Distribution of a recolonising species may not reflect habitat suitability alone: the case of the Eurasian otter (Lutra lutra) in southern Italy

Management of recolonising species needs precise knowledge concerning those environmental parameters that can influence the species current distribution at the margins of their range and their potential for further expansion. The otter (Lutra lutra) is one of the most endangered species of the Italian fauna. After a sharp decline, it has gradually recovered and, at present, it is confined to southern Italy. At the southern boundaries of the Italian otter range, the influence of two geographic and 15 habitat variables on otter distribution (estimated by 1-year monitoring of 18 spraint-sampling stations) was assessed, to improve management efficacy. Latitude represented the main source of variation in the spraint-collection data. The species was in fact well established in central and northern areas, whereas the southern and western sampling stations showed an unstable river occupancy (the average percentage of positive surveys being 94.5 and 35.8%, respectively; U = 0, P = 0.0014). Secondarily, otter relative abundance was positively associated with the degree of cover of aquatic macrophytes. Our results suggest that the whole analysed hydrographic network offers a suitable habitat for otters. Thus we argue that the discontinuous distribution observed in our study area could be a consequence of the sharp decline suffered by the Italian otter populations during the 1970s and 1980s. The positive trend of the last decades has resulted in the progressive recolonisation of empty habitat patches, a process that is still hindered by the lack of connectivity to extant populations. Our findings have implications for landscape restoration projects, suggesting that provision of suitable habitat alone may not be sufficient to effect recolonisation by some species.