Altitudinal gradient of Eurasian otter (Lutra lutra) food niche in Mediterranean habitats

The effects of geographical factors, such as latitude and altitude, on climate are known to influence the food habits of predator species distributed over a broad distribution range. A sharp latitudinal gradient in the diet of the Eurasian otter (Lutra lutra (L., 1758)), that is an increase of trophic diversity from temperate to Mediterranean Europe, has been described recently. Altitude, affecting the diversity of fish assemblages and hence prey availability for otters, could also be a key factor influencing otter diet. With the aim of testing for the presence of an altitudinal gradient in otter diet in the Mediterranean area, we assessed the composition of diet at 24 sampling reaches spread in three main river catchments of southern Italy. The analysis of 1885 spraints revealed that fish were the main prey of otters, followed by amphibians, while other food items were of lesser significance, although crustaceans could be locally important. The altitude of the sampling station was negatively correlated with fish consumption and positively correlated with otter trophic diversity; at higher altitudes, alternative prey (particularly amphibians) were included in otter diet. The reduction of fish availability with elevation plays a major role in shaping this dietary pattern. The ability of otters to exploit alternative prey, such as amphibians, could allow the colonization of small rivers sustaining a low fish biomass.