Shift in relative importance of complementarity and selection drives different effects of community evenness on richness-invasibility relationships

How biodiversity affects ecosystem functioning is context-dependent. Species richness of a plant community may influence its resistance to exotic plant invasions. However, it remains unexplored how species evenness affects richness-invasibility relationships and whether this effect is context-dependent. We constructed experimental communities with different richness and evenness levels and let them be invaded by the exotic annual Conyza canadensis or not in both xeric and mesic sites. Community invasibility was negatively correlated with species richness at both high and low evenness in the xeric site, but only at high evenness in the mesic site. In the xeric site, complementary use of soil water resources among native species was the main mechanism driving negative richness-invasibility relationships at both evenness levels. In contrast, in the mesic site, selection for dominant native species superior for light competition was the main driver. Here, changing evenness altered height differences between the dominant native species and the invader, leading to the different richness-invasibility relationships. Therefore, the relative importance of complementarity and selection effects drives different effects of community evenness on richness-invasibility relationships. Our study demonstrates that impacts of community evenness on richness-invasibility relationships depend on how changes in evenness alter the functional traits of communities governing resource utilization.