Past, present and potential distribution of the invasive Senecio inaequidens in the western Italian Alps (Aosta Valley)

North-western Italy, as many other European areas, is undergoing a rapid spread of Senecio inaequidens (Asteraceae), native to South Africa and unintentionally introduced to Europe in 1889 (Ernst 1998). The first occurrences were recorded in Italy in 1947. By 1970 it had become a weed in agricul- tural areas and by 1990 a serious threat to natural habitats. Moreover, this species is known for its toxicity to livestock (Dimande et al. 2007). In the native range, S. inaequidens grows on river banks and stony grasslands, between 1400 m and 2850 m asl. In the invaded range the species is found on ruderal sites, such as railroads, roadsides, river banks and disused quarries. According to Ernst (1998) S. inaequidens shows perennial behaviour and flowers from late spring to late autumn. The total amount of achenes during the flowering season is huge and seeds show a relatively high germination. In the native and natural habitats it is mainly wind dispersed, but in invasion areas human-mediated transport plays a key role. Since 1990 S. inaequidens has spread progressively across Aosta Valley and was recently included in the blacklist of the regional law on plant species conservation (L.R.45/09). Our work, supported by the Regione Autonoma Valle d’Aosta, Assessorato Istruzione e cultura, aims recording the present distribution of South African ragwort in the Valley and drawing a map of its potential range, fitting a predictive model based on distribution data available to date. Distribution data of S. inaequidens were obtained from database records (regional Flora database) and from an extensive field survey: more than 1200 km were covered collecting presence/absence data with a GPS tracker. In order to draw the potential distribution map, we considered spatial relation- ship among data and climate, digital terrain model and land use. Results suggest that invasion began from the lower valley, and continues inwards in the valley and at higher altitudes up to 1600 m asl. Present distribution analysis confirms previous observations indicating railroads as preferential spreading vectors. The model predicts that S. inaequidens is far from having reached all potentially suitable areas. The invasion is expected to shift from disturbed and ruderal sites to contiguous semi-natural habitats, with a further increase in altitude, as first findings in some pasturelands suggest.