Cave-obligate organisms usually have smaller ranges and their assemblages have higher beta diversity than their epigean counterparts. Phylogenetic and functional diversity is usually low in cave communities, leading to taxonomic and functional disharmony, with entire groups missing from the subterranean realm. The objective of this work is to compare range, beta diversity, phylogenetic and functional diversity, taxonomic and functional disharmony of epigean versus troglobiont spiders in the Iberian Peninsula.

The median extent of occurrence was found to be 33 times higher for epigean than for cave species. Beta diversity was significantly higher for troglobiont assemblages. Cave assemblages present lower phylogenetic and functional diversities than expected by chance. Taxonomic disharmony was noticeable, with many speciose families, namely Gnaphosidae, Salticidae and Lycosidae, absent in caves. Functional disharmony was equally high, with ambush hunters and sensing web weavers being absent in caves.

The small range and high beta diversity of troglobiont spiders in the Iberian Peninsula is typical of many cave-obligate organisms, caused by the fragmentation and isolation of cave systems and the low vagility and high habitat specialization of species. Caves were colonized mainly by pre-adapted lineages, with high proportions of eutroglophile species. Some families no longer occur in surface habitats, possibly since the last glaciations, and currently are restricted to caves in the region. Few hunting strategies and web types are efficient in caves and these dominate among the troglobiont species.

As troglobiont communities are of low alpha diversity, with low functional redundancy, have narrow ranges, present high levels of population fragmentation and are taxonomically unique, they should present higher proportions of imperilled species than epigean spiders in the Iberian Peninsula. Some species are probably endangered and require urgent conservation measures.