4.1 Effects of species diversity on invasion by P. kansuensis
Species richness in the area invaded by P. kansuensis was lower than that of the non-invaded area after removing the P. kansuensis population, which agreed with previous reports on how greater species diversity or species richness can effectively resist invasion by invasive species (Marvier and Smith, 1997), but the difference between the invaded and non-invaded sites was small. In contrast, the invaded area had a greater canopy cover, which means that the communities with low species diversities are more conducive to invasion by P. kansuensis . It is possible that species-rich communities are more likely to contain species with stronger competitive abilities, which allow them to effectively discourage invasive species (El-Barougy et al., 2020). The results are also related to the root hemiparasitic characteristics of P. kansuensis . Another reason might be that a community with higher species diversity has a stronger ability to compete for light, which makes it unfavorable for P. kansuensis survival and growth. Some researchers believe that invasion is a probabilistic process and that a community that contains native species will have a higher plant diversity level. They suggest that this improves the ability of a community to efficiently resist invasive plants (Tilman, 1999; Fargione et al., 2003; Fargione and Tilman, 2005). A further reason could be that communities with high species diversities reduce the likelihood of successful invasions by filling niche spaces with different resident species that jointly and efficiently use local resources (HilleRisLambers et al., 2012; El-Barougy et al., 2020). For example, a field study on Rhinanthus alectorolophus showed that the presence of R. alectorolophus increased total aboveground biomass in low diversity plots, but had no effect on total aboveground biomass in high diversity plots (Joshi et al., 2000).