Abstract
Individual diet specialization (IS) has important community- and population-level implications and its ecological drivers are actively investigated. Here, to test the hypothesis that local environmental conditions may influence IS in wild populations, we analyzed the stomach contents of 395 individuals from eight populations of five allopatric species of European cave salamanders (genus Hydromantes). We assessed whether their degree of individual diet specialization (1) scaled positively with the respective niche widths, in agreement with Van Valen's niche variation hypothesis (NVH), and (2) could be predicted by satellite-derived climatic and vegetation characteristics of the sites where the populations live. Consistent with the NVH, the degree of individual diet specialization increased with the populations' total niche width. Furthermore, two variables describing local nonarboreal vegetation cover and habitat heterogeneity successfully predicted the variation in individual specialization across the eight populations. Climatic factors had a generally low predictive power, with individual specialization in low- and high-elevation populations showing contrasting patterns of co-variation with air temperature in the warmest quarter of the year. However, independently from elevation, specialization peaked under conditions of high nonarboreal vegetation cover and high precipitation regimes. We discussed the results against two mutually nonexclusive scenarios hypothesizing different mechanisms linking environmental factors to salamanders' trophic strategy at an individual and population level. We concluded that satellite-derived climatic and vegetation variables to date generally adopted to model Grinnellian niches might also be useful in predicting spatial variations in dietary habits of populations, that is, their Eltonian niches.
Highlights
Inter-individual trait variability is widespread in wild populations, and there is an increasing recognition of its contribution in mediating species interactions, ecosystem functions, and ecoevolutionary dynamics (Melián et al 2014, Des Roches et al 2018)
We identified two potential non-mutually exclusive scenarios relying on different assumptions on how climate, vegetation cover, and habitat heterogeneity may potentially affect the degree of individual diet specialization and total niche width of Hydromantes populations
The highest and lowest degrees of specialization were observed for H. genei and H. flavus respectively, the former characterized by a proportion of specialized individuals 4 times higher than the latter (Vemp = 0.69 vs. 0.16; see Fig. 2 for examples of the respective bipartite networks)
Summary
Inter-individual trait variability is widespread in wild populations, and there is an increasing recognition of its contribution in mediating species interactions, ecosystem functions, and ecoevolutionary dynamics (Melián et al 2014, Des Roches et al 2018). Variations in ecological opportunity have been suggested to determine remarkable IS differences in marine mammals (Yurkowski et al 2016), and in multiple taxa between tropical and temperate regions (Araújo and Costa-Pereira 2013). Both studies used latitude as a proxy for ecological opportunity. It is apparent that the influence of bioclimatic drivers on IS variation across multiple species or populations over large spatial scales remains virtually unexplored
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