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

Abstract Climate change-induced range shifts can disrupt interactions between 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 between species and threaten co-existence. These impacts are particularly worrying at the tops of mountains where bumble bees are “trapped” and can no longer move up to higher elevations to track climate change. To better understand this issue, we investigated changes in bumble bee diet niche overlap between species along a 2296m elevation gradient in the southern Rocky Mountains. Additionally, we investigated how morphological and phenological traits impact diet composition (flower species visited) between bumble bee species and explored 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 between 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 between bumble bee species. Finally, we described how bumble bee species moving farther up mountaintops would lead to increased trait overlap for high-elevation species. These shifts could have implications for future competition for resident high-elevation species on mountaintops and exacerbate the effects of climate change on high-elevation bumble bees.