Ecological imperialism is the theory advanced first by Alfred Crosby that European settlers were successful in colonization of other regions because of their accidental or deliberate introduction of animals, plants, and disease leading to major shifts in the ecology of the colonized areas and to population collapses in the endemic peoples. The many pathogens they carried with them adversely affected the native populations of North America, Australia, and Africa, and were far more destructive than weaponry: it is estimated that disease wiped out up to 90 percent of indigenous people in some locations.
Cortes and the Aztecs
In the early 16th century, Spanish conquistador Hernán Cortés led an expedition to what is now Mexico, which resulted in the total destruction of the Aztec Empire within just two years (February 1519-August 1521). Notwithstanding the Europeans' superior weaponry, it is believed that the arrival of smallpox in the Americas in 1520 weakened the resistance the Aztecs people were able to mount.
"The New World"
In 1608, English settlers arrived in North America and established the Jamestown colony in Virginia. Though at first it seemed the colonists would not survive the harsh conditions of the New World, ultimately it was the natives who could not survive the diseases of the Old World. "The colonizers brought along plants and animals new to the Americas, some by design and others by accident. Determined to farm in a European manner, the colonists introduced their domesticated livestock--honeybees, pigs, horses, mules, sheep, and cattle--and their domesticated plants, including wheat, barley, rye, oats, grasses, and grapevines. But the colonists also inadvertently carried pathogens, weeds, and rats."  The introduction of these foreign species upset the balance of native species and severely hurt the way of life of the native population.
The first major smallpox outbreak among natives was between 1616 and 1619 in Massachusetts. Native Americans had never seen a disease like this, and it literally wiped out entire tribes, such as the Abenaki, the Pawtucket, and the Wampanoag. "By wiping out the Indians, smallpox helped the colonists help themselves to land and resources formerly controlled by unfriendly native people. The Europeans could and did colonize virtually unchallenged in some areas." In 1633 there was another devastating epidemic. William Bradford, governor of the Plymouth Colony, observed that: "They lye on their hard matts, ye pox breaking and muttering, and running one into another, their skin cleaving (by reason thereof) to the matts they lye on; when they turn them, a whole side with flea off at once…and they will be all of a gore blood, most fearful to behold. Then being very sore, what with cold and other distempers, they dye like rotten sheep."  Syphilis was also extremely devastating, and ran rampant when brought back to the Old World.
The fur trade was as detrimental to the survival of native people as it was imperative to the success of settlers due the high European demand. Trappers employed natives because of their knowledge of the terrain and wildlife, putting natives with no immunity to European diseases into close contact with Europeans.
The fur trade also upset the ecological balance of North America. "Restraint wasn't a hallmark of the fur trade. In 1822, in the north western regions of the country alone, the Hudson's Bay Company stockpiled 1500 fox skins, a paltry number compared with the 106,000 beaver skins, but too many none the less. The fur traders had miscalculated. As predators, they had failed to adapt to their prey, and their prey, in turn, retaliated with denial. Of course, the red fox didn't render himself extinct. His numbers merely shrank.".
Ecological Imperialism: The Expansion of Europe 900-1900, by Alfred Crosby
Historian and professor Alfred Crosby wrote Ecological Imperialism: The Biological Expansion of Europe, 900-1900 in 1986. He uses the term "Neo-Europes" to describe the places colonized and conquered by Europeans.
- D. N. Cook, Demographic Collapse: Indian Peru 1520-1620 (Cambridge, 1981), p. 116.
- Alan Taylor, American Colonies (Penguin Books: 2002), 280-300.
- Stephanie True Peters, Epidemic! Smallpox in the New World (Benchmark Books, 2005).
- Sharon Kirsch, What Species of Creatures (New Star Books: 2008)
- Alfred Crosby, Ecological Imperialism: The Biological Expansion of Europe, 900-1900. Cambridge University Press: 1993, 2nd edition 2004. ISBN 978-0521546188.
- Jared Diamond, Guns, Germs, and Steel. W.W. Norton & Company: 2005. ISBN 978-0393061314.
- Sharon Kirsch, What Species of Creatures: Animal Relations From the New World. New Star Books: 2008. ISBN 978-1554200405.
- Mark Elvin, The Retreat of the Elephants: An Environmental History of China. Yale University Press, 2006. ISBN 978-0300119930.
- Alan Taylor, American Colonies (Penguin Books: 2002), 280-300
- Stephanie True Peters, Epidemic! Smallpox in the New World. Benchmark Books, 2005. ISBN 978-0761416371.