Host and floral communities shape parasite prevalence and reproduction in intensively managed forests

Understanding how working landscapes can maintain viable wildlife populations is key to evaluating their conservation potential. We assessed the potential of intensively managed conifer forests for supporting healthy, productive bee populations in one of the major timber-growing regions of the world, the Pacific Northwest. We examined the direct effect of the number of years post-harvest and other forest characteristics on flowering plant and bee communities and their indirect effect on parasite prevalence (Apicystis spp., Ascosphaera spp., and Crithidia spp.) and reproduction of a native, forest-dwelling solitary bee (Osmia lignaria). Forest characteristics, including the time elapsed since harvest, influenced floral and bee community diversity and abundance and indirectly impacted parasite prevalence and offspring production. We found that increased bee diversity was associated with reduced parasite prevalence-consistent with a dilution effect-but the strength of the relationship varied across the different parasites. Additionally, bee abundance was more consistently associated with increased parasite prevalence, providing evidence of amplification. Floral abundance was only associated with lower Apicystis spp. prevalence. Across all parasite models, however, the R-2 values were <20%, indicating that additional factors shape bee communities beyond those we examined. Offspring production was positively related to floral diversity but not to parasite prevalence. Our results suggest that managing floral diversity is critical to enhancing the value of these landscapes for wild bee communities, both directly through promoting bee diversity and reproduction, and indirectly through facilitating parasite dilution.