Abstract: Species moving into historically cooler habitats can change the competition landscape with potential consequences for the persistence of resident species and community structure and function. While novel ecological interactions resulting from asymmetric range shifts are expected to impact populations, there has been little research into their effects on community structure and function. For instance, if range-shifting species have functional traits matching those of resident species, leading to trait homogenization, then we would expect to see reduced niche partitioning and lower modularity in the absence of competition-driven behavioral changes. Using bumble bee visitation data spanning 50 years, we tested whether documented the upward migration of subalpine bumble bees into alpine meadow communities modified resource partitioning and network structure. We tested for shifts in bumble bee niche overlap, and the modularity and robustness of bumble bee-plant networks constructed from visitation data collected in the 1960s and 2010s. To assess the impacts on pollination services, we tested for changes in pollinator service index (PSI) and pollen fidelity of individual foragers.
The modern network included 32 plant species, which is more than double the 14 plant species observed in the past network despite having fewer observed visits. Niche overlap among foraging bumble bees tended to decrease, and network modularity increased through time, which is consistent with increasing competition. Relative to the past network, the modern network was more robust to secondary extinctions of pollinators and plants, as predicted given increased modularity through time. Network PSI did not change through time, though colonizing taxa consistently had lower PSI than resident species, suggesting that the range-expanding species may be poorer pollinators of alpine plants than residents. Flower fidelity of individual bees within a foraging bout (as opposed to species-level visitation patterns captured by PSI) decreased through time, which could result in higher heterospecific pollen transfer. These results provide evidence for increased competition among alpine bumble bee species and reduced pollination services to alpine plants historically favored by bumble bees, which could compound the documented negative effects of warming on resident bee and plant populations. Still, alpine habitats serve as refugia for bees moving upslope, and several alpine plant species are now visited by colonizing bumble bees, which may increase pollination services to these plants.
