neither established under statutory authority nor enforced by agriculture departments (e.g.,
Georgia). In this study, only those NXW lists established and enforced under the Plant
Protection Act of 2000 are considered as regulations. Therefore, we have 24 and 36 NXW lists
in 1997 and 2002, respectively. A 48x48 matrix of NXW-regulation overlap similar to that of
NXWS lists is constructed with empty cells for comparison between states lacking NXW lists.
Indexes of Ecosystem Dissimilarities (Iij): In order to attribute differences in NXW/NXWS list
to ecosystems characteristics, we first quantify the latter (Rejmanek, 2001). An ecoregion is “a
relatively large unit of land or water containing a geographically distinct assemblage of species,
natural communities, and environmental conditions” (World Wildlife Fund, 1999). Several
methods to classify ecoregions have been developed, each with a set of criteria chosen for
specific objectives. For instance, Bailey’s (1995) ecoregions delineate continental United States
into a hierarchical system with four levels: domains, divisions, provinces and sections (Bailey,
1983; 1995). Leemans’ (1992) Holdridge Life Zone system uses biotemperature, mean annual
precipitation and potential evapotranspiration ratio to define provinces, while Omernick’s (1987)
ecoregions are based on land use, land surface form, potential natural vegetation, and soil types.
Bailey’s (1995) classification is most widely used (e.g., the US Forest Service) since it
includes many of the characteristics in the alternative classifications noted above. We follow
Bailey’s (1995) classification of the Ecoregions of the United States to derive measures of
ecological dissimilarities across states. Specifically, we use the data underlying the classification
such as land surface form, climate (temperature and precipitation), soil, and surface water
characteristics to measure ecosystem differences across US states. The data are taken from the
National Resources Inventory of the Natural Resource and Conservation Service, USDA. All
data at the county level are aggregated to obtain state-level indices using counties’ share of state
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