13Fig. 2.1 Ecoregions of the western United States that are characterized by distinct temperature
and soil moisture regimes and infl uence resistance to Bromus. Each ecoregion is an amalgam of
various level I, II, and III Ecoregions of North America ( http://www.epa.gov/wed/pages/ecore-
gions/na_eco.htm , accessed 17 Jun 2014; Wiken et al. 2011 ). Their specifi c delineations were
based on breaks in soil temperature and moisture (amount and seasonality) regimes associated
with ecological resistance to, and ecosystems threats posed by, Bromus invasions, and the avail-
ability of published information on these topics. The Western Forests ecoregion encompasses
Level I ecoregions 6.0 and 7.0 (the Northwestern Forested Mountains and the Marine West Coast
Forest). Mediterranean California corresponds to the Level I ecoregion 11.0 which bears the same
name. The Cold Deserts and Warm Deserts correspond to Level II ecoregions 10.1 and 10.2,
respectively. The Western Great Plains includes three level III ecoregions (Table 2.1 ) representing
parts of two Level II ecoregions, the West-Central Semi-Arid Prairies and South-Central Semi-
Arid Prairies (ecoregions 9.3 and 9.4)
to species interactions, both of which need to be understood to evaluate Bromus
invasions (Chambers et al. 2014a ). Temperature and precipitation amounts, and
precipitation seasonality, are key attributes that infl uence ecosystem resistance
2 Exotic Annual Bromus Invasions: Comparisons Among Species and Ecoregions...