Essay from the year 2002 in the subject Biology - Ecology, grade: 1.1 (A), Oxford University (New College), language: English, abstract: Biological invasion happens when an organism, any sort of organism, arrives somewhere beyond its previous range. Nowadays, most invasions come from human actions, deliberate or accidental. But natural invasions happen too, from minor changes of range to major invasions across continents. One reason for studying invasions is that many invasive species have become serious pests. The cumulative losses in the USA from some non-indigenous species were estimated at almost $100 billion by 1991. It has to be noted that most invaders fail and have small effects, but that the cumulative effect of those that succeed has been and will continue to be large. Another important fact is that invasions have been an important component of the evolutionary process throughout geological history. Researchers are therefore not only interested in what makes a successful invader, but also in the ecological as well as the genetic impacts of invasions