Changes in a pollinator food web in the face of climate change: effects of physiological limitations and species interactions

Understanding responses of food webs to climate change is vital, especially when those food webs influence important ecosystem services, like pollination, valued at over $3 billion globally. Historically the focus has been on single factors (e.g. temperature) and mechanisms (e.g. change in mortality). However, global climate change is predicted to alter temperature and moisture simultaneously. Additionally, thermal and hygric physiological performance and species interactions are both likely mechanisms underlying food web responses to changing climate. The current lack of a synergistic, mechanistic understanding of how food webs respond to key aspects of global climate change is a major research gap. Here we questioned how changes in temperature and moisture may alter food web composition through filtering of sensitive taxa (physiological limits) or by modifying consumption (trophic interactions). We placed bumblebees (Bombus impatiens) and tomato plants (Solanum lycopersicum) in 32 mesocosms within a greenhouse in Bowling Green, OH in July 2018. We explored differences in fruit set and tomato quality by excluding half of the flowers from buzz-pollination via bags. Additionally, all mesocosms were categorized in four abiotic treatments (cool/dry, cool/moist, hot/dry, hot/moist), and were paired based on predator presence (with or without Green Lynx spiders (Peucetia viridans)).We found that predatory spider body temperature was significantly higher when more moisture was available in the environment (SE=0.779, df=28.0, t-ratio=-3.661, p=0.005). Our findings also indicate that if predatory spiders are more hydrated, they change their behavior and expose themselves more to heat (χ2=4.028, df=1, p= 0.045). Furthermore, this behavioral change influences spider consumption of bumblebees. When more moisture was available in the environment, spiders ate significantly more bumblebees (χ2=8.924, df=1, p=0.003). However, there were no significant differences between the heat or water treatments in the mesocosms and the pollination of tomatoes (flowers covered vs. not covered) (χ2=2.163, df=3, p=0.539).These results demonstrate the importance of water balance in influencing animal behavior, species interactions, and food webs, but show that these effects can be complex, with tradeoffs between maintaining optimal hydration and optimal body temperature, altering food webs through behavioral changes. Our food web experiment was a simplistic, artificial model, so results may not be easily extrapolated to a natural system. However, this provides insight into the type of phenomenon that may occur in response to climate change, and hopefully will spur further research on this topic in more natural systems.