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Species introductions can alter local food‐web structure by changing the vertical or horizontal diversity within communities, largely driven by their body size distributions. Increasing vertical and horizontal diversities is predicted to have opposing effects on stability. However, their interactive effects remain largely overlooked. We investigated the independent and collective effects of vertical and horizontal diversities on food‐web stability in alpine lakes stocked with variable body size distributions of introduced fish species. Introduced predators destabilize food‐webs by increasing vertical diversity through food chain lengthening. Alternatively, increasing horizontal diversity results in more stable food‐web topologies. A non‐linear interaction between vertical and horizontal diversities suggests that increasing vertical diversity is most destabilizing when horizontal diversity is low. Our findings suggest that the size structure of introduced predators drives their impacts on stability by modifying the structure of food‐webs, and highlights the interactive effects of vertical and horizontal diversities on stability. [ABSTRACT FROM AUTHOR]

The European catfish (Silurus glanis) recently colonised large peri‐alpine lakes where its exploitation of the different lake habitats (i.e. littoral, pelagic and deep benthic) may be supported by interindividual trophic variations fostering its establishment. We investigated the prey and lake habitats supporting S. glanis diet in the largest French peri‐alpine lake (Lake Bourget) based on the contents of 231 stomachs combined with 217 individual stable isotope measurements (δ13C and δ15N). The interindividual diet variations in stomachs and isotopes were quantified to identify individual specialisation within the population. The effects of body size on trophic characteristics were also explored. For most individuals, a major reliance on littoral resources was determined based on both stomach contents (86% of prey items) and stable isotopes using Bayesian mixing models (BMMs; 70.3 ± 9.6% contributions of the littoral resources). However, pelagic and deep benthic prey (e.g. whitefish and burbot) were also identified, and BMMs indicated significant contributions of pelagic and deep benthic resources to S. glanis diets (15.8 ± 5.3% and 13.9 ± 5.2% respectively). Interindividual variations based on stomach contents were much higher than those estimated from stable isotopes, supporting an opportunistic trophic behaviour; although the population strongly relied on crayfish, perch and roach. Body size explained a minor fraction of S. glanis trophic characteristics, suggesting that ontogeny does not strongly structure the current population diet. Our results underscore the feeding plasticity of S. glanis, revealing that besides the littoral habitat, this invader exploits resources from all habitats in large peri‐alpine lakes (deep benthic and pelagic). Therefore, its establishment in these ecosystems may represent a growing threat to resident species, especially if the population and individual body size increase in the near future. [ABSTRACT FROM AUTHOR]

Trophic interactions are central in understanding ecosystem processes and the management of natural ecosystems but are frequently complex to estimate. To address this issue, body size has been shown to be a useful trait to reconstruct species interactions, particularly in aquatic ecosystems. An allometric niche model (aNM) considering body size as a niche trait is proposed to predict trophic interactions in temperate freshwater ecosystems. The aNM calibration was based on 26 ubiquitous freshwater species with known minimal and maximum prey body sizes that permitted the establishment of prey body size ranges for vertebrate and invertebrate consumers. The aNM inferences were validated for 13 empirical freshwater food webs, and the model was applied to an extensive inventory of 474 species (spanning six orders of body size magnitude) from the largest natural French lake (Lake Bourget). This application permitted to strengthen the aNM validation with predator‐prey mass ratio comparisons, predicted diet analyses, and allowed the exploration of the lake food web structure. The aNM provided appreciable intrinsic validity (specificity = 87 ± 12%, sensitivity = 59 ± 29%, accuracy = 81 ± 10%), and departures among inferred and empirical trophic interactions were explained by foraging specificities or limited sampling of stomach contents. In Lake Bourget, 26,037 trophic links were inferred. Predator–prey mass ratios for vertebrates and invertebrates were consistent with those empirically established and were occasionally higher for invertebrates as the aNM considers small prey (e.g., bacteria) as possible resources for invertebrates. The inferred diets for three species selected for their well‐known foraging ecology also revealed plausible outcomes of the aNM. The nested structure of the lake food web was determined by highlighting different topologies among the benthic and the pelagic food sub‐webs and the role of top predator fish in the coupling of both food sub‐webs. Due to the large number of species inventories available worldwide for freshwater ecosystems anchored in the ecological monitoring, the aNM may represent a valuable tool for both ecologists and managers to address complementary facets of applied biodiversity studies (e.g., reconstruct highly resolved food webs, predict pressures on important species or new interactions with invasive species). [ABSTRACT FROM AUTHOR]