Climate change can impact species interactions by changing the spatial and/or temporal overlap of interaction partners, but interspecific interactions can also vary when partners remain co-located in space and time. Understanding the effects of climate-driven environmental variation, such as rising temperatures, is particularly important for interactions that underlie key ecosystem functions like pollination. However, very little is known about how temperature variation may impact plant-pollinator interaction patterns in the absence of shifts in species presence or abundance, in part because environmental variation across multiple days or sites is often confounded with changing community composition. In this study, we examined the effects of temperature variation on pollinators' choice of floral partners within near-static ecological communities by sampling 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 pollinator floral choice independently of changes in species composition. We found differences in the choice of floral resources across pollinator taxa as temperature varied during individual days and further found that bumble bees, but not sweat bees, exhibited this pattern when analyzed independently. We also confirmed that our observed trends were not driven by variation in the number of interactions recorded, the relative abundance of pollinators, or in the overall attractiveness of flowers at different temperatures. We propose that thermal niche partitioning in this system could be driven by physiological and behavioral factors including energetics and competition for specific floral resources at different temperatures. These insights into thermal variation in pollinators' floral choice contribute to wider understanding of the fine-scale mechanisms through which climate change may impact ecological networks, community resilience, and ecosystem functioning.

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Food underpins fitness and ecological interactions, yet how nutrient availability shapes species interactions in natural communities remains poorly understood. Most nutritional ecology research focuses on laboratory or single-species systems, limiting insight into how nutrient use and nutrient niche dynamics occur in complex, multispecies assemblages in the wild. We combined long-term plant-pollinator interaction data with pollen macronutrient analyses to examine how wild bumble bees exploit macronutrients and whether they occupy distinct nutrient niches. Pollen macronutrient composition varied across plant species and over the season, with protein-rich pollen peaking in spring and lipid- and carbohydrate-rich pollen increasing by late summer. Across this nutrient landscape, bumble bee species occupied two distinct macronutrient niches: one high in protein and low in lipid and carbohydrate, and the other lower in protein but moderate in lipid and carbohydrate. Nutrient niche partitioning was associated with differences in feeding morphology and colony life stage (but not phenology). We found little evidence that nutrient niche breadth differed among species or was explained by feeding morphology or colony life stage. Our results extend nutritional ecology to a multispecies context, provide evidence for nutrient niche partitioning among wild pollinators and highlight the need to consider species-specific nutritional requirements in pollinator conservation.

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Many plants have evolved nutrient rewards to attract pollinators to flowers, but most research has focused on the sugar content of floral nectar resources. Concentrations of sodium in floral nectar (a micronutrient in low concentrations in nectar) can vary substantially both among and within co-occurring species. It is hypothesized that sodium concentrations in floral nectar might play an important and underappreciated role in plant-pollinator interactions, especially because many animals, including pollinators, are sodium limited in nature. Yet, the consequences of variation in sodium concentrations in floral nectar remain largely unexplored. Here, we investigate whether enriching floral nectar with sodium influences the composition, diversity, and frequency of plant-pollinator interactions. We experimentally enriched sodium concentrations in four plant species in a subalpine meadow in Colorado, USA. We found that flowers with sodium-enriched nectar received more visits from a greater diversity of pollinators throughout the season. Different pollinator species foraged more frequently on flowers enriched with sodium and showed evidence of other changes to foraging behavior, including greater dietary evenness. These findings are consistent with the "salty nectar hypothesis," providing evidence for the importance of sodium limitation in pollinators and suggesting that even small nectar constituents can shape plant-pollinator interactions.

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Megachile rotundata (F.) is an important pollinator of alfalfa in the United States. Enhancing landscapes with wildflowers is a primary strategy for conserving pollinators and may improve the sustainability of M. rotundata. Changing cold storage temperatures from a traditionally static thermal regime (STR) to a fluctuating thermal regime (FTR) improves overwintering success and extends M. rotundata's shelf life and pollination window. Whether floral resources enhance overwintering survival and/or interact with a thermal regime are unknown. We tested the combined effects of enhancing alfalfa fields with wildflowers and thermal regime on survival and macronutrient stores under extended cold storage (i.e., beyond one season). Megachile rotundata adults were released in alfalfa plots with and without wildflower strips. Completed nests were harvested in September and stored in STR. After a year, cells were randomly assigned to remain in STR for 6 months or in FTR for a year of extended cold storage; emergence rates were observed monthly. Macronutrient levels of emerged females were assessed. FTR improved M. rotundata survival but there was no measurable effect of wildflower strips on overwintering success or nutrient stores. Timing of nest establishment emerged as a key factor: offspring produced late in the season had lower winter survival and dry body mass. Sugars and glycogen stores increased under FTR but not STR. Trehalose levels were similar across treatments. Total lipid stores depleted faster under FTR. While wildflowers did not improve M. rotundata survival, our findings provide mechanistic insight into benefits and potential costs of FTR for this important pollinator.

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For many species and seed sources used in restoration activities, specific seed germination requirements are often unknown. Because seed dormancy and germination traits can be constrained by phylogenetic history, related species are often assumed to have similar traits. However, significant variation in these traits is also present within species as a result of adaptation to local climatic conditions. A growing number of studies have attempted to disentangle how phylogeny and climate influence seed dormancy and germination traits, but they have focused primarily on species-level effects, ignoring potential population-level variation. We examined the relationships between phylogeny, climate, and seed dormancy and germination traits for 24 populations of eight native, restoration-relevant forb species found in a wide range of climatic conditions in the Southwest United States. The seeds were exposed to eight temperature and stratification length regimes designed to mimic regional climatic conditions. Phylogenetic relatedness, overall climatic conditions, and temperature conditions at the site were all significantly correlated with final germination response, with significant among-population variation in germination response across incubation treatments for seven of our eight study species. Notably, germination during stratification was significantly predicted by precipitation seasonality and differed significantly among populations for seven species. While previous studies have not examined germination during stratification as a potential trait influencing overall germination response, our results suggest that this trait should be included in germination studies as well as seed sourcing decisions. Results of this study deepen our understanding of the relationships between source climate, species identity, and germination, leading to improved seed sourcing decisions for restorations.

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