Synoptic scale mammal density index map based on roadkill records

Globally there are few synoptic-scale (over 1000km) animal density index maps available, except for special cases such as important game species or alien species subject to control programs. Here, we propose a method of estimating synoptic-scale density index distribution of mammals, including non-game and non-pest species, by using roadkill records collected by municipalities.In order to correct biases caused by location, environment and traffic density to roadkill data, we constructed an animal density index model by using environmental factors as land use and climates, and fitted to roadkill records taking into account road types as delegate of traffic density. Since alien species are expanding geographic ranges and some native species were historically extinct regionally, we considered a regional factor not predictable by environment. We then estimated density indices over the research area for nine animals.Geographical distribution of density indices in nine mammal species were obtained. Density index maps of three of four animals agreed (AUC>0.7) with the data on previous presence/absence maps. The exception was that the red fox was distributed over almost of all the research area on previously published maps, but we found that its density was low in southern Pacific areas. The species was considered as endangered in some prefectures there, and it clearly showed the importance of our density index estimation. Significant differences in the regional factor explained ongoing range expansion of alien species (masked palm civet and raccoon) and the historical regional extinction of sika deer.Roadkill data were useful data source for deriving huge-scale density index maps of wildlife. Animal density index monitoring should now become feasible even at a synoptic spatial scale.