1. Ecologists continue to debate whether the assembly of communities of species is more strongly influenced by dispersal limitations or niche-based factors. Analytical approaches that account for both mechanisms can help to resolve controls of community assembly.
2. We compared littoral snail assemblages in Lake Tanganyika at three different spatial scales (5–25 m, 0.5–10 km and 0.5–27 km) to test whether spatial distance or environmental differences are better predictors of community similarity.
3. At the finest scale (5–25 m), snail assemblages shifted strongly with depth but not across similar lateral distances, indicating a stronger response to environmental gradients than dispersal opportunities.
4. At the two larger scales (0.5–27 km), both environmental similarity and shoreline distance between sites predicted assemblage similarity across sites. Additionally, canonical correspondence analysis revealed that snail abundances were significantly correlated with algal carbon-to-nitrogen ratio and wave energy.
5. Our results indicate that the factors governing assemblage structure are scale dependent; niche-based mechanisms act across all spatial scales, whereas community similarity declines with distance only at larger spatial separations.