Is the habitat of the native Ludwigia palustris suitable for the invasion of alien congeneric species? A comparative environmental analysis of Italian waterbodies

Context: Two invasive alien plants, Ludwigia hexapetala and Ludwigia peploides, are spreading rapidly in Italian wetlands, affecting water quality and native plant diversity. These opportunistic and plastic species could potentially compete with the native congeneric Ludwigia palustris, especially if they share environmental requirements. Aims: To assess whether the two alien Ludwigia species show any environmental overlap with the native one, by comparing the abiotic characteristics of Italian aquatic sites where the three Ludwigia species grow. Methods: Abiotic water parameters were analysed in 37 sites in Italian waterbodies where either species occurred. Human disturbance was also evaluated. Sites were compared by weighted principal component analysis and multiple ANOVA tests. Key results: L. palustris habitats showed many environmental similarities with those colonised by L. hexapetala in terms of pH, nutrients and substrate, whereas there was no overlap with L. peploides. Human disturbance was a critical factor differentiating sites colonised by the native and alien species. Conclusions: Habitats where L. palustris occurs are compatible with the invasion of L. hexapetala, and human disturbance could open a pathway for this invasive species. Implications: Preserving L. palustris habitats is crucial to limit the spread of L. hexapetala there. This approach can be generalised for cases where alien plant species and their congenerics coexist.