Graham, K. K. | bioRxiv Science

bioRxiv2026

Drought reduces solitary bee reproduction and skews sex ratios

The Intermountain West of the United States has experienced years of extreme drought and increased temperatures. Increasing temperature and droughts can negatively impact native species that are locally adapted to environmental conditions that have persisted prior to the Anthropocene. Bees, especially solitary bees, provide critical ecosystem services because they pollinate ~90% of all flowering plants. Here we asked how drought impacted the reproduction of Osmia bruneri, a solitary mason bee. We released 20 females in nesting blocks at 21 field sites across four years (2020 - 2023) in the Bear River Mountains of northern Utah. O. bruneri reproduction was positively correlated with winter precipitation, and sex ratio was skewed in years that had a yearlong drought from the expected female to male ratio of 1.3:1 to 0.33:1. These results suggest that O. bruneri is susceptible to winter precipitation droughts. Not only was there a decrease in the total number of cells provisioned but the overall number of females produced decreased significantly during season long drought conditions. Continuous droughts can lead to a local level population decline and could contribute to overall species declines. Identifying the effects of extreme drought on solitary bee fecundity is critical for supporting effective management practices and conservation prioritization. Additionally, the results suggest that preceding winter precipitation can act as an indicator for predicting nesting success in wild solitary bees and may be an overall indicator of habitat quality.

ecology↗

bioRxiv2023

A century of sampling at an ecological preserve reveals declining diversity of wild bees

We analyzed the wild bee community from 1921 to 2018 at a nature preserve in southern Michigan, USA using museum records and found significant shifts in the bee community. Across the near century of records, species richness peaked in the 1970s and 1980s. There was an intensive bee survey completed by F.C. Evans in 1972 and 1973. We attempted to replicate his effort in 2017 and 2018, and again found a significant decline in species richness and evenness. There was also evidence of declining abundance in many of the more common species. We also conducted traits analyses using neural networks, revealing that oligolectic ground-nesting bees and cleptoparasitic bees were more likely to be extirpated whereas polylectic cavity-nesting bees were more likely to have persisted. Additionally, larger body size was associated with increased probability of local extirpation for polylectic cavity-nesting species. Larger phenological range was associated with increased chances of persistence for polylectic species, while it was associated with extirpation for oligolectic ground-nesting species. Species in the contemporary samples also had a more southerly overall distribution compared to the historic one. Open Research StatementData used for analyses in this manuscript, including Evans original dataset from 1972/1973 with updated species nomenclature, will be permanently archived at the USDA Ag Data Commons after the acceptance of this manuscript and will be citable and accessible here: https://data.nal.usda.gov/dataset/century-sampling-ecological-preserve-reveals-declining-diversity-wild-bees. Complete instructions on how to access all data referenced in this manuscript can be found in Appendix S1.

ecology↗