Some species of plants, animals, and microorganisms have been spread by humans over much wider ranges than they occupied naturally. Some of these introductions have been deliberate and were intended to improve conditions for some human activity, for example, in agriculture, or to achieve aesthetics that were not naturally available in some place. Other introductions have been accidental, as when plants were introduced with soil transported as ships' ballast or insects were transported with timber or food. Most deliberate or accidental introductions have not proven to be successful, because the immigrant species were unable to sustain themselves without the active intervention of humans. (In other words, the introduced species did not become naturalized.) Some introduced species have become very troublesome pests, causing great economic damages or severe losses of natural values.

Deliberate introductions

The most common reason for deliberate introductions of species beyond their natural range...