Variability of seasonal trophic niche in two sympatric salamanders, the Italian cave salamander and the Fire salamander

The trophic niche is one of the most important ecological traits for any species, as it provides information about trophic position in a food web, preference on foraging sites, intra- and interspecific interactions. In this study we examined seasonal variations in the diet composition of two sympatric salamanders from central Italy, the Italian cave salamander (Speleomantes italicus) and the Fire salamander (Salamandra salamandra). Adults of these two species are generalist predators which mostly feed on terrestrial invertebrates. The diet of both species has been previously studied under allopatric conditions; only one study, performed in spring, aimed to assess their diet when the two species are in sympatry. In this follow-up study we surveyed the same sympatric population in autumn, performing two night surveys in rainy weather. Our study aimed to evaluate the species' autumn diet and compare it with data obtained in spring, to assess potential seasonal variations in their trophic niche. Captured salamanders of both species were measured (SVL) and we inspected the residuals of their last foraging activity through stomach flushing. Overall, we sampled 75 S. italicus and 34 S. salamandra, from which we recognized more than 1200 prey items belonging to 31 prey categories. Similarly to what has been observed in spring, there is a significant divergence in the diet composition of both species during autumn. Compared to spring, S. italicus reduced by 20% the diversity of consumed prey, while S. salamandra increased it by 13%. In spring, the Fire salamander consumed more slugs, larvae of Diptera and anellids, while in autumn it increased the consumption of millipedes. In spring, S. italicus consumed more adult insects (Diptera and Coleoptera), spiders, and Entomobryomorpha springtails, while in autumn the majority of consumed prey were Symphypleona springtails and adult Diptera. In both species larger individuals consumed more prey, while we did not find any significant predictor for the diversity of consumed prey. Analysis on individual diet specialization identified a high proportion of generalist individuals in both species, although the results were significant only for S. italicus. We conclude that also during autumn the trophic niche of these two sympatric salamanders do not significantly overlap, allowing their coexistence with low (if any) competition for food resources.