Modelling semi-aquatic vertebrates’ distribution at the drainage basin scale: The case of the otter Lutra lutra in Italy

Modelling habitat suitability of semi-aquatic vertebrates for large scale conservation purposes is a particularly challenging task, due to the fine-scale linearity of riverine habitats, and to the ecological continuum represented by the riparian and the aquatic ecosystems, on one side, and by a river and its tributaries, on the other.We advocate that habitat suitability models (HSMs) should represent the fine-scale complexity of the riparian and aquatic ecosystems at the drainage basin scale, which is the most relevant spatial unit for planning conservation strategies for semi-aquatic vertebrates. Hence, we developed a 3-step GIS-based modelling approach, and applied it to otter Lutra lutra in Italy to illustrate its potential applications for designing a nation-wide conservation strategy.First, we built a deductive HSM (resolution: 1:250,000, grain: 100 m × 100 m) of the riparian (100 m buffer around each river bank) and of the back-riparian (500 m buffer) areas along rivers. After, we created a synthetic index Si summarising this information within the cells of a 5 km × 5 km grid superimposed to the whole country, thus obtaining a nation-wide overview of habitat suitability. Both HSM and Si were validated by using data on otter distribution, by performing a sensitivity analysis. Finally, we scaled-up the information provided by Si to obtain a suitability index within each drainage basin (Sb). By overlapping a layer representing the basins occupied by otters to Sb we obtained an effective tool to identify basins to be prioritised for the conservation and expansion of existing populations, for rejoining isolated sub-populations, and for habitat restoration. The methodology developed here helps filling the gap between the urgent conservation needs of semi-aquatic species, and the inadequacy of traditional modelling approaches for species inhabiting one of the world's most endangered ecosystems.