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 2296m 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 thought experiment on 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.