Bumble bee niche overlap along an elevation gradient: how traits can inform novel competitive pressures under climate change

Climate change‐induced range shifts can disrupt interactions among species by moving them in and out of ecological communities. These disruptions can include impacts on competition for shared resources. Bumble bees ( Bombus spp.) are important pollinators shifting their range upwards in elevation in response to climate change. These shifts could lead to altered competition among species and threaten co‐existence. This could be particularly worrying at the tops of mountain ranges where bumble bees may no longer be able to move up to higher elevations to track climate change. To better understand this issue, we investigated changes in diet niche overlap among bumble bee species along a 2296 m elevation gradient in the southern Rocky Mountains. Additionally, we investigated how morphological and phenological traits impact diet composition (flower species visited) among bumble bee species and explored a simple simulation to understand how the continued upward movement of bumble bee species under climate change into the mountaintop may affect trait overlap of newly co‐occurring species. We found that diet niche overlap among bumble bee species increased with elevation. We also found that differences in morphological and phenological traits (body size, tongue length, date of activity) were correlated with differences in diet composition among bumble bee species. Finally, we described how the co‐occurrence of bumble bee species from lower elevations with mountaintop species would lead to increased trait overlap and likely more species sharing similar flowers. These shifts could lead to increased competition for high‐elevation restricted species on mountaintops and exacerbate the effects of climate change on high‐elevation bumble bees.