Multitrophic assembly influences β-diversity across a tripartite system of flowering plants, bees, and bee-gut microbiomes

Theoretical frameworks of terrestrial community assembly often focus on single trophic levels (e.g. plants) without considering how complex interdependencies across different trophic levels influence assembly mechanisms. Yet, when multiple trophic levels are considered (e.g. plant–pollinator, plant–microbe interactions) the focus is typically on network analyses at local spatial scales. As spatial variation in biodiversity (β-diversity) is increasingly being recognized for its relevance in understanding community assembly and conservation, considering how β-diversity at one trophic level may be influenced by assembly processes that alter abundance and composition of interacting communities at a different trophic level (multitrophic dependency) is critical. Here, we build on single trophic level community assembly frameworks to explore the assembly processes affecting β-diversity in multitrophic communities comprising flowering plants, their bee pollinators, and the corresponding bee-gut microbiota to better understand the importance of multitrophic dependency in community assembly. Using distance-based redundancy analysis and variation partitioning, we investigated community assembly processes across three interconnected trophic levels in two ecological regions in southern California: the Santa Monica Mountains and three islands of the Channel Island Archipelago. We found that the deterministic effects of multitrophic dependency are stronger on directly connected trophic levels than on indirectly connected trophic levels (i.e. flowers explain bee communities and bees explain bee-gut bacteria communities, but flowers weakly explain variation in bee-gut bacteria communities). We also found notable regional variation, where multitrophic dependency was weaker on the Channel Islands as ecological drift was more pronounced. Our results suggest that integrating the influence of multitrophic dependency on community assembly is important for elucidating drivers of β-diversity and that multitrophic dependency can be determined by the regional context in which β-diversity is measured. Taken together, our results highlight the importance of considering multiscale perspectives – both multitrophic and multiregional – in community assembly to fully elucidate assembly processes.