use more recent scientific names for weed species, many continued to use old names. For
example, Centaurea repens in Oregon’s list and Acroptilon repens in California’s list refer to the
same weed (Russian knapweed). Regulatory congruence is likely underestimated with such
inconsistencies in the global list. To overcome this problem, synonyms of each species in the
global list are obtained from the National Plant Database (http://plants.usda.gov/), and for each
species, the most recent scientific name is used.
The global list is coded using state and species indicators, which are used to identify
regulatory overlap. For instance, if a species in the global list appeared on two states’ lists, then
the overlap between the two states is equal to 1. Such a species comparison is hindered when
some states list only a genus name with all its species as a noxious weed, e.g., Allium spp., which
we referred to as the spp problem. Fortunately, the number of weeds listed this way is relatively
small (e.g., 31 in 2002), but a handful of genera had over 50 species each. Adding all species in
a genus would significantly inflate the NXWS list. For instance, the number of weeds in the
Alabama NXWS list with genus and species name is 23 and if the 5 weeds with only a genus
name (Allium spp., Cuscuta spp., Crotalaria spp., Rumex spp., and Xanthium spp.) were fully
enumerated, then its NXWS list would contain 240 species. Arkansas, which also has 23 weeds
with genus and species name and the same 5 weeds with only a genus name, would then have a
total of 240 species as well. Among the weeds with specific genus and species names, Alabama
and Arkansas have 15 weeds in common. If all species in the 5 genera (spp problems) are
included, the overlap between the two states jumps to 232, about a 1500% increase over the
overlap when weeds with genus and species names alone are considered. To resolve this
inflationary problem, we compare a species with a species and a genus with a genus. In the
above Alabama-Arkansas example, the overlap is then equal to 15 + 5, the latter addition
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