Introduced Species

Humans have also accidentally introduced many species to novel locations, and where the habitat was suitable these species became naturalized. For example, when cargo ships do not have a full load of goods they must carry some other heavy material as ballast, which is important in maintaining stability of the vessel in rough seas. The early sailing ships often used soil as ballast, and after a trans-oceanic passage this soil was usually dumped near the port and replaced with goods to be transported elsewhere. In North America, many of the familiar European weeds and soil invertebrates probably arrived in ballast, as is the case for water horehound (Lycopus europaeus), an early introduction to North America at the port of New York. In addition, ships have used water as ballast since the late nineteenth century, and many aquatic species have become widely distributed by this practice. This is how two major pests, the zebra mussel (Dreissena polymorpha) and spiny water flea (Bythothrepes cederstroemii), were introduced to the Great Lakes from European waters.

An important means by which many agricultural weeds became widely introduced is through the contamination of agricultural seed-grain with their seeds. This was especially important prior to the twentieth century when the technologies available for cleaning seeds intended for planting were not very efficient.