Purple loosestrife, Lythrum salicaria, is a beautiful, long-stemmed wetlands species brought to the United States from Europe as an ornamental plant. In the United States, it has no predators and crowds out cattails, Typha latifolia, another wetlands plant that's a valuable resource for many species. Purple loosestrife is an example of:

an introduced species.

Explanation:

An introduced species, often referred to as exotic species or invasive species, is term that describes species of either plants or animals that are not native to the place ore area in which they are being introduced, either as a result of accident or deliberate movement by human activity or external factors.

Common examples of animals that are introduced species are:

gypsy moth in eastern North America, the zebra mussel and alewife in the Great Lakes, the Canada goose and gray squirrel in Europe etc.

Notable example of introduced species of plants are:

the kudzu vine, Andean pampas grass, and yellow starthistle.

Hence, Purple loosestrife, Lythrum salicaria, being native to Europe, and taken to United States, is considered to an introduced specie.