The Relative Growth of Invasive Solanum rostratum Dunal Decreases with Increasing Competitive Species Richness Regardless of Resource Conditions

The biotic resistance hypothesis proposes that species-rich communities are more resistant to biological invasions due to the less available resources for invaders. The variation in available soil resources may affect the invasion resistance of community, but there is little evidence. Using invasive <i>Solanum rostratum</i> Dunal, a noxious invader in infertile habitats, and six co-occurring non-invasive species, we conducted a field experiment by testing the relationship between relative growth performance of invasives and richness of resident non-invasives, and the effects of available soil water and nutrients on the relationship. We found that relative aboveground biomass (hereafter relative biomass), relative coverage, community-weighted means (CWM) of specific leaf area (SLA), and photosynthetic rate (<i>P</i>_mass) of <i>S. rostratum</i> decreased with increasing resident non-invasive species richness. In contrast, for the non-invasive species, the CWM of SLA and <i>P</i>_mass significantly increased with increasing resident species richness. However, the available soil water and nutrients exerted limited effects on the relative biomass and coverage of <i>S. rostratum</i>. The piecewise structural equation model showed that non-invasive species richness has not only direct negative effects, but also indirect negative effects through promoting non-invasive biomass on the relative biomass of <i>S. rostratum</i>. Our findings indicate that the higher resident species richness is fundamental to resist invasion of alien <i>S. rostratum</i>, which operates through increasing biomass and resource acquisitive traits of a non-invasive community. Additionally, soil available resources exert limited effects on the invasion resistance. This study suggests the importance of conservation of biodiversity in preventing biological invasions.