Long-term monitoring by roadkill counts of mammal populations living in intensively cultivated landscapes

Elusive behaviour and financial constraints hamper the long-term monitoring of mammal population, particularly in intensively cultivated and urbanised landscapes, where most survey methodologies cannot be applied effectively. Provided that there is a direct relationship between each species’ density and frequency of road casualties, roadkill counts may represent a cost-effective alternative method to collect abundance data over long periods. We quantified the numbers of casualties of mammal species from 2001 to 2016 along two routes (65 km) crossing the heavily altered central River Po plain (N Italy). Each route was surveyed by car 10 times per month, covering 123,987 km and recording 15,589 road-kills from 15 species (15.3 roadkills/100 km/year). Most widespread mammals previously reported for the study area were recorded. Variation in each species’ roadkill numbers throughout the study period was consistent with available information on their distribution and abundance and the consistency of the patterns outlined on the two roads supported the hypothesis that the frequency of roadkills was related to each species’ density. Seasonal fluctuations in roadkill records could be related to either their reproductive cycles or dispersal patterns. For meso- and large species, the relationship between the occurrence of casualties and a set of 13 habitat variables was assessed by Logistic Regression Analysis. Based on our results, we believe that roadkill counts should be implemented to outline species’ population trends wherever high road density fragments wildlife habitats, and may represent a powerful citizen science-based method to collect large amounts of data over long periods.