The earlier the better? Nesting timing and reproductive success in subalpine cavity-nesting bees

1. Reproductive timing can affect an organism’s production of offspring
and its offspring’s success, both of which contribute to its overall
fitness. In seasonal environments, the timing of reproductive activity may
be restricted to short periods of the year owing to numerous potential
selective pressures such as variation in daylength, weather, food
availability, predation, or competition. 2. We documented the
relationships between reproductive timing and individual reproductive
success (total reproductive output and offspring success) in subalpine
populations of five cavity-nesting solitary bee species. We also examined
the relationships between bee reproductive success and environmental
variables that are likely ultimate drivers of bee phenology in subalpine
environments (i.e., seasonality of floral resource abundance and
temperature). 3. Over six years, we recorded solitary bee nesting timing,
egg production, and offspring success using artificial nesting structures
(“trap-nests”) established at multiple study sites. We also quantified
floral resources and recorded temperature throughout growing seasons. 4.
Bees nesting earlier in the season exhibited greater reproductive success.
Reproductive output generally increased with floral abundance, although
this relationship was weak and only significant for some bee species.
Elevated temperatures were associated with increased nest construction
rate, but not in greater reproductive output. These contrasting effects of
temperature may have been driven by the negative relationship between
temperature and bee longevity. Bees who nested for shorter durations of
time (a proxy for longevity) produced fewer offspring, and individuals
exhibiting the shortest nesting durations were also those that began
nesting late in the season. 5. Overall, bees who initiated nesting early
and sustained activity for a long duration had the highest reproductive
output. This work documents the relationship between reproductive
phenology and fitness in wild insect populations and highlights the ways
in which organisms can cope with the challenges of living in seasonal and
highly variable environments.