Meta menardi and M. bourneti (Araneae: Tetragnathidae) segregate along the altitudinal gradient of Mount Etna (Sicily, S–Italy)

Meta menardi (Latreille, 1804) and M. bourneti Simon, 1922 (Araneae: Tetragnathidae) are ubiquitous inhabitants of the twilight zone of most hypogean sites across Europe. Despite both species being mostly distributed in subterranean habitats, they show broad ranges of distribution compared to other cave-dwelling spiders which can be explained in the light of their life cycle, including an epigean phase of dispersal. The two species are broadly distributed in Italy, including Sicily, where they show a remarkable segregation along the altitudinal gradient of Mount Etna. Thanks to our recent sampling activities in this area, we built a georeferenced dataset allowing the application of Species Distribution Modelling aiming at evaluating the impacts of climate change on their local distribution. On the basis of SDM results, we predict a relatively wide suitable area for M bourneti, ranging from the sea level up to 1,100 meters a.sl.. whereas the predicted suitable area for M. menardi encompasses a narrow mid altitude strip, extending halfway between the lower areas, suitable for M. bourneti, and the higher ones, predicted as highly unsuitable and heavily impacted by ongoing volcanic activity. The averaged future predictions for 2070 under RCP 8.5 scenario, show that M. bourneti will expand its range upwards, in areas that are now suitable for M. menardi. In turn, predictions for M. menardi indicate an extreme reduction of the current strip of suitable habitat likely determining its local extinction. Our findings are further corroborated by the analysis of the bioclimatic niche of the two species assessed via multidimensional Hutchinsonian hypervolume, being much smaller in M. menardi compared to of M. bourneti. In light of our results it seems likely that having wider climatic preference, M. bourneti will substitute M. menardi in most of its current range in Sicily. Future interventions aiming at the conservation of M. menardi on Mount Etna are strongly advised.