The macrophyte swamp stonecrop has invaded a wide range of wetland habitats across Europe. An experiment was conducted within an invaded fen habitat, which tested whether the presence of grazing disturbance affected the relative abundance of swamp stonecrop, and whether any detected effect was suppressive or facilitative. The abundance of swamp stonecrop and co-occurring resident plants was monitored within fenced grazing exclosures and in adjacent unfenced plots. Swamp stonecrop abundance was higher in the unfenced plots compared to the fenced exclosures (t₍₈₇₎  =  28.974, P < 0.001), whereas the abundance of co-occurring plants was higher in the fenced exclosures compared to the unfenced plots (t₍₈₇₎  =  6.264, P < 0.001). These results indicate that the presence of large vertebrates could facilitate a higher abundance of swamp stonecrop in situations where competitive resident plant species were selectively removed by these grazing animals.

Nomenclature: Swamp stonecrop, rassula helmsii (Kirk) Cockayne.

Management Implications: It is important to understand the efficacy of management practices designed to increase native plant diversity in communities that have been invaded by a nonnative invasive plant. The results from this study suggest that in fen habitats, disturbance from vertebrate grazing animals along drawdown zones facilitates the development of high Australian swamp stonecrop (Crassula helmsii (Kirk) Cockayne) abundance. If replications of this experimental design produce concurrent results, it could be concluded that wetland habitat grazed by livestock is particularly vulnerable to C. helmsii colonization and the extensive spread of this species. It would therefore follow that land managers of such habitat should enforce biosecurity measures to reduce the likelihood of C. helmsii propagules being introduced, and should invest time into frequently checking grazed drawdown zones for colonizing C. helmsii and act quickly to eliminate any that is found. It is advisable to prevent grazing livestock access to drawdown zones where C. helmsii already occurs, or in areas near to where C. helmsii occurs, in order to limit C. helmsii plant community dominance. Such actions would, however, present a dilemma for land managers, who would otherwise use livestock to create patches of vegetation with an open structure in order to encourage native plant species that require this type of microhabitat.