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 seven years of bumble bee nest surveys spanning a period with two
historic droughts to estimate the impacts of drought on bumble bee nest
abundance and apparent survival. Nest abundance significantly declined
with drought (average of 19.3 vs. 11.3 nests in non-drought vs. 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 strategies and apparent
survival. Bombus impatiens abundance in forests increased during droughts
(~60% of nests in forests, 20%-40% in drought versus non-drought years).
Surprisingly, 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.