Pollination Ecology and Floral Traits in Alpine Plants

Foundational work

The RMBL pollination tradition was built on a series of now-classic papers in the late 1970s and 1980s. Waser showed that Delphinium nelsonii and Ipomopsis aggregata, both visited by Broad-tailed Hummingbirds, compete for pollinator services and that their staggered flowering times reduce wasteful interspecific pollen transfer (Waser, 1978) (Waser, 1978), while also functioning as effective mutualists by sustaining a shared pollinator across the summer (Waser & Real, 1979). Price and Waser then demonstrated that seed set in Delphinium peaks at intermediate outcrossing distances, establishing the empirical foundation for optimal outcrossing (Price & Waser, 1979), a pattern later confirmed for offspring lifetime fitness in Ipomopsis (Waser & Price, 1989).

Work in the 1980s and 1990s broadened the framework. Campbell quantified how selection acts simultaneously through male and female function on floral traits in Ipomopsis (Campbell, 1989), and Campbell and Halama showed that seed production is jointly limited by pollen receipt and soil resources, rather than by one or the other (Campbell & Halama, 1993). Inouye's terminology of floral larceny (Inouye, 1980) and the later synthesis on nectar robbing (Maloof & Inouye, 2000) reframed nectar thieves as sometimes-mutualists. Finally, Waser and colleagues (Waser et al., 1996) and a global test by Ollerton and colleagues (Ollerton et al., 2009) argued that real pollination systems are far more generalized than the classical syndromes predicted, with only about 1.6% of species in their global sample matching a textbook syndrome.

Key findings

Decades of work at RMBL have produced a coherent picture of how floral traits shape reproduction. Pollen-tracking studies confirm that visitation rate is the strongest direct driver of pollen receipt, explaining about a third of its variation (Engel & Irwin, 2003), and that hummingbirds dominate pollen delivery to Ipomopsis aggregata, contributing roughly 94% of pollinator visits in a seven-year study (Price et al., 2005). Floral robbers, despite their bad reputation, have weak and context-dependent effects: heavy robbing can cut male reproductive success in half (Irwin & Brody, 2000), and robbing matters most when plants are already pollen-limited or self-incompatible (Burkle et al., 2007), but across many studies the overall effect on female seed set is weakly negative (Maloof & Inouye, 2000) (Irwin et al., 2001). Dilute nectar in Ipomopsis appears to be an evolved compromise that deters robbing bumblebees without deterring hummingbird pollinators (Irwin et al., 2004).

Floral traits in these systems are clearly under pollinator-mediated selection. Bees and flies discriminate among flower shapes and colors of Polemonium foliosissimum (Campbell et al., 2014), UV bullseye size in flowers tracks elevation and pollinator identity (Koski & Ashman, 2015), and in the Ipomopsis hybrid zone, divergent selection on corolla length and anther position is consistent with pollinator-driven ecological speciation (Campbell et al., 2023). Floral traits diverge more sharply across the hybrid zone than vegetative traits, reflecting stronger selection on flowers (Faidiga, 2016), and corolla length has actually shifted measurably over five generations in a direction predicted by phenotypic selection and heritability (Campbell et al., 2018).

Climate, water, and disturbance shape these interactions. Later snowmelt years yield more seeds (Campbell, 2019), pollinator visits to Mertensia ciliata peak at intermediate water availability (Gallagher & Campbell, 2017), and hummingbird visitation to Ipomopsis declines following low-snow winters (Waser & Price, 2016). On the disturbance side, dust from unpaved roads shortens floral lifespan (Lopez, 2017), shortens corollas (Centeno Armenta, 2018), and changes Hylemya fly oviposition (Rosas, 2017); (Rosas, 2018) — a reminder that even local human activity reshapes pollination outcomes.

Current frontier

Early work in the 1970s and 1980s established the basic mechanics of pollination, selection, and outcrossing in Gothic's wildflowers. Recent studies since 2020 have shifted toward asking whether these systems can keep up with rapid climate change. A new generation of experiments manipulates snowmelt timing and summer precipitation directly. Powers and colleagues showed that floral volatile emissions of Ipomopsis aggregata respond to snowmelt advances of 3–11 days, with α-pinene as the dominant compound, and that selection on these volatiles depends on the climate scenario (Powers et al., 2025). Wu and colleagues used warming chambers to demonstrate that warming increases nectar production in Ipomopsis by 41% and reduces seed-predator oviposition by 72%, while also subtly shifting floral scent (Wu et al., 2025). Campbell and colleagues combined long-term selection estimates with demographic models to ask whether evolution of a single leaf trait could rescue Ipomopsis populations from drought-driven decline, finding that rescue is possible in only one of two populations and requires both selection and plasticity (Campbell et al., 2025).

A second frontier is methodological. Reproducible workflows for floral scent analysis (Eisen et al., 2022), quantum-dot pollen tracking (Thoresen, 2024), and integration of camera-trap visitation data with long-term floral censuses (Pantoja Alfaro, 2025); (Granier, 2024) are expanding the questions ecologists can ask. Comparative studies are also placing RMBL findings into broader context: Opedal and colleagues showed that floral traits diverge less among populations than vegetative traits, but in proportion to their standing genetic variation (Opedal et al., 2023), and Kooyers and colleagues synthesized evidence that herbaceous model species — including those studied at RMBL — show widespread maladaptation to current climate, with limited dispersal constraining evolutionary rescue (Kooyers et al., 2025).

Open questions

Several questions will define the next decade. First, can plastic and evolutionary responses in floral traits — scent, nectar, corolla length, phenology — keep pace with the increasing variability of snowmelt and summer precipitation, or will mismatches with hummingbirds and bees deepen (Campbell et al., 2025); (Granier, 2024)? Second, how do floral microbes, road dust, and other under-studied actors modify pollination outcomes in ways that long-term studies have largely missed (Garcia, 2025); (Thoresen, 2024); (Lopez, 2017)? Third, what is the fate of hybrid zones as climate shifts the relative fitness of parental species and their hybrids, and could hybridization itself supply the genetic variation needed for adaptation (Morrison, 2025); (Campbell et al., 2022)? Answering these questions will require sustaining the multi-decade datasets that make Gothic uniquely valuable while integrating new tools in chemical ecology, genomics, and remote sensing.

REFERENCES

Campbell, D.R. (1989). Measurements of selection in a hermaphroditic plant: variation in male and female pollination success. Evolution. →

Campbell, D.R. et al. (2025). Predicting the contribution of single trait evolution to rescuing a plant population from demographic impacts of climate change. Evolution Letters. →

Campbell, D.R., Halama, K.J. (1993). Resource and pollen limitations to lifetime seed production in a natural plant population. Ecology. →

Inouye, D.W. (1980). The terminology of floral larceny. Ecology. →

Kooyers, N.J. et al. (2025). Responses to climate change – insights and limitations from herbaceous plant model species. New Phytologist. →

Maloof, J.E., Inouye, D.W. (2000). Are nectar robbers cheaters or mutualists? Ecology. →

Ollerton, J., Alarcón, R., Waser, N.M., Price, M.V., Watts, S., Cranmer, L., Hingston, A., Peter, C.I., Rotenberry, J. (2009). A global test of the pollination syndrome hypothesis. Annals of Botany. →

Opedal, Ø.H. et al. (2023). Evolvability and trait function predict phenotypic divergence of plant populations. Proceedings of the National Academy of Sciences. →

Powers, J.M. et al. (2025). Natural selection on floral volatiles and other traits can change with snowmelt timing and summer precipitation. New Phytologist. →

Price, M.V., Waser, N.M. (1979). Pollen dispersal and optimal outcrossing in Delphinium nelsoni. Nature. →

Waser, N.M. (1978). Competition for hummingbird pollination and sequential flowering in two Colorado wildflowers. Ecology. →

Waser, N.M. (1978). Interspecific pollen transfer and competition between co-occurring plant species. Oecologia. →

Waser, N.M., Chittka, L., Price, M.V., Williams, N.M., Ollerton, J. (1996). Generalization in pollination systems, and why it matters. Ecology. →

Waser, N.M., Price, M.V. (1989). Optimal outcrossing in Ipomopsis aggregata: seed set and offspring fitness. Evolution. →

Waser, N.M., Real, L.A. (1979). Effective mutualism between sequentially flowering plant species. Nature. →

Wu, Y. et al. (2025). Effects of experimental warming on floral scent, display and rewards in two subalpine herbs. Annals of Botany. →

B. occidentalis

M. L. McCutcheon