RMBL Pollination Research

Sketch by Danny DeSantiago
Pollination is the movement of pollen (which contains the plant sperm) from the male sex parts of a flower to the female sex parts (which contain the ovules or eggs) of the same or a different flower. Pollen can be moved by wind or even water, but in about 90% of flowering plant species it is transported by flower-visiting animals – pollinators. Animal pollination is ubiquitous in terrestrial environments; the seeds and fruits it produces are essential to our diets and to the health of natural ecosystems.

Pollination biology is the scientific study of this critical ecological interaction between flowering plants and pollinators. It was invigorated in the 1960s and 1970s by interest in coevolutionmutualism, community ecology, and foraging theory. During this period, pollination biology began to move rapidly beyond its roots as a mostly descriptive science. Place-based research, illustrated by studies of Ipomopsis aggregata, Delphinium nelsonii, Ipomopsis tenuituba, native bumblebees, hummingbirds, hawkmoths, and other species at the Rocky Mountain Biological Laboratory, has made major contributions to this progress of pollination biology research.


Current pollination research at the RMBL is in large part the legacy of a small group of graduate students who arrived in the 1970s to conduct their doctoral research on flower-visiting animals and communities of flowering plants. As they characterized the floral food resources available to hummingbirds and pollinating insects, and as they observed patterns of bloom and visitation in the wildflower-rich meadows around RMBL, these people laid the foundation for continuing discoveries about ecological and evolutionary interactions between plants and pollinators, plants and herbivores, and among plants. What follows are seven “mini-stories” that illustrate pollination research at RMBL.

1. Pollinators as Careful Shoppers

08 - Bombus appositus
The Mountain Bumblebee (Bombus appositus). Photo by Jimmy Lee

The meadow and forest habitats near RMBL contain many species of plants, and subsets of those plants produce flowers at the same time. Thus pollinators at the RMBL (and this is generally true of pollinators) have many choices of flowers to visit – it’s as if they are shopping in a “floral supermarket”. What determines which flowers pollinators choose? Scientists at RMBL have shown that many pollinators such as bumble bees and hummingbirds are “careful shoppers” – they quickly learn what nectar or other rewards different plant species offer, and whether they can reach these rewards (often concealed inside the flowers), and they tend to choose the flowers that will yield the best rewards per unit of time or energy cost.

Can you explain why a behavior of “careful shopping” might evolve in pollinating animals?

Read more about the “shopping habits” of pollinators:

  • Gori, D. F. 1989. Floral color change in Lupinus argenteus (Fabaceae): Why should plants advertise the location of unrewarding flowers to pollinators? Evolution 43:870-881.
  • Pleasants, J. M., and N. M. Waser. 1985. Bumblebee foraging at a “hummingbird” flower: Reward economics and floral choice. American Midland Naturalist 114:283-291. pdf
  • Hodges, C. M. 1981. Optimal foraging in bumblebees: Hunting by expectation. Animal Behaviour 29:1166-1171.

2. Pollination Interactions as a Web of Nature

Photo from RMBL Archives
Pollination biology has been revolutionized by the realization that many pollinators behave as careful shoppers who can detect flowers with diverse colors, shapes, and scents and assess their economic value. (Before, it was thought that plant-pollinator interactions were specialized, with each plant having “its” pollinator, and each pollinator “its” preferred plant.) If pollination interactions are usually less specialized, it becomes interesting to ask, “What do the connections between plants and pollinators look like for an entire community at the RMBL?” Imagine yourself an eagle gliding far above a meadow, with eyesight keen enough to detect which animals visit which flowers. What do you see? Scientists at the RMBL helped to introduce this “eagle-eye” perspective to pollination biology. What it reveals is a richly-connected web of interactions in which plants typically are pollinated by several types of pollinators (for example, bees as well as hummingbirds), and pollinators typically visit flowers with multiple colors and shapes. Pollination webs at the RMBL show some surprising features that turn out to be shared by pollination webs around the world – these features hold clues as to how the webs are “assembled” by ecological and evolutionary processes, and how human impacts might “disassemble” them.

Draw a “web” of interactions between a few plants and pollinators, as you imagine it. Is this the only possible way in which plants and pollinators might interact – or can you draw some alternatives?

Read more about pollination interactions:

  • L. A. Burkle and R. Alarcon. 2011. The future of plant-pollinator diversity: Understanding interaction networks across time, space, and global change. American Journal of Botany 98:528-538. pdf

3. Climate Change, Flowering Phenology and Pollination

The glacier gily (Erythronium grandiflorum). Photo from RMBL Archives
Climate change is altering the phenology, the geographic distribution, and the migration patterns of plants and animals on a worldwide scale. In the high Rockies, snow melt date initiates the plant growing season. The snow melt date is arriving earlier at RMBL, and this in turn is causing earlier flowering in species such as Erythronium grandiflorum (glacier lily). Such changes in phenology of wildflowers may change resource availability for pollinators and herbivores.

Could changes in flowering phenology caused by climate change decouple plant-pollinator relationships?

Read more about climate change and phenology:

  • Aldridge, G, Inouye D. W, Forrest J.R., Barr, W.A. and Miller-Rushing A.J. 2011. Emergence of a mid-season period of low floral resources in a montane meadow ecosystem associated with climate change. Journal of Ecology 99:905-913.
  • Thomson, J.D. 2010. Flowering phenology, fruiting success and progressive deterioration of pollination in an early-flowering geophyte. Phil Trans. R. Soc. B 365, 3187-3199. pdf
  • Memmott, J., P. G. Craze, N. M. Waser, and M. V. Price. 2007. Global warming and the disruption of plant-pollinator interactions. Ecology Letters 10:710-717. pdf
  • Price, M. V. and N. M. Waser. 1998. Effects of experimental warming on plant reproductive phenology in a subalpine meadow. Ecology 79:1261-1271. pdf

4. Floral Larceny

Illustration by RMBL researcher Marieke Perchik
Animal pollination is an important factor in the evolutionary success of flowering plants, and it’s important for the animals too. Animals visit flowers to get resources (for example, nectar), and plants produce costly rewards to attract pollinating visitors. So, the interaction is mutually beneficial. But that does not mean it is a cooperation! Plants produce less reward than the animals would like, and animals “cheat” when it is profitable to do so. Nothing shows this better than flower visitors that take nectar without pollinating. Scarlet gilia, for example, is often visited by a native bumblebee, Bombus occidentalis, that has a tongue too short to reach nectar at the base of the long floral tube from the front of the flower. Undeterred, this bee crawls to the base of the flower, chews a hole, and licks up the sugary nectar through this hole. These “nectar robbers” do not pollinate the flower because they do not contact the sexual parts of the flower. Furthermore, they harm the plants because robbed flowers receive fewer hummingbird visits and less pollen, and as a consequence produce fewer seeds.

Can you explain floral larceny in terms of the ideas outlined above about pollinators as careful shoppers?

Read more about floral larceny:

  • Irwin, R. E., A. K. Brody, and N. M. Waser. 2001. The impact of floral larceny on plant individuals, populations, and communities. Oecologia 129:161-168. pdf
  • Irwin R.E. and Brody A.K. 1998. Nectar robbing in Ipomopsis aggregata: effects on pollinator behavior and plant fitness. Oecologica 116: 519-527.
  • Inouye D.W. 1980a. The terminology of floral larceny. Ecology 61: 1251-1253.

5. The Paradox of Being Eaten

Photo from RMBL Archives
Many animals exist by eating plants, an interaction known as herbivory. For example, scarlet gilia is commonly browsed by ungulates such as mule deer (Odocoileus hemionus). Herbivory should be bad for plants, right? Not necessarily, suggest some scientists – they point to cases (scarlet gilia in Arizona is one) where browsed plants appear to produce more flowers and more seeds than unbrowsed plants! RMBL scientists have tried to replicate these surprising results, but have been unable to discover any benefit to Ipomopsis plants from being eaten.

Is it “good” or “bad” for plants to be eaten? Can you imagine any circumstance in which it could be “good”? (Hint: see Vail, cited and discussed in Pulliam and Waser).

Read more about the paradox of being eaten:

  • Pulliam, H. R., and N. M. Waser. 2010. Ecological invariance and the search for generality in ecology. In The Ecology of Place: Contributions of Place-Based Research to Ecological Understanding, eds. I. Billick and M. V. Price, University of Chicago Press, pp. 69-92. pdf
  • Sharaf K. and Price M.V. 2004. Does pollination limit tolerance to browsing in Ipomopsis aggregata? Oecologia 138: 396-404. pdf
  • Bergelson, J., Crawley M.J. 1992b. Herbivory and Ipomopsis aggregata: the disadvantages of being eaten. The American Naturalist 139: 870-882.

6. Interactions Between Plants Visited by the Same Pollinators

Photo from RMBL Archives

Because pollinating animals are careful shoppers, they may go to flowers with the best rewards, leaving other flowers without pollination. This is one way in which plants may compete for pollination. This possibility was recognized a century ago, but RMBL scientists provided the first solid evidence. They found experimentally that scarlet gilia and dwarf larkspur (Delphinium nelsonii) compete for hummingbird pollination – each species produces fewer seeds when it blooms in the company of the other species. Likewise, RMBL scientists were the first to demonstrate that plants can facilitate each others pollination in some cases – each species producing more seeds when blooming with others.

Facilitation can occur if pollinators are attracted to dense flower patches – why might they be attracted this way?

Read more about how plants compete for the attention of pollinators:

  • Caruso, C. M., and M. Alfaro. 2000. Interspecific pollen transfer as a mechanism of competition: Effect of Castilleja linariaefolia pollen on seed set of Ipomopsis aggregata. Canadian Journal of Botany 78:600-606.
  • Mitchell, R. J., R. J. Flanagan, B. J. Brown, N. M. Waser, and J. D. Karron. 2009. New frontiers in competition for pollination. Annals of Botany 103:1403-1413. pdf
  • Thompson, J.D. 1981. Spatial and temporal components of resource assessment by flower-feeding insects. The Journal of Animal Ecology 50: 49-59. pdf
  • Waser, N.M. 1978. Competition for hummingbird pollination and sequential flowering in two Colorado wildflowers. Ecology 59: 934-944.

7. Hybridization and Speciation

Photo from RMBL Archives
We often are told that biological species are defined by their complete reproductive isolation from other species. But the biological world is less tidy, and more interesting, than that! Species of flowering plants, for example, often can reproduce with other, related species, producing hybrid offspring. Studying this process gives insights into the evolution of new species. Around the RMBL, hybrid offspring between two species of Ipomopsis are produced when hawkmoths and hummingbirds visit and transfer pollen between the two species. The pollinators also visit the hybrid offspring, which are fertile and produce viable offspring themselves. When RMBL scientists examined survival of the hybrids relative to that of parental individuals, however, they found that each parent survived best at the elevation where it grows naturally. RMBL scientists continue to explore this system, joining other workers around the world who are taking advantage of plant hybridization to study fundamental evolutionary questions.

How might research on a system such as Ipomopsis shed light on concerns about “escape” of engineered genes from genetically modified crop plants?

Read more about hybrids and speciation:

  • Campbell, D. R., N. M. Waser, G. Aldridge, and C. A. Wu. 2008. Lifetime fitness in two generations of Ipomopsis hybrids. Evolution 62: 2616-2627. pdf
  • Aldridge, G., Campbell, D.R. 2007. Variation in pollinator preference between two Ipomopsis contact sites that differ in hybridization rate. Evolution 61: 99-110.
  • Campbell, D.R. 2004. Natural selection in Ipomopsis hybrid zones: implications for ecological speciation. New Phytologist 161: 83-90.

Next step – learn about the Ipomopsis Research Group at RMBL.

Additional References

Billick, I and M.V. Price, Eds. 2010. The Ecology of Place: Contributions of Place-based Research to Ecological Understanding. Chicago: University of Chicago Press.