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Infectious disease is a major driver of biodiversity loss, but how disease threatens pollinator communities remains poorly understood. Here, we review the plant–pollinator–pathogen literature to identify mechanisms by which plant and pollinator traits and community composition influence pathogen transmission and assess consequences of transmission on plant and pollinator fitness. We find that plant and pollinator traits that increase floral contact can amplify transmission, but community-level factors such as plant and pollinator abundance are often correlated and can counteract one another. Although disease reduces pollinator fitness in some species, little research has assessed cascading effects on pollination, and taxonomic representation outside of honey bees and bumble bees remains poor. Major open challenges include (a) disentangling correlations between plant and pollinator abundance to understand how community composition impacts pathogen transmission and (b) distinguishing when pathogen transmission results in disease. Addressing these issues, as well as expanding taxonomic representation of pollinators, will deepen our understanding of how pathogens impact diverse pollinator communities. 

Abstract Flowers can be transmission platforms for parasites that impact bee health, yet bees share floral resources with other pollinator taxa, such as flies, that may be hosts or non-host vectors (i.e., mechanical vectors) of parasites. Here, we assessed whether the fecal-orally transmitted gut parasite of bees,Crithidia bombi, can infectEristalis tenaxflower flies. We also investigated the potential for two confirmed solitary bee hosts ofC. bombi,Osmia lignariaandMegachile rotundata, as well as two flower fly species,Eristalis arbustorumandE. tenax,to transmit the parasite at flowers. We found thatC. bombidid not replicate (i.e., cause an active infection) inE. tenaxflies. However, 93% of inoculated flies defecated liveC. bombiin their first fecal event, and all contaminated fecal events containedC. bombiat concentrations sufficient to infect bumble bees. Flies and bees defecated inside the corolla (flower) more frequently than other plant locations, and flies defecated at volumes comparable to or greater than bees. Our results demonstrate thatEristalisflower flies are not hosts ofC. bombi, but they may be mechanical vectors of this parasite at flowers. Thus, flower flies may amplify or diluteC. bombiin bee communities, though current theoretical work suggests that unless present in large populations, the effects of mechanical vectors will be smaller than hosts. 

Abstract AimAnimal migration is often explained as the result of resource tracking in seasonally dynamic environments. Therefore, resource availability should influence both the distributions of migratory animals and their seasonal abundance. We examined the relationship between primary productivity and the spatio‐temporal distributions of migratory birds to assess the role of energy availability in avian migration. LocationNorth America. Time periodFull annual cycle, 2011–2016. Major taxa studiedNocturnally migrating landbirds. MethodsWe used observations of nocturnally migrating landbirds from the eBird community‐science programme to estimate weekly spatial distributions of total biomass, abundance and species richness. We related these patterns to primary productivity and seasonal productivity surplus estimated using a remotely sensed measure of vegetation greenness. ResultsAll three avian metrics showed positive spatial associations with primary productivity, and this was more pronounced with seasonal productivity surplus. Surprisingly, biomass showed a weaker association than did abundance and richness, despite being a better indicator of energetic requirements. The strength of associations varied across seasons, being the weakest during migration. During spring migration, avian biomass increased ahead of vegetation green‐up in temperate regions, a pattern also previously described for herbivorous waterfowl. In the south‐eastern USA, spring green‐up was instead associated with a net decrease in biomass, and winter biomass greatly exceeded that of summer, highlighting the region as a winter refuge for short‐distance migrants. Main conclusionsAlthough instantaneous energy availability is important in shaping the distribution of migratory birds, the stronger association of productivity with abundance and richness than with biomass suggests the role of additional drivers unrelated to energetic requirements that are nonetheless correlated with productivity. Given recent reports of widespread North American avifaunal declines, including many common species that winter in the south‐eastern USA, understanding how anthropogenic activities are impacting winter bird populations in the region should be a research priority.