Plant-pollinator interaction network traits are affected by phenology

Background/Question/Methods

In a changing climate, it is important to consider how the structure and resilience of mutualistic plant-pollinator networks is related to variables that are likely to change in response, such as the timing of life history events.  The timing of emergence for a pollinator, or flowering in a plant, could greatly affect the structure of the networks in which they are embedded. Using plant-pollinator networks from 33 sites in southern British Columbia, Canada, we asked a) how duration of activity in a network, and timing of first appearance in the network, were related to network structure, and b) how network robustness is related to phenologically-biased species loss.

Pollinators were collected directly off of flowers and identified to the lowest taxonomic level possible.  For each plant or pollinator species within each network, we determined how duration and first appearance were related to several species-level network metrics:  standardized specialization (d’) interaction strength asymmetry (ia), and two-species level counterparts of modularity: within-module degree and connectivity (z and c, respectively).   We additionally evaluated the robustness of whole networks to loss of species, if loss was related to either duration in a network or relative timing (early or late) of appearance in the network.

Results/Conclusions

Later flowering plants were more connected within modules, as were later-emerging pollinators and those with longer duration in the network.  Later-emerging and high-duration pollinators affected the plant species they interacted with more than the plant species affected them (higher interaction asymmetry), and pollinators with high-duration had more interactions among network modules. 

Loss of plants with longer blooming periods significantly reduced robustness relative to loss of plants with short blooming times. Network robustness was significantly greater if late-appearing pollinators were lost than if early-appearing pollinators were lost, and was also significantly greater when pollinators with short flight periods were lost compared to when long-duration species were lost.

Climate change is largely expected to result in losses or phenological mismatches for organisms with early activity within seasons.  Our results indicate that losses of early-flying pollinators will reduce the robustness of networks, as will loss of pollinators with long activity in the network.  In our region, some early-appearing pollinators are generalized social species with long flight durations that are important within- and among-module connectors.  Further consideration of such phenological shifts is needed to understand how mutualistic networks will respond to climate change.