Role of abandoned grasslands in the conservation of spider communities across heterogeneous mountain landscapes

In the last decades, the transformation of agriculture and the associated decline in semi-natural grasslands have been major drivers of the loss of spider diversity across Europe, in particular in mountain regions. In the early stages of forest succession, abandoned grasslands exhibit a complex vegetation structure sharing environmental conditions with both open habitats and forests. At the landscape scale, the occurrence of abandoned patches can potentially provide novel habitats and niches for ground-dwelling spiders. Here, we used two complementary methods: traditional species diversity analyses and species-habitat networks to understand how abandoned grasslands can interact with the surrounding habitat mosaic in supporting spider communities. We selected six landscapes along an elevational gradient and sampled spiders in all the major agricultural and semi-natural habitats occurring across the landscapes. At all elevations, we found a similar species richness and activity density between agricultural habitats (i.e. meadows, pastures, and crop fields) and abandoned grasslands. On the contrary, we consistently found lower species richness and activity density in forests than in all open habitats. Despite the similar diversity between open habitats, most species-habitat networks exhibited a high and constant modularity, i.e. the same species tended to occur in a specific habitat type and not in the others. Only a few species were shared between abandoned and agricultural habitats, indicating that abandoned grasslands cannot support populations of most species typical of open managed habitats. Early abandoned patches increased landscape habitat heterogeneity providing unique niches to spider species that, however, rarely occur in other habitats. The maintenance of multiple habitat types across the landscape is expected to increase the regional species pool, while the role of abandoned patches as surrogate habitat for the conservation of spider species typical of semi-natural grasslands appeared limited. Similarly, abandoned patches are not expected to help maintaining the populations of spider species occurring in agricultural habitats.