Biology Reference
In-Depth Information
9.2.2 Terminology
The interpretation of
invasive species
by horticulturalists, policy makers, and scien-
tists often varies depending on stakeholder interests. The American Nursery and
Landscape Association (ANLA, http://www.anla.org/industry/index.htm. Accessed
23 May 2008) and the Weed Science Society of America define invasive plants as
“plants that have or are likely to (1) spread into native plant communities and cause
environmental harm by developing self-sustaining populations and disrupting those
systems; or, (2) spread into managed plant systems and cause economic harm”
(Hall 2000). The legal definition of an invasive species, and the official position of
the U.S. Government (Federal Register - Presidential Documents 1999), is “an
alien species whose introduction does or is likely to cause economic or environ-
mental harm or harm to human health.” In a strict interpretation, any plant outside
its native ecosystem is considered nonindigenous. For example, black locust
(
Robinia pseudoacacia
) is a common tree native to the central US Appalachian and
Ozark Mountains but is considered an invasive species in California. Thus, plants
native to one state can be invasive in another state.
Richardson et al. (2000) define a minimum set of terms that describes the inva-
sion/naturalization process of plant species: “
Introduction
means that the plant
(or its propagule) has been transported by humans across a major geographical
barrier.
Naturalization
starts when abiotic and biotic barriers to survival are sur-
mounted and when various barriers to regular reproduction are overcome.
Invasion
further requires that introduced plants produce reproductive offspring in
areas distant from sites of introduction (approximate scales: >100 m over <50
years for taxa spreading by seeds and other propagules; >6 m per 3 years for taxa
spreading by roots, rhizomes, stolons, or creeping stems).” Colautti and MacIsaac
(2004) further refine the definition of “invasive species” with biogeographically
based terminology, where an invasive species can be placed in three stages of
invasion: widespread but rare (stage IVa), localized but dominant (stage IVb), or
widespread and dominant (stage V). Thus, invasion biologists are moving toward
more explicit terms to accurately define an invasive species. The dynamic nature
of the lexicon is characterized by certain terms being abandoned due to their
potential xenophobic link such as
alien
being replaced by the more objective
nonindigenous
(Simberloff 2003).
9.2.3
How Do Invasive Species Harm the Environment?
The definition of the impact of an invasive species has evolved quickly within the
last few years yet policy makers and the gardening public have not been apprised
of these scientific advances in terminology. Parker et al. (1999) characterize impact
on the basis of range, abundance, and the per-capita or per-biomass effect of the
invader. Daehler (2001) contends that the notion of impact depends on the variable
being studied and the scale of study. He concludes that, “All species that meet the spread
