Invasive Species

Defining biologicalinvasions is challenging due in part to the proliferation of terms, especially among biological disciplines, that usually describe a set of ranging and different concepts. Depending on the author, a species in the invasion might be referred to as: alien, exotic, invasive, non-indigenous, imported, weedy, introduced, nonnative versus naturalized, endemic or indigenous. Some of the terms employed evoke anthropocentric concepts such as aggression, assault, and attack, which have normative implications. These implications and the lack of consistent uses of terms contribute to confusion.

Invasive species in general are defined as species that occupy or are in the process of occurring in regions where they have not been present historically. Specifically, invasive species are defined by their origin and distribution. An invasive species might spread into native plant communities and cause environmental harm by developing self-sustaining populations and disrupting the structure and functioning of the system. What characterizes invasive organisms is the ability to take hold of a habitat and become aggressive and dominant. It could be that either invasive species have extraordinarily wide distributions around the world, or that they are [Page 949]distributed locally with very high population densities affecting the endemic biota of specific regions. Invasive species are usually alien species, meaning they are able to reproduce outside their native ecosystems, and whose introduction is more likely to cause environmental harm.

Plants and insects are the most common orders in terms of their invisibility. From a population biology point of view, invasive plants tend to produce reproductive offspring, often in very large numbers (e.g., seeds, spores) and disperse them at considerable distances from parent plants. Invasive species also spread successfully through the use of roots or rhizomes (for example, more than 2 meters/year for taxa spreading), and such strategies enable them to spread over a considerable area. This definition also concerns species that have spread previously but not currently because of competition. These species are still considered invasive species because once local competition disappear, it may lead to re-invasion.