Species‐rich old grasslands have beneficial effects on the health and gut microbiome of bumblebees

Abstract Recent studies have shown that old, traditionally managed semi‐natural grasslands (SNGs) harbour specific plant assemblages characterized by high species richness, diversity, evenness and a great abundance and diversity of floral resources. As nectar and pollen from many of these plants contain a multitude of favourable phytochemical compounds, we hypothesized that pollinator health and survival would improve in old, species‐rich hay SNGs compared to younger SNGs. We monitored experimental colonies of bumblebees placed in hay SNGs of different ages. The survival of bumblebees increased with grassland age, thus they lived longer and produced more workers in the old SNGs. The abundance of actinomorphic flowers (with radial symmetry) increased with the age of the SNGs and was positively correlated with the body size, body mass and lipid reserves of the bumblebees. The taxonomic composition of gut microbiota changed during the experiment, with a significant decrease of core taxa Bombiscardovia and Bifidobacterium in young SNGs, while the abundance of Bombilactobacillus increased in old SNGs. At the end of the experiment, the bumblebees of young SNGs hosted lower abundances of Gilliamella than those of old SNGs. In old SNGs, there was a positive relationship between floral richness and the abundance of six taxa, including the three core taxa Bombiscardovia , Bifidobacterium and Snodgrassella. Microbiota α‐diversity decreased and microbiota β‐diversity increased over time in young SNGs, while both remained stable in old SNGs. Both deterministic and stochastic processes acting simultaneously in bumblebee microbiomes within young SNGs seemed to explain these changes. Compared to old forests, very little attention has been paid to old, species‐rich grasslands. Considering their importance for pollinator health, as well as their high ecological and cultural values, it is crucial that the rare and endangered old, species‐rich hay SNGs are protected. Read the free Plain Language Summary for this article on the Journal blog.