Alfred Crosby articulated the concept of ecological imperialism in The Columbian Exchange and later expanded on this work in Ecological Imperialism: The Biological Expansion of Europe, 900-1900 (1986). European imperialism in the Americas resulted in the exchange of biological materials—including plants, animals, and diseases—between the continents. This biological exchange was most devastating for Indigenous peoples in North and South America, while the impact on Europeans was marginal.

Foremost, European diseases that arrived in the Americas shortly after Columbus’s arrival destroyed Indigenous populations. While syphilis was introduced into Europe through contact with the Americas, its long-term impact was much less devastating than that of smallpox, measles, and typhus, among others, for populations in the Americas. Within just the first century after contact, the Indigenous population was reduced substantially, and this population reduction due to biological factors continued into the 20th century.

Smallpox was the deadliest killer brought by European colonizers. Recent estimates hold that by 1650, European diseases may have killed up to 90% of the Indigenous population in the Americas. Crosby postulates that there might have been as many as 17 epidemics in Peru alone between 1520 and 1600. He counters the theory that European brutality caused the majority of Indigenous deaths.