Resource availability, competitor abundance and specialization affect competition among bumblebees.

The strength of interspecific and intraspecific competition depends on resource availability, competitor abundance, and specialization. Most studies are conducted with single species pairs, constraining our understanding of what drives competition in multispecies communities. We investigated the effect of floral density, competitor abundance, flowering period, and trait specialization (proboscis length) on the potential for interspecific and intraspecific competition between seven common bumblebee species in the Hengduan Mountains of southwest China. We estimated the potential for interspecific and intraspecific competition using resource partitioning indices in plant-bumblebee networks (Müller's index) over five consecutive years, quantifying changes in floral densities and bumblebee abundance throughout the season. The potential for interspecific competition increased with bumblebee abundance, particularly when floral density was low later in the season. On the other hand, the potential for intraspecific competition increased with abundance of focal bumblebee species and for specialized long-tongued bumblebees, especially when floral density was low. This suggests that resource scarcity regulates species coexistence by limiting abundant species dominance via interspecific competition and intraspecific competition. Our results indicate the importance of intraspecific competition and specialization in maintaining diversity in multispecies communities.