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Aim: Quantifying the relative contribution of environmental filtering versus limiting similarity in shaping communities is challenging because these processes often act simultaneously and their effect is scale‐dependent. Focusing on caves, island‐like natural laboratories with limited environmental variability and species diversity, we tested: (i) the relative contribution of environmental filtering and limiting similarity in determining community assembly in caves; (ii) how the relative contribution of these driving forces changes along environmental gradients. Location: Europe. Time period: Present. Major taxa studied: Subterranean spiders. Methods: We used data on distribution and traits for European cave spiders (n = 475 communities). We estimated the trait space of each community using probabilistic hypervolumes, and obtained estimations of functional richness independent of the species richness of each community via null modelling. We model functional diversity change along environmental gradients using generalized dissimilarity modelling. Results: Sixty‐three percent of subterranean spider communities exhibited a prevalence of trait underdispersion. However, most communities displayed trait dispersion that did not depart significantly from random, suggesting that environmental filtering and limiting similarity were both exerting equally weak or strong, yet opposing influences. Overdispersed communities were primarily concentrated in southern latitudes, particularly in the Dinaric karst, where there is greater subterranean habitat availability. Pairwise comparisons of functional richness across caves revealed these effects to be strongly scale‐dependent, largely varying across gradients of cave development, elevation, precipitation, entrance size and annual temperature range. Conversely, geographical distance weakly affected trait composition, suggesting convergence in traits among communities that are far apart. Main conclusions: Even systems with stringent environmental conditions maintain the potential for trait differentiation, especially in areas of greater habitat availability. Yet, the relative influence of environmental filtering and limiting similarity change with scale, along clear environmental gradients. The interplay of these processes may explain the assembly of species‐poor subterranean communities displaying high functional specialization. [ABSTRACT FROM AUTHOR]

Species traits are an essential currency in ecology, evolution, biogeography, and conservation biology. However, trait databases are unavailable for most organisms, especially those living in difficult-to-access habitats such as caves and other subterranean ecosystems. We compiled an expert-curated trait database for subterranean spiders in Europe using both literature data (including grey literature published in many different languages) and direct morphological measurements whenever specimens were available to us. We started by updating the checklist of European subterranean spiders, now including 512 species across 20 families, of which at least 192 have been found uniquely in subterranean habitats. For each of these species, we compiled 64 traits. The trait database encompasses morphological measures, including several traits related to subterranean adaptation, and ecological traits referring to habitat preference, dispersal, and feeding strategies. By making these data freely available, we open up opportunities for exploring different research questions, from the quantification of functional dimensions of subterranean adaptation to the study of spatial patterns in functional diversity across European caves. Measurement(s) morphological trait • ecological trait Technology Type(s) literature extraction • observation Sample Characteristic - Organism Agelenidae • Amaurobiidae • Anapidae • Cybaeidae • Dysderidae • Hahniidae • Leptonetidae • Linyphiidae • Liocranidae • Mysmenidae • Nesticidae • Pholcidae • Pimoidae • Segestriidae • Sicariidae • Sparassidae • Symphytognathidae • Telemidae • Tetragnathidae • Theridiidae Sample Characteristic - Environment caves • subterranean habitats Sample Characteristic - Location Europe [ABSTRACT FROM AUTHOR]

1. Dark diversity represents the set of species that can potentially inhabit a given area under particular ecological conditions, but are currently 'missing' from a site. This concept allows characterisation of the mechanisms determining why species are sometimes absent from an area that seems ecologically suitable for them. 2. The aim of this study was to determine the dark diversity of hoverflies in south‐eastern Europe and to discuss the role of different functional traits that might increase the likelihood of species contributing to dark diversity. Based on expert opinion, the Syrph the Net database and known occurrences of species, the study estimated species pools, and observed and dark diversities within each of 11 defined vegetation types for 564 hoverfly species registered in south‐eastern Europe. To detect the most important functional traits contributing to species being in dark diversity across different vegetation types, a random forest algorithm and respective statistics for variable importance were used. 3. The highest dark diversity was found for southwest Balkan sub‐Mediterranean mixed oak forest type, whereas the lowest was in Mediterranean mixed forest type. Three larval feeding modes (saproxylic, and phytophagous on bulbs or roots) were found to be most important for determining the probability of a species contributing to hoverfly dark diversity, based on univariate correlations and random forest analysis. 4. This study shows that studying dark diversity might provide important insights into what drives community assembly in south‐eastern European hoverflies, especially its missing components, and contributes to more precise conservation prioritisation of both hoverfly species and their habitats. [ABSTRACT FROM AUTHOR]

Macroecologists seek to identify drivers of community turnover (β-diversity) through broad spatial scales. However, the influence of local habitat features in driving broad-scale β-diversity patterns remains largely untested, owing to the objective challenges of associating local-scale variables to continental-framed datasets. We examined the relative contribution of local- versus broad-scale drivers of continental β-diversity patterns, using a uniquely suited dataset of cave-dwelling spider communities across Europe (35–70° latitude). Generalized dissimilarity modelling showed that geographical distance, mean annual temperature and size of the karst area in which caves occurred drove most of β-diversity, with differential contributions of each factor according to the level of subterranean specialization. Highly specialized communities were mostly influenced by geographical distance, while less specialized communities were mostly driven by mean annual temperature. Conversely, local-scale habitat features turned out to be meaningless predictors of community change, which emphasizes the idea of caves as the human accessible fraction of the extended network of fissures that more properly represents the elective habitat of the subterranean fauna. To the extent that the effect of local features turned to be inconspicuous, caves emerge as experimental model systems in which to study broad biological patterns without the confounding effect of local habitat features. [ABSTRACT FROM AUTHOR]

Background Spiders (Arachnida: Araneae) are widespread in subterranean ecosystems worldwide and represent an important component of subterranean trophic webs. Yet, global-scale diversity patterns of subterranean spiders are still mostly unknown. In the frame of the CAWEB project, a European joint network of cave arachnologists, we collected data on cavedwelling spider communities across Europe in order to explore their continental diversity patterns. Two main datasets were compiled: one listing all subterranean spider species recorded in numerous subterranean localities across Europe and another with high resolution data about the subterranean habitat in which they were collected. From these two datasets, we further generated a third dataset with individual geo-referenced occurrence records for all these species. New information Data from 475 geo-referenced subterranean localities (caves, mines and other artificial subterranean sites, interstitial habitats) are herein made available. For each subterranean locality, information about the composition of the spider community is provided, along with local geomorphological and habitat features. Altogether, these communities account for > 300 unique taxonomic entities and 2,091 unique geo-referenced occurrence records, that are made available via the Global Biodiversity Information Facility (GBIF) (Mammola and Cardoso 2019). This dataset is unique in that it covers both a large geographic extent (from 35° south to 67° north) and contains high-resolution local data on geomorphological and habitat features. Given that this kind of high-resolution data are rarely associated with broad-scale datasets used in macroecology, this dataset has high potential for helping researchers in tackling a range of biogeographical and macroecological questions, not necessarily uniquely related to arachnology or subterranean biology. [ABSTRACT FROM AUTHOR]

Despite their ecological importance and diversity, spiders (Arachnida: Araneae) are underrepresented in conservation policies in comparison to other groups. We review all extant conservation tools focusing on spiders in Europe, highlighting general patterns, limitations, gaps, and future directions. We assembled a comprehensive online database reporting all available information concerning the legal protection and conservation status of 4,154 spider species. Existing international legislation has limited coverage, with only one species listed in the Bern Convention and EU Habitats Directive. At the national and subnational levels, 178 species are formally mentioned in the legislation of 19 European countries. Moreover, the International Union for Conservation of Nature (IUCN) includes assessments for 301 species worldwide, 164 of these threatened and eight native to Europe. In addition, spiders are mentioned in Regional Red Lists and Red Books in 28 out of 42 European countries considered in this review. Northern and Central European countries have the highest percentage of species assessed at the regional level in Red Lists and Red Books. The Mediterranean basin has the highest spider diversities in Europe but conservation efforts are lacking, both in terms of assessments and national or subnational legislation. Among European species, Dolomedes plantarius , Argyroneta aquatica and Eresus kollari are the most frequently mentioned in European conservation measures, possibly due to their ecological traits and their strict association with declining habitats. Considering the current threats to spiders in Europe, the protection of large areas of suitable habitat should be considered as the most effective approach to spider conservation. • Spiders are largely underrepresented in conservation policies in Europe. • Only one spider species is listed in the Bern Convention and EU Habitats Directive. • 178 species of spiders are mentioned in the legislation of 19 European countries. • 28 European countries have spider Red Lists or Red Books. • Conservation efforts are lacking in Mediterranean countries. [ABSTRACT FROM AUTHOR]

Through the Habitats Directive (92/43/EEC) and the financial investments of the LIFE projects, Europe has become an experimental arena for biological conservation. With an estimated annual budget of €20 billion, the EU Biodiversity Strategy for 2030 has set an ambitious goal of classifying 30% of its land and sea territory as Protected Areas and ensuring no deterioration in conservation trends and the status of protected species. We analysed LIFE projects focused on animals from 1992 to 2018 and found that investment in vertebrates was six times higher than that for invertebrates (€970 versus €150 million), with birds and mammals alone accounting for 72% of species and 75% of the total budget. In relative terms, investment per species towards vertebrates has been 468 times higher than that for invertebrates. Using a trait-based approach, we show that conservation effort is primarily explained by species' popularity rather than extinction risk or body size. Therefore, we propose a roadmap to achieve unbiased conservation targets for 2030 and beyond.