representing a genus comparison. Most of the spp problems were fixed with our taxonomic
resolution, but in a few cases a state (A) has listed some species within a genus, while the
comparator state (B) has only the genus name. In this case, the number of overlap is set equal to
the number of explicit species in the concerned genus in state A.
Each state’s NXWS list has two components: prohibited and restricted lists. Most states
imposes zero tolerance on the prohibited noxious weed species, while restricted species have
defined tolerance (e.g., number per 100 seeds). Therefore we developed three 48x48 overlap
matrices, one each for prohibited, restricted and the combined (NXWS) lists:
z' AL ∩ AL AL ∩ AR ... AL ∩ WY y
. WY ∩ AL WY ∩ AR - WY ∩ WY .
48×48
where AL, AR and WY denote Alabama, Arkansas and Wyoming, respectively. The overlap
matrix is symmetric, where each row corresponds to a state’s number of common weed species
with all states (including itself). For example, the first row corresponds to the overlap of
Alabama’s list with itself and all other 47 state lists. Note that the diagonal elements of the
overlap matrix are the number of noxious weeds listed in that state. Since the number of the
noxious weeds listed by each state is different, we calculated percent overlap by dividing the
overlap of noxious weeds in each row by the size of the corresponding state’s list. For instance,
the first row of the 48x48 overlap matrix is divided by the number of noxious weeds listed in
AL. The diagonal elements of the matrix are equal to 1, while off-diagonal elements vary
between 0 and 1 depending on the degree of overlap.
The case of NXW lists is uneven relative to that of the NXWS lists. Some states do not
have a NW list as of 2004 (e.g., Mississippi, New Jersey), while others have NW lists that list the
same weeds in the NXWS list (e.g., Louisiana, Massachusetts). In other cases, the NW list is
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