Community context mediates effects of pollinator loss on seed production

A critical goal for ecologists is understanding how ongoing local and global species losses will affect ecosystem functions and services. Diversity-functioning relationships, which are well-characterized in primary producer communities, are much less consistently predictable for ecosystem functions involving two or more trophic levels, particularly in situations where multiple species in one trophic level impact functional outcomes at another trophic level. This is particularly relevant to pollination functioning, given ongoing pollinator declines and the value of understanding pollination functioning for single plant species like crops or threatened plants. We used spatially replicated, controlled single-pollinator-species removal experiments to assess how changes in bumble bee species richness impacted the production of fertilized seeds in a perennial herb-Delphinium barbeyi-in the Rocky Mountains of Colorado, USA. To improve predictability, we also assessed how traits and abundances in the plant and bumble bee communities were related to D. barbeyi reproductive success. We hypothesized that trait-matching between pollinator proboscis length and D. barbeyi's nectar spurs would produce a greater number of fertilized seeds, while morphological similarity within the floral community would dilute pollination services. We found that the effects of pollinator removal differed depending on the behavioral patterns of pollinators and compositional features of the plant and pollinator communities. While pollinator floral fidelity generally increased D. barbeyi seed production, that positive effect was primarily evident when more than half of the Bombus community was experimentally removed. Similarly, communities comprising primarily long-tongued bees were most beneficial to D. barbeyi seed production in tandem with a strong removal. Finally, we observed contrasting effects of morphological similarity in the plant community, with evidence of both competition and facilitation among plants. These results offer an example of the complex dynamics underlying ecosystem function in multitrophic systems and demonstrate that community context can impact diversity-functioning relationships between trophic levels.

Automated summary

A critical goal for ecologists is to understand the impact of ongoing species losses on ecosystem functions. The relationship between diversity and ecosystem functions involving multiple trophic levels is less predictable. This study focuses on pollination functioning and assesses the effects of bumble bee species richness on seed production in a perennial herb. The results demonstrate the complex dynamics of ecosystem function in multitrophic systems and the importance of community context in diversity-functioning relationships.