Horses have been an important and influential part of North American and European history. In his book, Ecological Imperialism: The Biological Expansion of Europe, Alfred W. Crosby argues that horses helped to bring about European's successful colonization of a number of temperate regions such as North America, Australia, New Zealand, and some parts of South America. He argues that the profound success of horses in these regions resulted from the filling of an empty biological niche, and that the arrival of horses on the plains in North America resulted in profound changes in the lives of North American Indians. In his article, The Rise and Fall of Plains Indian Horse Cultures, Pekka Hamalainen argues that the common view that horses brought success to Native Americans is fundamentally oversimplified. He suggests that the common focus on only the successful incorporation of horses by the Lakota people has distorted modern understanding of plains history, and obscures the damaging impact of the arrival of horses on native American culture and ecology. In Ecological Imperialism: The Biological Expansion of Europe, Crosby argues biology played a large part in the fact that Europeans displaced the native people of many temperate zones in the world (including North America, New Zealand, temperate South America and Australia). While the success of European imperialism is commonly thought to stem from military might, and advanced technologies, it can be better explained by other factors, notes Crosby. Essentially, Crosby argues that the native biology of these conquered temperate places (including humans) was not equipped to deal with European invaders. European diseases like smallpox decimated native populations, and European weeds and agriculture brought large scale reductions in native flora

Overall, the animals, weeds, and diseases that Europeans brought to the New World allowed them to dominate the native peoples of these lands. European plants squeezed out native plants, and European diseases like smallpox and measles dramatically reduced populations of indigenous peoples (Crosby).

Similarly,...

European cattle and pigs and horses easily adapted to biological niches. Crosby notes, "Old World livestock prospered in the Neo-Europes. In fact, they did amazingly better in the Neo-Europes than in their homelands." Horses, along with cattle, rabbits and sheep escaped and soon established themselves in the abundant grasslands of their new homes.
Horses, in particular, found their new homes rich in grazing lands, abundant with space, and relatively free of natural predators. Australia was populated with kangaroos, and the South American pampas with flightless birds: species that offered little danger to horses. Further, they did not have to compete, on a large scale, with existing animals for their niche. In the New World, horses from the old world found a welcoming ecological climate that was similar to that of Europe. Mountains, especially the Appalachian Mountains in the United States, provided large open grazing for horses, cattle, and other animals (Crosby).

The success of horses in adapting to temperate zones that were colonized by Europeans soon made them indispensable to those of European descent. Writes Crosby, "Horses thrived so famously in Australia that the Neo-Europeans forgot what a miracle it was to have mounts for next to nothing, and cursed the excess of their own good fortune."

Crosby notes that the native populations of the Americas were forced to adjust their way of life as Europeans took over their land. Horses, along with other European animals, played an important role in these changes. Notes Crosby "If the Europeans arrived in the new world ... with twentieth-century technology in hand, but no animals, they would not have made as great a change as they did by arriving with horses, cattle, pigs, goats, sheep, asses, chickens, cats, and so forth. Because these animals are self-replicators, the efficiency and speed with which they can alter environments, even continental environments, are superior to those for any machine we have thus far…