Divergent phenological responses to climate cues in a 20-year record of syrphid flies and flowers

Background/Question/Methods

Climate change may lead to temporal mismatches among interacting species because of interspecific variation in responses to abiotic phenological cues.  Studies have shown that both plant and pollinator phenologies respond to recent climate change, but datasets containing plants and pollinators known to interact at the same location are rare.  Syrphid flies are widely considered to be the second most important group of pollinating insects, following bees, and here we examine a long-term record of syrphid fly and flower phenology, spanning the timeframe from 1992-2011 at the Rocky Mountain Biological Laboratory.  We investigated the abiotic environmental cues associated with the timing of peak abundance of syrphid flies and of flowers of six plant species commonly visited by syrphids at our site.  We used simple linear regression to describe relationships of phenology to climate variables (air temperature, precipitation, and timing of snowmelt).  Because insects require a certain degree of heat accumulation or vernalization to break diapause, we also included the number of days on which temperatures warmed above freezing as a potential explanatory variable. Important predictors were chosen based on AIC scores.  

Results/Conclusions

Syrphid flies and the flowers they visit respond to different phenological cues (heat accumulation and precipitation vs. snowmelt, respectively). The timing of syrphid peak abundance is shifting later in the season (10.0 ± 4.6 days/decade; mean ± 1 SE throughout). Syrphids peaked 1.8 ± 0.9 days earlier in years with each additional day that warmed above freezing in the early spring. Counterintuitively, there is a significant trend toward a decreasing number of days that warm above freezing in the early spring, because of decreasing average maximum temperatures (and no change in average temperatures). Syrphids peaked earlier in years with less rainfall in June (5.4 ± 0.9 days earlier/ 1 cm decrease). Rainfall is decreasing more slowly than the trend in days warming above freezing, resulting in delayed syrphid phenology. Most plant species trended toward advanced flowering, but there were no significant changes in flowering phenology.  Interspecific variation in peak flowering relative to peak syrphid abundance resulted in converging (3 cases), diverging (1 case), and no change (2 cases) in syrphid phenology relative to flowers. Widespread phenological mismatch, then, seems unlikely. Our results highlight the complex interplay between climate change and the responses of interacting species to climate-related phenological cues.