Comparison of historic and contemporary plant-pollinator interaction networks: Changes in phenology, loss of specialist bee species, and decreased pollinator fidelity

Background/Question/Methods

Climate change is one of the main drivers of anthropogenic global environmental change, potentially disrupting community structure and ecosystem services like pollination. Warmer temperatures are altering the phenologies of plants and insects worldwide, but we know virtually nothing about how species interactions, especially plant-pollinator interactions, are changing at the community level due to phenological mismatches. By recollecting plant and pollinator phenologies and their interactions, first sampled at the same location over 120 years ago, we tested the degree to which plant and pollinator phenologies have changed over time and the degree to which phenological changes have disrupted their interactions at the community level. Focusing on the bee pollinators of Claytonia virginica, an essential floral resource during the spring season, we also investigated how bee species richness, visitation rate, and fidelity have changed as well as the extent to which alterations in land-use over the last 40 years have contributed to the loss of bee species.

Results/Conclusions

We found that the strong, climate-driven phenological changes of flowering plants and their bee visitors alone were not the cause of massive changes in plant-bee interaction networks. Instead, changes in the network over time reflect the local extirpation of half of all historic bee pollinator species, mostly specialists. For Claytonia virginica, both visiting bee species richness as well as visitation rate have declined over time. Loss of forest (and corresponding increases in development) over the last 40 years was significantly related to loss of bee species richness visiting C. virginica. Additionally, pollen sampled from the bodies of bee visitors to C. virginica illustrated that the fidelity and possible effectiveness of bee species as pollinators have declined since the 1880’s. Current plant-pollinator interaction networks are being held together by a core group of generalist interactors and redundancy of species that overlap in flowering and activity times. Though loss of pollinator species thus far has not resulted in community collapse due to the nested nature of plant-pollinator interactions and surprising levels of flexibility in pollinator foraging and diet breadth, nestedness is measurably decreasing, and further loss of species has foreseeable, disastrous consequences.