© 2006 The Authors DOI: 10.1111/j.1366-9516.2005.00228.x 195 Journal compilation © 2006 Blackwell Publishing Ltd www.blackwellpublishing.com/ddi Diversity and Distributions, (Diversity Distrib.) (2006) 12, 195–204 BIODIVERSITY RESEARCH ABSTRACT Many studies have quantified plant invasions by determining patterns of non-native species establishment (i.e. richness and absolute cover). Until recently, dominance has been largely overlooked as a significant component of invasion. Therefore, we re-examined a 6-year data set of 323 0.1 ha plots within 18 vegetation types collected in the Grand Staircase-Escalante National Monument from 1998 to 2003, including dominance (i.e. relative cover) in our analyses. We specifically focused on the non- native species Bromus tectorum, a notable dominant annual grass in this system. We found that non-native species establishment and dominance are both occurring in species-rich, mesic vegetation types. Therefore, non-native species dominance may result despite many equally abundant native species rather than a dominant few, and competitive exclusion does not seem to be a primary control on either non-native species establishment or dominance in this study. Unlike patterns observed for non- native species establishment, relative non-native species cover could not be predicted by native species richness across vegetation types (R 2 < 0.001; P = 0.45). However, non-native species richness was found to be positively correlated with relative non-native species cover and relative B. tectorum cover (R 2 = 0.46, P < 0.01; R 2 = 0.17, P < 0.01). Analyses within vegetation types revealed predominantly positive rela- tionships among these variables for the correlations that were significant. Regression tree analyses across vegetation types that included additional biotic and abiotic variables were a little better at predicting non-native species dominance (PRE = 0.49) and B. tectorum dominance (PRE = 0.39) than at predicting establishment. Land managers will need to set priorities for control efforts on the more productive, species-rich vegetation types that appear to be susceptible to both components of invasion. Keywords Biological invasions, Bromus tectorum, cheatgrass, dominance, Grand Staircase- Escalante National Monument, non-native species. INTRODUCTION Contradictory findings among experimental and observational studies conducted across multiple spatial and temporal scales have hindered our ability to determine the major factors controlling species invasion patterns (Levine & D’Antonio, 1999; Stohlgren, 2002). Most small-scale experimental studies have shown species-rich plots to be more resistant to invasion (Robinson et al., 1995; Tilman, 1997; Knops et al., 1999; Levine, 2000; Naeem et al., 2000; Prieur-Richard et al., 2000; Hector et al., 2001; Lyons & Schwartz, 2001; Troumbis et al., 2002), but a lack of resistance in similar experiments has also been seen (Palmer & Maurer, 1997; Crawley et al., 1999; Foster et al., 2002). Few large-scale observational studies have shown a negative relationship between native and non-native species richness (Fox & Fox, 1986; Woods, 1993; Morgan, 1998), whereas a positive relationship has been seen in most cases (Pickard, 1984; Planty-Tabacchi et al., 1996; Stohlgren et al ., 1998b, 1999, 2003; Wiser et al ., 1998; Lonsdale, 1999; Smith & Knapp, 1999; Levine, 2000; Brown & Peet, 2003). These contradictions may be the consequence of differences in primary controls on invasion at different scales. At smaller spatial scales, native and non-native species rich- ness may be negatively correlated because of competitive exclu- sion (Tilman, 1997; Kennedy et al., 2002). At larger spatial scales, the effects of competition are likely masked because of increasing habitat heterogeneity that allows competitors with similar habitat requirements to coexist (Levine & D’Antonio, 1999; Stohlgren, 2002; Jiang & Morin, 2004). Other factors include disturbance, resource availability and productivity. Disturbance increases susceptibility to invasion (Fox & Fox, 1986; Hobbs, 1989; Hobbs 1 Natural Resource Ecology Laboratory, Colorado State University, Fort Collins, Colorado 80523- 1499, USA, 2 Fort Collins Science Center, US Geological Survey, 2150 Centre Avenue, Fort Collins, Colorado 80526, USA *Corresponding author. Alycia W. Crall, Natural Resource Ecology Laboratory, Colorado State University, Fort Collins, Colorado 80523-1499, USA. E-mail: mawaters@nrel.colostate.edu Blackwell Publishing, Ltd. Evaluating dominance as a component of non-native species invasions Alycia W. Crall 1 *, Gregory J. Newman 1 , Thomas J. Stohlgren 2 , Catherine S. Jarnevich 2 , Paul Evangelista 1 and Deb Guenther 1