Physical and biotic factors driving the diversity of spider assemblages in tree hollows of Mediterranean Quercus forests

Tree hollows are keystone structures that promote forest biodiversity. This study analyses the spatio-temporal diversity of spiders in these microhabitats. Forty-eight emergence traps were installed in tree cavities of Quercus pyrenaica forests in the Iberian Peninsula. Traps were collected monthly during a complete year. Generalised linear models and canonical correspondence analysis were used to evaluate the effect of two physical (hollow volume and height above ground) and three biotic variables (beetle richness and abundance and isopod abundance) on the spider diversity and composition of the entire assemblage and of the foraging guilds. Moreover, we examined the temporal segregation of the species using beta diversity metrics. Tree hollows hosted a rich spider assemblage (87 species) including 10 endemic and one vulnerable species. We registered four tree-bark obligate and one hollow-dependent species. Hollow height and beetle richness were the variables that better explained richness, while all variables analysed influenced spider abundances. However, contrary to the observed effect for other taxa (i.e. beetles), bigger cavities did not host more spider species. Temporal beta diversity throughout the year was very high and primarily explained by species turnover suggesting a temporal species segregation. We conclude that tree hollows constitute a key habitat for forest spiders and that special attention must be paid to the preservation of this microhabitat in forest ecosystems.