Balloon milkweed gomphocarpus physocarpus distribution and drivers in an internationally protected wetland

Abstract

Invasive species continue to spread and alter ecological function and structure in natural systems. Invasive alien plant species can be particularly ecologically damaging and costly to control, yet their success might be influenced by key habitat characteristics which can be empirically measured. The present study employs field surveys and laboratory analyses to examine whether the abundance and key characteristics (i.e. height and number of stems) of the non-native balloon milkweed Gomphocarpus physocarpus E. Mey are influenced by wetland zonation (i.e. permanent, temporary and seasonal zones) and soil characteristics, within a designated Ramsar wetland. Significant site-and zonation-specific differences were observed in a range of soil parameters (e.g. pH, conductivity, Na, Ca, Mg, Cu, Zn, Mn, Fe). Overall, milkweed numbers did not differ significantly according to wetland zonation, but differed significantly among sites. However, in turn, both plant heights and stem numbers related significantly to habitat zonation and sampling sites. Whilst unrelated to most soil properties, milkweed variables were found to relate significantly positively to Mn (abundance), negatively to Cu and positively to P (stem numbers). Furthermore, principal components analyses concerning milkweed abundances indicated clear patterning across sites, finding strong associations with soil variables (i.e. soluble S, Mn, pH, Na, SOM and conductivity). The present study illustrates distributions of a non-native plant species and assesses how its characteristics relate to environmental properties in an internationally recognised protected area. This information could be employed to help predict future distributions and better target management efforts towards sites at high-risk of invasion.