Bumble bee nest density is lower in drought years.

Climate change is driving more frequent and intense droughts in many parts of the world. While we hold a broad understanding of how drought impacts plant populations, predicting drought's demographic effects on animal populations remains a challenge, particularly for invertebrates. Here, we use 7 years of bumble bee nest surveys spanning a period with two historic droughts to estimate impacts of drought on bumble bee nest abundance and apparent survival. Nest abundance significantly declined with drought (overall average of 19.3 versus 11.3 nests in non-drought versus drought years). Relative nest abundances of Bombus impatiens and Bombus griseocollis, two common species in this system, did not change with drought despite differences in life history and apparent survival. B. impatiens abundance in forests increased during droughts (approx. 60% of nests in forests versus 20-40% in drought versus non-drought years). Nests of rarer species were absent from our survey area after the first of the two droughts, but the two common species recovered quickly in the year after each drought. This study demonstrates that well-known effects of drought on floral resources likely translate to population-level impacts in bumble bees. It also highlights the importance of long-term monitoring for detecting impacts of intermittent environmental disturbances.