Reproductive timing can significantly impact an organism’s total offspring production and success, both of which contribute to overall individual fitness. The importance of reproductive timing is often accentuated in seasonal environments as favorable periods for reproduction are constrained by short growing seasons. Organisms must synchronize reproductive activity with periods of adequate food abundance and favorable weather conditions but also leave enough time for offspring to undergo sufficient development prior to the end of the season. Over six years, we recorded nesting phenology and reproductive success in subalpine populations of five cavity-nesting bee species. Given that bees require floral resources to provision offspring and elevated temperatures for flight, we also quantified floral resource abundance and recorded temperature as factors that potentially constrain bee nesting phenology. Our study addresses the three following questions: (1) What are the consequences of nesting timing for individual reproductive output and offspring success?, (2) How do selective pressures acting on nesting timing such as resource abundance and weather vary seasonally? and (3) Do the fitness consequences of nesting timing reflect the effects of seasonally varying selective pressures such as resource abundance and weather conditions?
Results/Conclusions
Bees that initiated nesting early in the season exhibited greater reproductive success despite imperfect synchrony with seasonal peaks in floral abundance and temperature. Reproductive output generally increased with floral abundance, although this relationship was only significant in one of five species. Increasing temperatures negatively affected bee reproductive output. We further investigated this latter result post-hoc by considering bee longevity as an additional factor influencing reproductive output. We found that bees exhibiting the shortest longevity were also those that began nesting late in the season. Elevated temperatures reduced longevity. Bees with reduced longevity had lower reproductive output.
While these results seemingly contradict the findings of phenological studies that emphasize the importance of synchrony with resource abundance and favorable weather, the brevity of growing seasons in our study region may ultimately limit the phenological flexibility of short-lived organisms. As such, cavity-nesting bees may maximize reproductive output not necessarily via synchrony with optimal conditions, but rather by initiating nesting early and sustaining activity for a long period. Although the underlying mechanisms driving these results are unclear, our study demonstrates the consequences of variation in reproductive phenology and highlights the potential ways in which organisms cope with the challenges of living in seasonal environments.