Native solitary bee population dynamics little affected by variation in fecundity or brood parasitism

Background/Question/Methods

Understanding influences on the populations of native bees is central to pollinator conservation.  Research shows that bee diversity and abundance responds to landscape-level habitat distributions and local resources, but few studies have investigated the demographic causes.  Fecundity and brood parasitism are the most studied and easily measured sources of demographic variability. We examined the demography of the solitary bee, Osmia pumila (Hymenoptera: Megachilidae), throughout its annual life cycle to determine factors most important for population regulation.  Specifically, we contrasted resource-related factors, such as brood cell production, with mortality at stages vulnerable to brood parasitism.  Fates of brood cells made within artificial domiciles were followed throughout their development in old field sites on Long Island, New York, USA over two years.  We divided the life cycle into five stages based on the developmental sequence for Osmia (Egg, Larva, Prepupa, Pupa, Adult), and constructed matrices to describe the transition probabilities from one stage to the next, as well as survival and fecundity. We used periodic matrix models to project growth of three cohorts corresponding to early, mid, and late reproductive period.  The effects of variation in the transition probabilities, survival, and fecundity on population growth were examined using elasticity analysis.   

Results/Conclusions

Despite large variation in fecundity and mortality caused by brood parasites, population growth (λ) at most sites was stable (λ ≥ 1.0).  The stages with the largest impact on population growth were similar among sites. The elasticity of λ to fecundity and mortality of eggs and prepuae (stages affected by parasites) were all less than 0.02.  The mid-reproductive period appeared most susceptible to parasitism, but also had the highest fecundity.  Elasticity values of λ to transitions affected by brood parasites declined from early to mid-reproductive periods, while the elasticity of λ to fecundity was always highest during the mid-period.  The highest elasticities of λ were for survival of pupae with values ranging from 0.24 – 0.29.  We conclude that the stages typically studied in the dynamics of solitary bee populations, fecundity and egg and larval mortality by parasitism, contributed little to variation in population growth of O. pumila, whereas survival of pupae was critical.  Pupal survival depends on the quality of food provided at the time of brood cell construction, but also on abiotic conditions experienced by pupae at the end of the growing season.  Variation in environmental factors, such as temperature, during this time could greatly influence survival.