Urbanization and the expansion of human activities foster radical ecosystem changes with cascading effects also involving host-pathogen interactions. Urban pollinator insects face several stressors related to landscape and local scale features such as green habitat loss, fragmentation, and availability reduction of floral resources with unpredictable effects on parasite transmission. Furthermore, beekeeping may contribute to the spread of parasites to wild pollinators by increasing the number of parasite hosts. Here we used DNA-based diagnostics tools to evaluate how the occurrence of parasites, namely microsporidians (Nosema spp.), trypanosomatids (Crithidia spp.) and neogregarines (Apicystis bombi), is shaped by the above-mentioned stressors in two bumblebee species (i.e, Bombus terrestris and B. pascuorum). Infection rates of the two species were different and generally higher in B. terrestris. Moreover, they showed different responses towards the same ecological variables, possibly due to differences in body size and foraging habits supposed to affect their susceptibility to parasite infection. The probability of infection was found to be reduced in B. pascuorum by green habitat fragmentation, while increased along with floral resource availability. Unexpectedly, B. terrestris had a lower parasite richness nearby apiaries probably because parasites are prone to be transmitted among the most abundant species. Our finding supports the need to design proper conservation measures based on species-specific knowledge, as suggested by the variation in the parasite occurrence of the two species. Moreover, conservation policies aiming at safeguarding pollinators through flower planting should consider the indirect effects of these measures for parasite transmission together with pollinator biodiversity issues.
Habitat fragmentation is known to affect biodiversity, but the impact on pollinators and their interactions with plants is still unclear in anthropized landscapes. Islands are open-air laboratories for ecological studies with simplified communities and interactions, suitable to disentangle how land-use alteration impacts pollination ecology and its ecosystem service. Here, we used Maldives islands as model systems to investigate how pollinator richness, their mutualistic interactions with plants, and pollination efficiency are shaped by the degree of green area fragmentation (i.e., gardens, parks and semi-natural green covered patches), by considering both community- and species-level responses. To do this, we surveyed pollinators from 11 islands showing a gradient of green area fragmentation. In order to characterize the interactions between plants and pollinators and obtain a novel and comprehensive view of the key ecological dynamics, a DNA metabarcoding approach was adopted to identify the pollen carried by pollinators. We found that green area fragmentation at intermediate levels played positive effects on pollinator richness. However, fragmentation decreased interaction network complexity. Intriguingly, body size mediated the effect of landscape alteration on plant-pollinator interactions, as only the largest bee species expanded the foraging breath in terms of transported pollen richness at increasing fragmentation. In parallel, the pollination efficiency increased with pollinator species richness in two sentinel plants. This study shows that moderate landscape fragmentation of green areas shapes the ecosystem service of pollination, where in spite of interactions being less complex and mediated by pollinator body size, pollinator biodiversity and potential plant reproduction are supported.