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criterion probably have ecological and environmental impacts, although for most
non-indigenous species, these impacts have not been adequately quantified.” Davis
and Thompson (2001) counter Daehler's contentions by stating that, “outside of the
discipline of ecology, 'invasive species' are usually explicitly defined on the basis
of their impact.” Davis and Thompson also contend that consolidating all NIS into
the “invader” category contributes “to a belief that invasions are a unique ecological
phenomenon, which we believe has hindered ecologists' efforts to understand the
invasion process.” They see value in segregating invasive plants on the basis of
impact which may then lead to discovering the traits that are unique to “high
impact” invaders. Lodge et al. (2006) aptly note that a best attempt to quantify net
“harm” by an invasion to the environment, industry, or to human health requires the
collective input of economists, public health experts, and ecologists.
To lump all invasive NIS into one group is, from a practical perspective, too inclu-
sive since the impact of invasive NIS can range from relatively innocuous to very
environmentally disruptive (Fox et al. 2003). This is an especially relevant point con-
sidering that the ornamental horticulture industry sells many plants that spread
outside of planted sites and that the level of environmental impact of these widespread
species has rarely been determined. Coulatti (2005), discussing the inclusion of
impact in the invasive definition, concludes “there is a large intellectual rift between
ecologists on one side, and resource managers and politicians on the other. This cre-
ates confusion for newcomers to the discipline, and impedes the rapid and unambigu-
ous dissemination of knowledge from ecological experiments to the formation of
strategies designed to protect natural habitats from problematic invaders.”
Although NIS are a major environmental concern, the proportion of NIS plants
that become invasive is quite small. Rejmanek et al. (2005) make a pertinent case
that “not all naturalized plant taxa, and not even all invaders, are harmful...”
Williamson and Fitter (1996) developed the “tens rule,” which states that one in ten
imported plant or animal species (brought into the country) appear in the wild
(introduced, feral), and one in ten of those become established (self-sustaining
population), and one in ten of established plants become a pest (negative economic
effect). Thus, if 1,000 species were imported, then 100 species would escape into
the wild, 10 species would establish in the wild, and only one species would be
become a pest. These authors acknowledge that this is a relatively gross prediction
and qualified that 1 in 10 actually represents the range of 1 in 5 to 1 in 20. They
noted that crop plants did not follow this rule and had a higher incidence of becom-
ing a pest than predicted by the tens rule. Lockwood et al. (2001) determined the
proportion of naturalized (self-sustaining populations) NIS that were classified as
“the most harmful exotics” or “natural area invaders” in three US states. These
authors found that 5.8, 9.7, and 13.4% of nonnative plants in California, Florida,
and Tennessee, respectively, were natural area invaders. Thus, their findings are in
general agreement with the tens rule of Williamson and Fitter (1996). Despite the
relatively low percent of plants that ultimately become serious invaders, the large
number of garden plants for sale makes the potential invasive, nonindigenous plant
list quite sizable. Dave's Garden - Plant Files ( Dave's Garden - Guides and
Information , http://davesgarden.com/guides/. Accessed 23 May 2008), an Internet
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