Waiting on a mule deer crossing…
Examines how floral traits, nectar chemistry, and pollinator interactions — especially between hummingbirds and Ipomopsis aggregata — shape plant reproductive success in subalpine Colorado, with growing attention to how climate change and nutrient availability alter these relationships.
Flowering plants in the mountains around Gothic, Colorado depend on animals — hummingbirds, bumblebees, solitary bees, flies, and even nectar-stealing insects — to move pollen from flower to flower. The study of these interactions, and of the floral traits (shape, color, scent, nectar) that mediate them, is one of the oldest and richest research traditions at the Rocky Mountain Biological Laboratory (RMBL). Because subalpine wildflower meadows are tightly tuned to snowmelt timing, summer monsoons, and the seasonal schedules of their pollinators, they are an unusually sensitive system in which to ask how plant reproduction works — and how it is changing.
A handful of concepts recur throughout this literature. Reproductive success in plants is usually measured as fruit set and seed set — the number of fruits and viable seeds a plant produces. When plants make fewer seeds than they could because they did not receive enough pollen, ecologists call this pollen limitation. Pollinator-mediated selection refers to the way pollinators, by visiting some flowers more than others, drive the evolution of floral morphology (corolla length and shape), nectar traits (volume and sugar concentration), and floral scent (the blend of volatile organic compounds released by flowers). Pollination effectiveness varies enormously among visitors: a hummingbird probing a Scarlet Gilia flower delivers pollen very differently than a bumblebee chewing a hole in its base to steal nectar — a behavior called nectar robbing, one form of floral larceny.
Two more ideas tie the rest of the primer together. Optimal outcrossing distance is the observation that the best mates for a plant are usually neither its closest neighbors (risking inbreeding) nor very distant individuals (risking the breakup of locally adapted gene combinations). And in places where two related species meet, such as the Scarlet Gilia (Ipomopsis aggregata) and its close relative I. tenuituba, they form a hybrid zone whose floral traits, pollinator visits, and ecological tolerances offer a natural laboratory for studying ecological speciation — the origin of new species through divergent natural selection.
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) , while also functioning as effective mutualists by sustaining a shared pollinator across the summer . Price and Waser then demonstrated that seed set in Delphinium peaks at intermediate outcrossing distances, establishing the empirical foundation for optimal outcrossing , a pattern later confirmed for offspring lifetime fitness in Ipomopsis .
effects of behavioral or life history decisions on individual reproductive success and survival
Plant fitness measured through various components including fruit set, seed production, and seeds per plant
Nectar volume, nectar sugar concentration, and total nectar sugar content measured from individual flowers
The ability of an organism to successfully transfer pollen between flowers for plant reproduction
Natural selection on floral traits that occurs through differential pollinator visitation and resulting differences in reproductive success
The structural characteristics of flowers, categorized as complex or open based on accessibility to pollinators
Foraging behavior where floral visitors bypass the floral opening and access nectar by chewing holes in nectar spurs or using pre-existing holes
Comprehensive measurement of floral traits (corolla dimensions, anther insertion, petal color, nectar production) and vegetative traits (specific leaf...
Collection and microscopic analysis of pollen grains deposited on wildflower stigmas using pink fuchsin gel mounting and reference collection for iden...
A method to experimentally add nectar rewards to naturally pollen-only rewarding flowers by depositing measured volumes of artificial nectar solution ...
Experimental addition of outcross pollen to flowers to test whether natural pollination limits plant reproductive success, comparing reproductive outp...
Comprehensive evaluation of seed production including total fruit counts, viable seed counts per fruit, individual seed mass measurements, and visual/...
Multi-site common garden experiment using controlled crosses to partition genetic and environmental variance in vegetative and floral traits across a ...
1. Plants interact simultaneously with both mutualists and antagonists. While webs of plant-animal interactions in natural systems can be highly compl...
In western North America, hummingbirds can be observed systematically visiting flowers that lack the typical reddish color, tubular morphology, and di...
Climate change can impact plant fitness and population persistence directly through changing abiotic conditions and indirectly through its effects on ...
Individual differences in fecundity often serve as proxies for differences in overall fitness, especially when it is difficult to track the fate of an...
Climate change is likely to alter both flowering phenology and water availability for plants. Either of these changes alone can affect pollinator vi...
Vegetative traits of plants can respond directly to changes in the environment, such as those occurring under climate change. That phenotypic plastici...
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.
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.
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).
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.
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. →
The rate at which plants convert carbon dioxide and water into glucose using light energy, measured as gas exchange
The hypothesis that predispersal seed predators preferentially attack high-quality flowers, potentially masking negative effects of environmental stre...
Changes in ecological patterns and processes across different time scales
Interbreeding between taxa that alters the pool of genetic variation available for population responses to changing environments
The measure of seed quality based on embryonic development and potential for successful germination
Chemical compounds emitted by flowers that contribute to floral scent and can influence pollinator attraction and plant-herbivore interactions
Chemical compounds emitted by plants as signals to other organisms, interrelated with atmospheric chemistry and climate change
Reproduction in which pollen from a flower fertilizes ovules in the same flower or on the same plant
Statistical approach allowing estimation of directional relationships among variables and comparison of direct versus indirect effects
A geographic region where genetically distinct populations meet and produce offspring of mixed ancestry
Mathematical models that represent how different pollinators perceive color based on their photoreceptor sensitivities and opponent color processing
The ability of plants to tolerate or compensate for herbivore damage through increased growth or reproduction
Percentage of leaf biomass composed of water, calculated as difference between wet and dry weight divided by wet weight
Fine particulate matter generated by vehicular traffic on unpaved roads that settles on plant surfaces
Study of ecological effects of roads and traffic on natural systems
The organization of genetic variation within and among populations, reflecting patterns of gene flow, drift, and selection
The impacts of dust from unpaved roads on plant physiology and reproduction
Computational approach to predict how visual signals are perceived by animal visual systems
Reduced fitness in F2 hybrids compared to F1 hybrids, often attributed to genetic incompatibilities between nuclear genes
The extent of possible variation in a particular phenotypic trait where different dimensions represent the extent of phenotypic variation in two trait...
Distance range that maximizes pollen fertilization success and offspring survival
Differential responses of organisms to different stereoisomeric forms of the same chemical compound
F-statistics (FIS, FST, FIT) measuring genetic differentiation and inbreeding at different hierarchical levels
Constrained ordination method used to analyze multivariate patterns among predicted variables with environmental predictors
Crosses between two species where each species serves as both maternal and paternal parent, allowing study of cytoplasmic inheritance effects
Population growth rate (λ) integrating all vital rate components equivalent to average fitness component across a lineage or population
The location of extrafloral nectaries relative to plant reproductive structures acts to distract flower-damaging ants or other visitors from flowers, ...
Analytical technique combining gas chromatography separation with mass spectrometry identification for analyzing volatile compounds
Systematic tendency for historical plant collectors to preferentially collect rare or uncommon species while avoiding common, dominant species
Lower than expected frequency of heterozygous genotypes in a population
Research practices that allow independent researchers to reproduce results using the same methods and data
Selection pressures that oppose each other within or between levels of biological organization
Behavioral patterns acquired through experience rather than innate programming
Experimental manipulation of summer precipitation levels (doubled, halved, ambient) to assess effects on plant physiological and fitness traits in a c...
Paired experimental design comparing floral development and longevity between plants receiving daily road dust applications versus undusted controls, ...
Time-lapse photography of individual flowers using motion-activated cameras to capture and quantify hummingbird feeding visits. Combined with machine ...
Controlled experiment using artificial flowers with discrete bullseye phenotypes exposed to UV-present vs UV-absent treatments to test UV-mediated sel...
Quantitative measurement of dust particle deposition rates using filter paper disks placed at varying distances from unpaved roads, with microscopic p...
Standard enzyme electrophoresis technique used to determine genotypes at the 6PGD-2 locus for parentage assignment. Distinguishes between homozygous m...
Path analysis using structural equation modeling to test competing hypotheses about causal relationships between nectar traits, robbing, pollination, ...
Experimental clipping treatment designed to reduce plant photosynthetic capacity by approximately 50% through removal of stems. Used to simulate herbi...
Standard protocol for measuring leaf traits by scanning fresh leaves, counting trichomes using ImageJ, then drying and weighing leaves to calculate sp...
Small-scale removal of invasive plant flowers to test effects on native plant pollination through paired treatment and control plots within experiment...
Plants were subjected to progressive water stress by withholding irrigation to create a gradient of soil moisture conditions. This experimental design...
Controlled application of synthetic pinene enantiomers via cotton-filled microcentrifuge tube emitters to test effects of specific volatile compounds ...
Systematic collection of seeds from plants along established transects for genetic analysis. Seeds collected within defined distances of sampling poin...
Production of F2 hybrid generations with alternate cytoplasmic backgrounds through controlled hand-pollination crosses. Allows testing of cytoplasmic ...
Statistical approach to test causal pathways between herbivory, plant characteristics, and reproductive output using multiple linked regression models...
PCR amplification of microsatellite loci followed by capillary electrophoresis for allele sizing and polymorphism assessment.
Long-term monitoring of plant morphological traits across permanent transects spanning a hybrid zone between two Ipomopsis species along an elevationa...
Development of deterministic and stochastic population models incorporating empirical estimates of selection, heritability, and phenotypic plasticity ...
Seed-burial and greenhouse germination experiments to estimate seed bank-related vital rates including seed germination and stasis.
Non-destructive estimation of leaf chlorophyll concentration using reflectance spectroscopy and vegetation indices.
Standardized protocol to transfer dust from plant surfaces to microscope slides and count particles in defined areas under magnification. Used to cali...
Direct measurement of leaf stomatal conductance using porometer to assess plant water stress and physiological responses to watering treatments. Measu...
Weekly photography from fixed viewpoints to map snowmelt progression and delineate zones with different snow-free periods. Contours drawn on detailed ...
Replicated split-plot design with snowmelt manipulated at plot level and precipitation manipulated at subplot level.
In ecological speciation, incipient species diverge due to natural selection that is ecologically based. In flowering plants, different pollinators co...
Organismal traits often influence fitness via interactions with multiple species. That selection is not necessarily predictable from pairwise interact...
Premise of the Research. Seed production by flowering plants depends on abiotic and biotic factors who...
Reproductive isolation due to pollinator behavior is considered a key mode of speciation in flowering plants. Although floral scent is thought to medi...
Premise of the Research. Seed production by flowering plants depends on abiotic and biotic factors whose interacting effects may be hidden. We previ...
Altered precipitation patterns associated with anthropogenic climate change are expected to have many effects on plants and insect pollinators, but it...
Many hummingbird-pollinated plant species evolved from bee-pollinated ancestors independently in many different habitats in North and South America. T...
We examined invasive, casual (found occasionally outside cultivation) and non-invasive (found only in cultivation) species to investigate the role of ...
How climate change influences the dynamics of plant populations is not well understood, as few plant studies have measured responses of vital rates to...
Gene flow from transgenic crops allows novel traits to spread to sexually compatible weeds. Traits such as resistance to insects may enhance the fitne...
This dataset contains data and scripts that supplement the publication Lichtenberg et al. (2020) Competition for nectar resources does not affect bee ...