Lessons learnt from large-scale eradication of Australian swamp stonecrop Crassula helmsii in a protected Natura 2000 site

The eradication of the aquatic invasive Australian swamp stonecrop Crassula helmsii in North-western Europe usually fails. This is especially true for areas where this plant species is abundant and wide spread or the probability of re-infestation is high due to hydrological connectivity with other infested surface waters or wetlands. Therefore, the large-scale eradication of this invader is often assumed not to be cost-effective. In 2018, C. helmsii was eradicated in a wetland of 4.5 ha covering interconnected humid dune valleys on the Wadden Island of Terschelling in the Netherlands. The total costs of this large-scale project were estimated to be 1.5 million euro. The applied method was excavation of infested locations and replenishing these areas with clean (uncontaminated by C. helmsii) sand to restore boundary conditions suitable for recovery of the treated habitats in this Natura 2000 site. An eight step approach was implemented to eradicate the invasive plant species. Aftercare consisted of a monitoring program with six-weekly vegetation surveys for early detection of any regrowth of C. helmsii and assessment of the effectiveness of the eradication measures. The eradication of C. helmsii was regarded successful since this fast growing plant species was not observed in the areas of concern during a period of at least two years. The results of a strength, weaknesses, opportunities and threads (SWOT) analysis revealed that the method for eradication of this invasive plant species was effective to secure nature values but also complex, time consuming and costly. Our lessons learnt and recommendations for management will help other nature managers making the right decisions in determining appropriate eradication measures for C. helmsii.