Diversified but not redundant: Upslope migrating bumble bees restructure pollination services to alpine plants

Climate-induced range shifts lead to novel interactions between resident
and migrating species. Understanding the outcomes of such novel
interactions can allow for informed predictions of the fates of ecological
communities experiencing a warming climate. Long-term studies of
ecological communities are rare, limiting our ability to understand how
novel interactions under climate change affect community structure and
function. Using bumble bee visitation data collected before and after the
onset of accelerated anthropogenic global warming, we examined the impact
of upslope bumble bee migration on an alpine plant-bumble bee community
structure and pollination services. We predicted that an influx of
competitors would result in higher pollination services through increased
niche partitioning. Contrary to our predictions, our findings reveal a
substantial reduction in potential pollination services to historically
preferred host plants, despite an increase in resource partitioning by
alpine bumble bees. Direct measures of foraging fidelity reveal a 351%
increase in individuals carrying mixed pollen loads (from 27.4% to 96.1%
of foragers), potentially affecting seed production in historically
preferred plant species. Similarly, network size increased as bumble bees
foraged from 22 novel alpine plant species. Network modularity increased,
potentially enhancing network robustness to secondary extinctions despite
increased competition between bumble bee species. In summary, our study
underscores that climate-induced range shifts can restructure alpine
communities and ecosystem services, potentially favoring colonizing
species and their novel interaction partners. While alpine habitats may be
refugia for subalpine organisms, the indirect effects of climate change on
species interactions could compound direct effects of climate stress on
specialist resident species.