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

Abstract

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 dense C. helmsii growth 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 of C. helmsii in a greenhouse experiment with two native competitor species of the same habitat type, Pilularia globulifera and Littorella uniflora. Sods dominated by either of the native species, as well as bare soils, were collected from the field and manually infested with propagules of C. helmsii. Settlement and growth of C. helmsii was assessed after five weeks. In addition, the effect of nutrient enrichment by water bird feces on competition was studied by adding waterfowl droppings. C. helmsii was able to settle successfully in all treatments, but P. globulifera and L. uniflora dominance reduced settlement success and growth of C. helmsii. On vegetated sods, the addition of waterfowl droppings had a low effect on the performance of C. 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 of C. helmsii. Given the fact that eradication of C. helmsii is 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 of C. helmsii and enhance resistance by native plant communities.

Quantifying the ecological impacts of alien aquatic macrophytes: A global meta‐analysis of effects on fish, macroinvertebrate and macrophyte assemblages

“…Although responsibility for the replacement of native vegetation is often ascribed to the alien invader itself, the competitive dominance of invasive alien macrophytes does not arise in a vacuum. Rather, competitive interactions are often mediated by extraneous factors such as climate change (Calvo et al, 2019;Hussner et al, 2014;You et al, 2014) and anthropogenic nutrient/contaminant loading (Chase & Knight, 2006;Galatowitsch et al, 1999;Richburg et al, 2001;van der Loop et al, 2020;You et al, 2014;Zedler & Kercher, 2004).…”

“…For example, eutrophication has been demonstrated to facilitate the replacement of native macrophytes by M. spicatum (Chase & Knight, 2006) and promote colonisation of bare soil by Crassula helmsii (van der Loop et al, 2020), whilst eutrophication and warming interactively promote the growth and clonal propagation of E.…”

The ecosystem resilience approach to control the invasive alien species Australian swamp stonecrop (Crassula helmsii)

“…Results of greenhouse experiments already confirm that ERA is promising for lowering the invasibility of ecosystems for C. helmsii . The establishment and performance of this species is strongly reduced by a high abundance of native species (Brouwer et al 2017; Van der Loop et al 2020). Competition for space and nutrients by a high cover of native species (e.g.…”

“…Competition for space and nutrients by a high cover of native species (e.g. Littorella uniflora L. and Pillularia globulifera L.) reduced stem fragments from settling and biomass production (Van der Loop et al 2020). Below ground competition for nutrients by L. uniflora and Hypericum elodes L. proved to be effective to reduce growth of settled C. helmsii (Brouwer et al 2017).…”

“…Below ground competition for nutrients by L. uniflora and Hypericum elodes L. proved to be effective to reduce growth of settled C. helmsii (Brouwer et al 2017). Reducing nutrients increased the competitive strength of native species (Brouwer et al 2017; Van der Loop et al 2020). Evidence of competition limiting C. helmsii invasions has also been demonstrated in the field, where a survey of >40 sites invaded by C. helmsii showed that the vegetation cover and biomass of this species negatively correlated with that of native species (Van Kleef et al 2017).…”