Co-occurrence patterns of introduced terrestrial flatworms in metropolitan France revealed by citizen science

Terrestrial flatworms are a group of successful carnivorous invaders that have been introduced to many regions outside their native range. In France, six different species have been recorded in significant numbers over the past 20 years, with a recent increase in observations. These include predators of earthworms, gastropods and soil arthropods. Their presence raises concerns about their potential to disturb communities of soil-dwelling organisms. Here, we aimed to describe patterns of co-occurrence between these species in France, using a national inventory of alien flatworms coupled with estimates of potential co-occurrence derived from ecological niche models trained at the global scale. We showed that, although alien predatory flatworms are relatively widespread in France, they co-occur less frequently than expected simply based on each species’ respective environmental suitability, i.e., they tend to be found alone and not in association with other alien flatworm species. However, ecological niche models indicate a high potential for more frequent co-occurrences in the future. We hypothesize that their limited dispersal ability and their relatively recent introduction has confined most species close to their introduction sites. However, if they eventually fill out their potential range, predators of different taxonomic groups will co-occur more frequently, leading to greater predation pressure in invaded ecosystems. Our conclusions shed light on the current and future co-occurrence patterns of alien predatory flatworms in France. Understanding how they translate in the field in terms of predation and competitive interactions will be crucial for predicting the impact of flatworm invasion on soil ecosystems.