The co-occurrence of different grassland communities increases the stability of pollination networks

High heterogeneity of grassland communities supports a high diversity of species and represents a key point for the retention of pollinators in agricultural landscapes. In the present study, we explored whether the co-occurrence of different grassland communities has any effect on the stability of the network of pollination interactions. We monitored pollination interactions in two co-occurring grassland communities, differing in disturbance history and water and nutrient supply. The monitoring was carried out during the summer season (7 surveys). For each survey we compared the role in the pollination networks (i.e., keystone vs. peripheral species) of habitat-specialist and habitat-generalist plant and pollinator species. We found that plant and pollinator species of the two different grassland communities were highly interconnected, revealing that pollination interactions occur at a level of organization above that of the single community. The co-occurrence of the two grassland communities increased the type, number and frequency of contacts, thereby contributing to networks stability. The role of habitat-specialist and habitat-generalist plant and pollinator species in pollination networks was asymmetric, with habitat-specialist plants and habitat-generalist pollinators being keystone species, while habitat-generalist plants and habitat-specialist pollinators being peripheral in the pollination networks. Our results showed that the stability of the network does stem from the co-occurrence of different species pools having different but complementary roles in the pollination networks. From a conservation perspective, the maintenance of different grassland communities is important not only because they allow the conservation of habitat-specialist species, but specifically because plant species specialized in either grassland community are also keystone for the maintenance of the stability of the pollination networks.