Generalisation within specialization: inter-individual diet variation in the only specialized salamander in the world

Abstract

Specialization is typically inferred at population and species level but in the last decade many authors highlighted this trait at the individual level, finding that generalist populations can be composed by both generalist and specialist individual. Despite hundreds of reported cases of individual specialization there is a complete lack of information on inter-individual diet variation in specialist species. We studied the diet of the Italian endemic Spectacled Salamander (Salamandrina perspicillata), in a temperate forest ecosystem, to disclose the realised trophic niche, prey selection strategy in function of phenotypic variation and inter-individual diet variation. Our results showed that Salamandrina is highly specialized on Collembola and the more specialized individuals are the better performing ones. Analyses of inter-individual diet variation showed that a subset of animals exhibited a broader trophic niche, adopting different foraging strategies. Our findings reflects the optimal foraging theory both at population and individual level, since animals in better physiological conditions are able to exploit the most profitable prey, suggesting that the two coexisting strategies are not equivalent. At last this species, feeding on decomposers of litter detritus, could play a key role determining litter retention rate, nutrient cycle and carbon sequestration.

Figure 1. Costello’s modified graphical interpretation of the foraging strategy (Amundsen et al., 1992).

Autumn data are represented by squares, while spring data are represented by triangles. Labels of prey categories with both values of Prey importance and Frequency of occurrence lower than 0.30 are not shown.

Figure 2

Vanderploeg and Scavia’s (1979) electivity index E* for the most representative prey categories in autumn (a) and spring (b). n.s. = not significant values.

Figure 3. Histograms of distributions of within-individual component/trophic niche width (WIC/TNW) ratio and individual specialization (IS) indices obtained by Monte Carlo resampling procedure, both for autumn and spring data.

Vertical broken lines show the 95% confidence limits of the simulated distribution, while the vertical solid line shows the actual index value for the original data.

Figure 4. Scatterplot of the probability of use of Collembola, obtained from resource selection probability functions (RSPF), in relation to body condition index (SMI).