Temperature influences plant-pollinator interaction patterns independently of community composition

Climate change can impact species interactions directly and indirectly. Indirect effects of climate change may include changes to the spatial and/or temporal overlap of interaction partners, while direct effects can occur when interaction partners remain co-located in space and time. Understanding the effects of climate-driven environmental perturbations, such as rising temperatures, is particularly important for interactions that underlie key ecosystem functions like pollination. However, very little is known about temperature variation may impact plant-pollinator interaction patterns in the absence of shifts in species presence or abundance. This research gap has arisen in part because environmental variation across multiple days or sites is often confounded with changing community composition. We examined the direct effects of temperature variation on plant-pollinator interactions within near-static ecological communities by sampling plant-pollinator interactions at multiple temperatures within individual days---a method that allowed us to disentangle the effects of temperature variation and species turnover on patterns of plant-pollinator interactions. The substantial temperature variation both within and across days in each growing season also enabled us to largely disentangle temperature variation from time-of-day effects. With this sampling protocol, we show that temperature can influence interaction patterns independently of any change in species composition. We found differences in partner choice across pollinator taxa as temperature varied during individual days and further found that bumble bees, but not sweat bees, exhibit this pattern when analyzed independently. We also confirmed that our observed trends were not driven by variation in either the composition of pollinators foraging at different temperatures nor by variation in the overall resources produced by flowers at different temperatures. We propose that thermal niche partitioning in this system could be driven by physiological and behavioral factors including pollinator energetics and competition for specific floral resources at different temperatures. The effects of temperature that we observed were highly interaction-specific, making this study just the first step in any effort to identify more general trends linking temperature and plant-pollinator interaction occurrence. Nonetheless, our insights into the thermal variation in plant-pollinator interaction patterns contributes to wider understanding of how climate change may impact ecological networks, community resilience, and ecosystem functioning.