Associations between habitat quality, body size, and reproductive fitness in the alpine endemic spider Vesubia jugorum

Aim. In theory, the most suitable habitat across a species range should support individuals displaying the highest fitness, making it possible to provide a biological verification of the reliability of species distribution models by exploring the relationships between habitat suitability and functional traits. However, relationships between habitat suitability and functional traits have been tested almost exclusively in plants, resulting in a general lack of consensus. We investigated the degree to which morphological and reproductive traits of an alpine habitat-specialist spider vary along a gradient of habitat suitability projected via species distribution models (SDMs), testing whether we could provide a biological verification of the model performance grounded in field-collected data. Location. South-western Alps (France and Italy) Time period. 2007–2018 Major taxa studied. Vesubia jugorum (Simon 1881) (Arachnida: Araneae: Lycosidae) Methods. We used a combination of climatic, topographical and geomorphological variables at a resolution of 250 m to model the habitat suitability of V. jugorum, using multiple SDM algorithms (Generalized Additive Models, Boosted Regression Trees, Maximum Entropy Models). In parallel, we carried out field samplings aimed at obtaining fresh specimens for measuring functional traits related to size and reproductive success. We tested the relationship between functional traits and habitat suitability using Linear Mixed Models (LMM). Results. We found that the species distribution could be adequately approximated via a combination of habitat and climatic variables. We further demonstrated that optimal habitat conditions maximized both body and egg clutch size. Main conclusions. Bridging biogeography and functional ecology is a promising avenue for obtaining a realistic construction of species niches, achieving a better understanding of the interplay between climate, functional traits, species distribution and individual fitness. We emphasize the importance of using field-collected data for obtaining objective evaluations of SDM performance.