Biology Reference
In-Depth Information
(worldwide) database of garden plants, lists 38,779 species and 100,685 cultivars.
Isaacson (1996), in an inventory of North American seed and nursery catalogs
(1988-1989), records almost 60,000 plant taxa sold. Applying the tens rule to the
38,779 species number, approximately 3,800 plants would escape, 380 species
would establish in the wild, and 38 would become pests. Thirty-eight species can
be construed as a relatively small number; however, this apparently low count belies
the negative ecological effects of even a single species. One only has to consider the
serious environmental effects caused by nonindigenous US landscape species such
as Japanese honeysuckle (
Lonicera japonica
), English ivy (
Hedera helix
), cape ivy
(
Delaireia odorata
), and Chinese tallow (
Triadica sebifera
) to realize that a single
species can cause ecological havoc. The tens rule also does not take into account
the unique situation of garden plants in which plants are sold year after year and
planted in all parts of the country. Such repeated introductions (invasion pressure)
will be discussed in the next section. Thus, the many NIS that are queued in lists of
nongovernmental organizations and states vary considerably in their reproduction,
rate and ecological region of spread, and impact. Hence, management decisions and
regulatory actions should be species and region specific.
There are numerous governmental and nongovernmental lists, which queue
invasive plant species by locality (i.e., county, state, region). These lists vary con-
siderably in the criteria used to list a species and to rank a species' invasiveness.
Thus, the usefulness of some of these lists is questionable. In an attempt to assess
the criteria for published invasive plant lists, Fox and Gordon (2004) conducted a
meta-analysis of 113 invasive plant lists from states, regions, and countries. They
found that there was a gross lack of consistency of invasive criteria used to classify
species as invasive or to rank invasiveness. Only 10% of the lists used invasive
scoring systems that provided consistent application of criteria whereas two-thirds
of the lists incorporated vague terms to describe environmental impact. In essence,
their analysis showed that most invasive plant lists lack verifiable criteria that offer
consistent interpretation and application. These authors, as part of a workshop
effort, endorsed a standard system for invasive plant lists that (1) has a robust, scientific
basis, (2) only lists species already present in an area, and (3) is flexible enough
to be useful relative to the purpose of the list (e.g., regulatory or advisory). Fox
and Gordon (2004) acknowledged the formidable challenges (i.e., complexity, col-
laboration between agricultural and natural system experts, continued data acqui-
sition) to develop a standardized system with flexible options. Other than plants
banned by the federal or state governments, most invasive plants lists in the USA
carry no regulatory weight and serve to advise against the use of listed species. In
contrast, New Zealand has a three-part invasive plant list system, i.e., banned
plants, plants that require monitoring, and species-specific and site-specific weed
control, which are clearly delineated and defined (Timmins 2004). New Zealand
has 2,350 indigenous land plant species and 2,020 nonindigenous naturalized plant
species; over 70% of invasive weeds were imported as ornamental plants
(Department of Conservation 2002). While the USA has not adopted such an
approach, some state governments, such as Montana, have tried stricter regulations
(Simberloff et al. 2005). Additionally, New Hampshire (as of January 2007; New
