Background/Question/Methods Patterns of local community assembly reflect dispersal, abiotic conditions, and species interactions, but largely depend on the collection of all potential residents available: the species pool. Yet, determining the spatial scale at which the species pool is defined remains challenging, one reason being that this scale can vary over both time and space, even within the same system. Currently, we lack a complete understanding of how the spatial scale of species pools contributes to determinism and stochasticity in community assembly. Using nectar-inhabiting microbial communities of the hummingbird-pollinated shrub, Diplacus aurantiacus, we investigated the effect of the species pool spatial scale in shaping the composition and function of local communities. We manipulated nectar microbial communities across a 1-km landscape within the Jasper Ridge Biological Preserve in California by mixing nectar communities to create small- and large-scale microbial species pools. We also varied the number of donor nectar communities contributing to the species pool mixtures by using nectar from either 2 or 12 donor flowers. We then inoculated newly opened flowers with the different species pools in the field. Once flowers began to senesce, we harvested and characterized microbial communities in the recipient flowers in comparison to the original inoculant mixture using both culture-dependent and culture-independent methods. We also quantified resulting nectar pH and sugar composition to infer functional consequences of the inoculations. We predicted that communities assembled under the large-scale pool composed of 12 donors would be the most homogenized in terms of community composition and function. Results/Conclusions Nectar communities that received the large-scale species pool inoculant had the highest abundance of bacteria, while those that received small-scale species pool mixtures had the highest abundance of yeast. Inoculation with species pools composed of 12 donor nectar communities in comparison to mixtures made with two donor communities led to both higher bacterial and fungal abundance in recipient nectar communities. The largest magnitude difference between bacteria and yeast abundance occurred in recipient communities that were inoculated with the large-scale species pool. Contrary to our expectation, the communities that had the highest variability in nectar pH, which indicates bacterial metabolism, were those that received the large-scale mixture created with 12 donor communities, whereas the least variable communities were those that received the large-scale mixture composed of two donor communities. Our results suggest that, in this system, stochasticity may be most important when communities are assembled under a large-scale species pool composed of many donor communities.