Can the Eltonian niche be predicted? A test with Sardinian cave salamanders

Individual diet specialisation (IS) has significant ecological and evolutionary implications, yet its causes are still debated. We tested whether the degree of individual diet specialisation in five species of European cave salamanders (genus Hydromantes) can be predicted by the bioclimatic and topographic features of the sites where the populations live. We analysed the stomach contents of 395 individuals belonging to eight allopatric populations from all the five Hydromantes species living in Sardinia (Italy). The degree of individual diet specialisation increased with the populations' total niche width, with a slope significantly steeper than that obtained by a null model. Furthermore, IS variation across multiple salamander populations was determined by bioclimatic variables, being highest in sites with more precipitation and high vegetation index. These results indicate that individual diet specialisation in populations may be influenced by local environmental conditions, either directly, via changes in ecological opportunity, or indirectly, via effects on physiological or metabolic conditions. Climatic variables are generally recognized to influence the salamanders' Grinnellian niche, but they are also successful in predicting trophic strategy at individual and population level, i.e., their Eltonian niche.