Alien and native plant species play different roles in plant community structure

Although impacts of alien plants on species richness are widely reported, it remains unclear how common such effects might be and whether they differ from those due to native species. To date, too few studies have examined the impacts of a wide enough spectrum of native and alien species to address the importance of geographic origin on the richness of plant communities. We investigated how total plant species richness, as well as its separate alien and native components, varied with the abundance of 115 alien and 146 native plant species across Banks Peninsula in New Zealand. Using null models, we tested whether significant correlations between the abundance of each focal species and species richness were more frequent than expected by chance. Examination of the differences in the relationships with species abundance found for native and alien richness can shed light as to whether alien species might be acting as drivers of biodiversity change or passengers responding to external environmental factors. Significant relationships were rarely found for total richness due to contrasting trends of native and alien richness with species abundance. In contrast to natives, an increase in the abundance of an alien species revealed more frequent negative relationships with native richness and positive correlations with alien richness. The relationship with species abundance found for native and alien components of richness differed between grasslands and woodlands. While we found putative support for negative impacts on native richness for several alien species in grasslands, the similar negative correlations in woodlands were more likely to reflect the shade intolerance of alien grasses where native species richness was high. Synthesis. Our analyses reveal that widespread alien and native species play different roles in the plant communities in which they co-occur. Separately analysing the relationship with species abundance for alien and native components of richness can help distinguish between situations where aliens may be acting as the primary drivers in plant community changes or simply passengers. This is an essential first step in designing further experimental studies to determine the underlying ecological processes and potential ecosystem impacts of alien species. Our analyses reveal that widespread alien and native species play different roles in the plant communities in which they co-occur. Separately analysing the relationship with species abundance for alien and native components of richness can help distinguish between situations where aliens may be acting as the primary drivers in plant community changes or simply passengers. This is an essential first step in designing further experimental studies to determine the underlying ecological processes and potential ecosystem impacts of alien species