Background/Question/Methods Two prominent mechanisms proposed to explain local species composition and biodiversity are niche-based environmental filtering and chance immigration from local and regional species pools. Both mechanisms are hypothesized to play important roles in high-diversity communities, where local species membership can reflect environmental filtering from large species pools and where many species are rare and recruitment limited. We examined how two niche-based abiotic filters, climate (rainfall) and disturbance (fire intensity), interact with seed arrival to structure local biodiversity in high-diversity, fire-frequented longleaf pine savanna groundcover assemblages (>30 spp/m²). In a two-year factorial field experiment, we manipulated: (1) rainfall using rain shelters and supplemental irrigation to mimic dry and wet years, respectively; (2) local fire intensity by increasing pre-fire fine-fuel loads (pine needles); and (3) seed arrival by increasing seed densities of 30 species of forbs and sedges. Results/Conclusions Seed arrival increased species richness and the abundance of most species in the presence and absence of both abiotic filters. However, the positive effects of seed arrival on species richness, and especially abundance, were reduced by drought. Irrigation, in contrast, increased abundance but not species richness. Locally-intense fires temporarily reduced species richness when seed arrival was low, but not when seed arrival was high. This pattern suggests that intense fires filter some resident species from local sites, yet simultaneously increase microsite availability for recruitment of immigrating species. In this high-diversity community, seed arrival plays a key role in structuring local biodiversity, but the net importance of this mechanism is influenced by both climate and disturbance dynamics. Our results have implications for understanding community assembly and the maintenance of biodiversity in the face of environmental change.