Occurrence and abundance of invasive and native Arion slugs in three types of habitats in urban area of Wrocław (SW Poland)

Arion vulgaris and Arion rufus are two cryptic slug species whose ranges overlap in vast areas of Europe. In Poland, A. rufus is native; it reaches the eastern border of its range near Wrocław, while the invasive A. vulgaris was previously not recorded in this area. The study aimed to map the distribution of both species in the city of Wrocław and determine the size and abundance of their populations. Twenty-six sites were surveyed in 2019-2020. They represented three types of habitats: natural, semi-natural and anthropogenic. Because the investigated species are externally indistinguishable, their identification was based on the genital organs. Among 280 analysed specimens, 72% were identified as A. vulgaris, 23% as A. rufus and 5% were classified as hybrids. The hybrids and A. vulgaris were most abundant in semi-natural (83%) and anthropogenic habitats (95%), while A. rufus most often occurred in natural habitats (51%). Arion vulgaris occupied most of the sites (88.5%), and in 50%, it was collected alone. Arion rufus occurred in 46% of the sites (in 11.5% alone), and the hybrids were noted in 27%. In Wrocław A. vulgaris dominates in most sites, and its populations are much more abundant than those of A. rufus where the two species co-occur. This finding indicates that A. vulgaris, with its better adaptive skills and competitive abilities, may negatively impact the native species and, consequently, the latter’s displacement. Although the recorded frequency of hybridisation was very low (5%), it may also have an effect on the local extinction of A. rufus. Interspecific hybridisation is assumed to foster invasions, and climate change may further exacerbate displacement; therefore, they should continue to be monitored.