DOI: 10.5061/dryad.7h44j1018
title: Pollinator visitation on Na-enriched plants in a subalpine meadow

description: 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. 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 have gone largely
 unexplored. Here, we investigate whether enriching floral nectar with
 sodium influences the composition, diversity, and abundance of 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 pollinator visitors throughout the season. Each pollinator
 species foraged more frequently on flowers enriched with sodium and showed
 evidence of other changes to pollinator 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.
description: <strong>Experimental design.
 </strong>We selected four focal species for this experiment:
 <em>Delphinium barbeyi</em> (Ranunculaceae),
 <em>Erigeron</em> <em>speciosus</em> (Asteraceae),
 <em>Helianthella</em> <em>quinquenervis</em>
 (Asteraceae), and <em>Heliomeris</em>
 <em>multiflora</em> (Asteraceae). These plant species have
 varied floral morphologies and have an abundant and diverse community of
 floral visitors (CaraDonna and Waser 2020; Bain et al. 2022). All focal
 plant species co-occur in this subalpine meadow, are abundant, and
 co-flower (in July). For each species, we selected 5 paired plants; pairs
 were typically within 1 meter of one another, similarly sized, and had a
 similar number of flowers. We labeled <em>D. barbeyi
 </em>and<em> E. speciosus </em>individuals but
 considered adjacent groupings (2-5 individuals) of <em>H.
 quinquenervis </em>and<em> H. multiflora</em> a “single
 plant” because each individual has few or one inflorescence. For each
 pair, one plant received the sodium-enriched nectar treatment and the
 other received the control treatment. Control nectar contained 50% sucrose
 (weight : volume) and sodium-enriched (NaCl) experimental nectar contained
 50% sucrose + 0.5% NaCl (weight : volume). To make the artificial nectar
 solutions, we combined 170 g of sugar and 200 ml of water for 0.5 °Bx. For
 the sodium-enriched treatment, we added 1.55 g NaCl (~ 0.5% NaCl w:v). The
 amount of artificial nectar applied to the flowers approximates naturally
 occurring nectar volumes and sugar concentrations observed at the Rocky
 Mountain Biological Laboratory (Table S1; Hiebert and Calder 1983; Luo et
 al. 2014; Kirschke and CaraDonna, unpublished data). Observations of
 sodium concentrations in floral nectar of 14 species in the Colorado Rocky
 Mountains range from &lt; 0.001% Na to 0.12% Na (Hiebert and Calder
 1983). Therefore, the 0.5% NaCl (0.077% Na) sodium-enriched nectar was
 approximately 5× naturally occurring concentrations of sodium in
 <em>D. barbeyi </em>at the RMBL (Table S1). <strong>Sodium-enrichment and pollinator
 observations.</strong> We applied the experimental nectar treatments
 and observed interactions between flowering plants and insect pollinators
 on warm, sunny days. Nectar treatments were randomly assigned to one of
 the plants in each pair and applied by gently inserting a pipette tip into
 the nectar spur (<em>D. barbeyi</em>), into individual florets
 (<em>H. quinquenervis, H. multiflora</em>), or spread across
 the surface of the capitulum (<em>E. speciosus</em>) for all
 open flowers. We replaced the pipette tip between each experimental
 addition to avoid cross-pollination and cross-contamination of nectar
 solutions. Our experimental plants received the same nectar treatment for
 the duration of the experiment. After applying nectar treatments to a pair of
 plants, we conducted 20-minute observations of pollinators at each plant
 in random order, recording the identity of each pollinator visitor, the
 total number of flowers visited, and the duration of its foraging bout. We
 recorded an interaction when an insect contacted the reproductive parts of
 an open flower and identified visitors in the field to the finest
 taxonomic unit possible. If species-level identification was not possible
 in the field, pollinators were recorded as distinct morphospecies (e.g.,
 Bain et al. 2022). We did not destructively sample pollinators to avoid
 influencing interactions during other observation periods.
description: # Pollinator visitation on Na-enriched plants in a subalpine meadow
 [https://doi.org/10.5061/dryad.7h44j1018](https://doi.org/10.5061/dryad.7h44j1018) #### Contributors: Ethan VanValkenburg (contact): evanvalk@stanford.edu Nathan J Sanders Paul CaraDonna Here, we investigate whether enriching floral nectar with sodium influences the composition, diversity, and abundance of 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 pollinator visitors throughout the season. Each pollinator species foraged more frequently on flowers enriched with sodium and showed evidence of other changes to pollinator 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. ## Description of the data and file structure Describe relationships between data files, missing data codes, other abbreviations used. Be as descriptive as possible. This main file (Pollinator\_Observations.csv) provides all of the pollinator observations recorded during this study. The data are counts of pollinator visits to flowers that had their nectar artificially spiked with NaCl (Na Treatment) or not (C Treatment). **Variables include:** date: mm/dd/yyyy weather: factor with general description flower.sp: AQCO (Aquilegia caerulea), DEBA ( *Delphinium barbeyi)*, HEQU (*Helianthella quinquenervis*), ERSP (*Erigeron speciosus*\*\*)\*\*, HEMU (*Heliomeris multiflora)*, MECI (Mertensia ciliata) pair.num: integer 1-5 treatment: : C (control), Na (sodium-enriched) start: sample period start (hhmm) end: sample period end (hhmm) fl.count: number of open flowers on the plant pollinator.sp: Species (or finest taxonomic unit possible) for floral visitors (see methods). Missing values (i.e. observation period with no species) is "0". no.visits: number of pollinator visits on open flowers with contact to the reproductive parts handling: time (in seconds) of the foraging bout. Missing values include: "0" for no pollinator, "-" if we could not measure time.