Assessing range re-expansion and recolonization of human-impacted landscapes by threatened species: a case study of the otter (Lutra lutra) in Italy

Detecting and understanding recoveries of threatened species from past and recent presence/absence data is essential to improving conservation efforts. However, false positive trends may be reported because of false absences in past data, therefore appropriate testing is needed. We tested an expansion/recolonization of the otter (Lutra lutra) populations in Italy from a fragmented distribution outlined in the 1984-1985 period by using Monte Carlo simulations. Land-use changes, human depopulation and decrease in influence of anthropogenic features were investigated as potential drivers of recolonization by using GIS modeling. A survey of the entire Italian range of the otter, conducted in 2002-2004, recorded a 2.2-fold increase in the proportion of occupied 10-km squares and 49 local colonizations. The range size and proportion of urban land-use in the 1984-1985 distribution were smaller (P < 0.05) than those derived from simulated surveys based on 49 random false absences versus colonization events. The distribution range expanded southward and nearly reached the southern margins of the peninsula. Recolonization of moderately urbanized landscapes was not accompanied by human depopulation. According to partial logistic regressions, the pure effect of urban land proportion and industry proximity on otter occurrence probability declined by about 48%, suggesting a decreased impact on the stream habitat. Conversely, natural factors, increased in importance by about 116%. The absence of a significant northward expansion in the 20 year period suggests that northward habitat restoration should be planned to encourage further recolonization. On the other hand, potential urban threats in the newly occupied landscapes should be controlled.