An invasive species otherwise known as an alien is an introduced organism that becomes overpopulated and harms its new environment. Although most introduced species are neutral or beneficial with respect to other species, invasive species adversely affect habitats and bioregions, causing ecological, environmental, and/or economic damage. The term can also be used for native species that become harmful to their native environment after human alterations to its food web – for example the purple sea urchin (Strongylocentrotus purpuratus) which has decimated kelp forests along the northern California coast due to overharvesting of its natural predator, the California sea otter (Enhydra lutris). Since the 20th century, invasive species have become a serious economic, social, and environmental threat.

Invasion of long-established ecosystems by organisms is a natural phenomenon, but human-facilitated introductions have greatly increased the rate, scale, and geographic range of invasion. For millennia, humans have served as both accidental and deliberate dispersal agents, beginning with their earliest migrations, accelerating in the age of discovery, and accelerating again with international trade. Notable examples of invasive plant species include the kudzu vine, Andean pampas grass, English ivy, Japanese knotweed, and yellow starthistle. Examples of invasive animals include the New Zealand mud snail, feral pig, European rabbit, grey squirrel, domestic cat, carp, and ferret.

Some popular reference sources now name Homo sapiens, especially modern-age humans, as an invasive species, but broad appreciation of human learning capacity and their behavioral potential and plasticity argues against any such fixed categorization.

Alien or naturalized species are those species which are not native to an area but established, and those that are a threat to native species and biodiversity are often called invasive species. The term "invasive" is poorly defined and often very subjective, Invasive species may be plants, animals, fungi, and microbes; some also include native species that have invaded human habitats such as farms and landscapes. Some broaden the term to include indigenous or "native" species that have colonized natural areas. The definition of "native" is also sometimes controversial. For example, the ancestors of Equus ferus (modern horses) evolved in North America and radiated to Eurasia before becoming locally extinct. Upon returning to North America in 1493, during their human-assisted migration, it is debatable as to whether they were native or exotic to the continent of their evolutionary ancestors.

While the study of invasive species can be done within many subfields of biology, the majority of research on invasive organisms has been within the field of ecology and geography where the issue of biological invasions is especially important. Much of the study of invasive species has been influenced by Charles Elton's 1958 book The Ecology of Invasion by Animals and Plants which drew upon the limited amount of research done within disparate fields to create a generalized picture of biological invasions. Studies on invasive species remained sparse until the 1990s when research in the field experienced a large amount of growth which continues to this day. This research, which has largely consisted of field observational studies, has disproportionately been concerned with terrestrial plants. The rapid growth of the field has driven a need to standardize the language used to describe invasive species and events. Despite this, little standard terminology exists within the study of invasive species which itself lacks any official designation but is commonly referred to as "invasion ecology" or more generally "invasion biology". This lack of standard terminology is a significant problem, and has largely arisen due to the interdisciplinary nature of the field which borrows terms from numerous disciplines such as agriculture, zoology, and pathology, as well as due to studies on invasive species being commonly performed in isolation of one another. Invasive species often thrive because there are no predators that hunt them in their new locations. Many invasive species are destroying habitats where plants and animals naturally live.

Regards,
Tony Wilson
Journal coordinator
Research Journal of Plant Pathology