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.