Forecasting the impact of invasions is nevertheless challenging because it is difficult to identify the mechanisms underpinning the success of alien plant species once established in new environments (Shea & Chesson, 2002 Levine et al., 2003 MacDougall et al., 2009). Invasion by alien plant species can profoundly alter community structure and ecosystem functioning (Vilà et al., 2011 Pyšek et al., 2012). It highlights that trait differences can underlie several coexistence processes and that the demonstration of trait differences between aliens and natives is only a first step to understanding the role of biotic interactions in structuring invaded communities. Our study explains why some alien species coexist with natives, whereas others have strong impacts on native communities.Only a few alien species with a particular set of traits were able to jeopardize species coexistence when grazing was ceased. Importantly, although aliens and natives differed in their trait values, they did not differ in their competitive response, highlighting the importance of equalizing mechanisms in structuring invaded communities. Species coexistence was determined by a trade-off between species susceptibility to herbivory vs competitive hierarchy and facilitation. Trait differences reflected marked competitive hierarchy, suggesting average fitness differences.
Alien and native plant species often differ in functional traits.