal species seem to be most serious on isolated islands where the native organisms have not evolved defenses against the mammals common to the major continents. In Great Britain, an island close to mainland Europe, only 22% of the mammal species are considered exotic. But on New Zealand's islands, which were only settled by humans within the last 2,000 years, 92% of the mammal species are recent introductions. Indeed, New Zealand and some other islands were free from mammalian predator pressure for so long that flightless bird species evolved.

In North America and the islands of the Pacific, other mammal species accompanied the human invasions. The first Asians entering the Americas brought along dogs. When the Polynesians set sail for new Pacific islands, they brought along their pigs, plants, and stowaways like lizards and rats. European colonialism from the fifteenth to twentieth centuries was a major driving force behind biological invasions of North America, Australia, New Zealand, and other areas. Like the ancient Polynesian voyagers, European colonists brought along their plants and animals to help settle these "new worlds." Besides livestock like sheep, goats, cattle, pigs, and horses, there were stowaway species like rats. Later, predators like the mongoose were deliberately introduced to help control the rats.

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