Reducing nutrient availability and enhancing biotic resistance limits settlement and growth of the invasive Australian swamp stonecrop (Crassula helmsii)

摘要

The invasive Australian swamp stonecrop,Crassula helmsii, is a perennial amphibious herb originating from Australia and New Zealand. In freshwater wetlands of North-western Europe, this alien plant species is invasive due to its efficient colonization of empty niches. The establishment of denseC. helmsiigrowth is threatening native biodiversity and functioning of freshwater ecosystems, especially oligotrophic wetlands with high disturbance and nutrient enrichments. As the effects of these potential drivers of ecosystem degradation are generally difficult to determine in the field, we tested the competitive strength ofC. helmsiiin a greenhouse experiment with two native competitor species of the same habitat type,Pilularia globuliferaandLittorella uniflora. Sods dominated by either of the native species, as well as bare soils, were collected from the field and manually infested with propagules ofC. helmsii. Settlement and growth ofC. helmsiiwas assessed after five weeks. In addition, the effect of nutrient enrichment by water bird feces on competition was studied by adding waterfowl droppings.C. helmsiiwas able to settle successfully in all treatments, butP. globuliferaandL. unifloradominance reduced settlement success and growth ofC. helmsii. On vegetated sods, the addition of waterfowl droppings had a low effect on the performance ofC. helmsii, however, this treatment significantly increased biomass production on bare soils with low nutrient availability. We conclude that both absence of native competitors and eutrophication, including guanotrophication by waterfowl, explain the establishment success and invasiveness ofC. helmsii.Given the fact that eradication ofC. helmsiiis very challenging, our results imply that management should focus on a combination of increasing local species densities and abating eutrophication. This will strongly limit the window of opportunity for invasion ofC. helmsiiand enhance resistance by native plant communities.