Could alien Impatiens capensis invade habitats of native I. noli-tangere in Europe? – Contrasting effects of microhabitat conditions on species growth and reproduction

Recent declines in the population size and habitat of the native European Impatiens noli-tangere have led to its observed disappearance. The spread of the North American I. capensis in Europe presents a potential additional threat. However, comparative studies on natural populations of the two species are still lacking. Our research aimed to elucidate the determinants of potential habitat segregation between the two Impatiens species in the natural environment and to identify key factors influencing variations in their growth and reproduction traits. Fine spatial scale studies allowed for a detailed description of microhabitat conditions and demonstrated the high explanatory power of our multidimensional models. The two species have overlapping environmental distributions, but the range of habitats supporting I. noli-tangere is narrower than that of I. capensis . The contrasting patterns in species traits result from different responses of each Impatiens species to environmental factors. A comparison of RDA results showed the environmental factors to explain 86.4% of the trait variability in I. noli-tangere , the soil Pb content (56%) and habitat fertility (21%) being most influential, followed by shading and the soil pH and Ca content. In I. capensis , environmental variables accounted for 70.7% of the variability, with shading being the most significant (29%), followed by anthropogenic pressure, aphid presence, soil pH, and moisture content. Our findings suggest no strong habitat barriers preventing I. capensis from entering habitats occupied by I. noli-tangere . While I. capensis invasion in Poland is still in the early stages and no co-occurrence of both species has been reported in European forests, I. capensis may eventually colonize forest habitats, given its preference for understory conditions in its native range. The higher sensitivity of I. noli-tangere to environmental changes and its weaker performance raise concerns about its ability to persist if I. capensis continues to spread.