Interspecific and interpopulation variation in individual diet specialization: Do environmental factors have a role?

Abstract

Individual diet specialization (IS) has important community-and populationlevel 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.

Updating salamander datasets with phenotypic and stomach content information for two mainland Speleomantes

“…Another advantage of “living” collections is its repeatability, namely the possibility that individuals can be digitally collected multiple times, allowing to perform long term studies on single individuals and populations as well. The data related to stomach contents can be analysed to study the species’ trophic niche and the multiple related traits 34 , 38 , 39 . Nonetheless, comparing datasets produced over different time allows to assess potential variation affecting specific species traits and infer on the possible causes 40 , 41 .…”

“…Salamanders were sampled following protocols aiming to avoid the spread of potential pathogens 46 ; specifically, we used disposable gloves and disinfected with bleach equipment and boots before changing location. During each month, all surveys were performed within 4 days to limit the variation of environmental conditions which may alter the local ecological opportunity 38 , 41 . To limit pseudoreplication, each site was surveyed only once per month.…”

“…To combine the data on stomach contents with the previous dataset: 9 S_ambrosii2 = Cave_ambrosii1, S_ambrosii3 = Cave_ambrosii3, S_ambrosii4 = Cave_ambrosii2. The data on stomach contents allows to assess different characteristics of the populations trophic niche 34 , 38 , 41 . However, only after verifying whether the single individuals were captured during the two different surveys, it is possible to evaluate the variation of individuals’ trophic niche over time.…”

Same Diet, Different Strategies: Variability of Individual Feeding Habits across Three Populations of Ambrosi’s Cave Salamander (Hydromantes ambrosii)

“…The presence of individual specialisation in the use of trophic resources has been assessed in most of the European Hydromantes. Within the six studied species (H. strinatii, H. flavus, H. supramontis, H. imperialis, H. sarrabusensis and H. genei) each population had "true specialists", although in different proportions [35][36][37]. Individuals may specialise to use a subset of the available resources as a consequence of negative biological interactions, because a high prey diversity promotes individuals' preference, or of both conditions acting synergistically [26,38].…”

Distributive stress: individually variable responses to hypoxia expand trophic niches in fish

“…This is often because of practical limitations on the ability of researchers to observe or reconstruct environmental histories for individuals over specific life stages (Stamps and Groothuis 2010). Importantly, the exposure to abiotic conditions is rarely quantified at the individual level and subsequently paired with trait measurements in order to assess impacts of abiotic stress on individual specialization (Lunghi et al 2020). Individual variation in the exposure, sensitivity, or behavioral response to an environmental stress may lead to a distributive stress model (DSM).…”