How Does Pollination Vary with Flower Color

Most flowering plants rely upon animal pollinators for fertilization. Plant species that can attract reliable animal pollinators should reproduce more frequently than plant species whose pollinators visit other species and therefore "waste" pollen. From the pollinators’ perspective, flowers provide food sources (pollen and energy-rich nectar) and other benefits such as breeding sites. Some of the best (and often the most bizarre) examples of tight mutualistic interactions between species come from plant-pollinator associations. For example, tropical orchid species have become so specialized in flower morphology, fragrance, color and timing of nectar production so as to exclude all but one or a few species of animal pollinators.

Flower traits obviously vary widely among different plant species. But such traits can also vary with a plant population. A visit to any local nursery will testify that flower color varies greatly within a species (roses, for example), and also may heritable (e.g., one can buy seeds of many different flower colors).

In this activity, we will investigate how variation in flower color within a population of globemallow shrubs influences the types and frequency of animal pollinators. We will generate hypotheses to explain our observations and think about how these ideas can be tested. We will also explore the evolutionary implications of observations and those of other plant-pollinator interactions.

• To develop a sampling procedure to estimate the number of species and the frequency and duration of each species visitation to three color varieties of globemallow.
• To discover patterns of visitation among three color varieties.
• To propose alternative hypotheses to explain any observed patterns of pollinator visitation.
• To consider the evolutionary implications of variation in flower traits.

Stopwatches clipboards

Data sheets insect nets

Collecting jars binoculars

Meter tape population of globemallow

1. Each pair of students should choose a 2-3 meter section of globemallows according to your randomly-assigned color. How should your choice of a section within a population be determined?
2. One student with stopwatch is the observer; the other is the data recorder.
3. For 1-3 hours, the observer should note which pollinator (bumble-bee, honeybee, hummingbird, wasp, etc.), and the length of time remaining in the section. If there is more than one pollinator at a time, use the second stopwatch. Assume that any individual entering a section is different form any previously observed ones.
4. Record all data on the data sheets provided.
5. Also record which part of the inflorescence each pollinator visits. Approximate whether the pollinator visits the top, middle, or bottom 1/3 of the inflorescence.
6. At the designated time, return to the lab and assemble the class data.
7. Analyze combined data with descriptive statistics (means and standard deviations) and use appropriate statistical test to determine the probability of rejecting your null hypothesis.
8. Use graphs, if necessary to depict your results, be prepared to discuss your results and your classmates.

 

9. Questions to consider:

1. In comparing your data with those of others observing the same flower color, what differences were noted? What factors might cause these differences?
2. If differences exist between pollinator visitation among flower colors, what factors might have caused these differences? What experiments/observations could you perform to test these hypotheses?
3. If differences exist among pollinator species for certain flower colors, discuss how changes in both the pollinator and flower type could occur over evolutionary time.
4. Based upon the results of the class, what predictions would you make about the frequency of flower colors in future generations of this population of globemallow? What assumptions did you make in arriving at these predictions?

Study Questions:

1. A tropical orchid species emits a aromatic compound similar to the females sex pheromone of euglossine bees and the flower shape resembles a female bee. Male bees attempt to copulate with flowers and in the process pick up pollen that may be delivered in another flower in the next (probably very frustrating) copulation attempt. Is this a stable interaction over evolutionary time? Propose an hypothesis as to why the interactions persists and propose ways to which you would test it.
2. Baker and Baker (1973) surveyed the nectar composition of 266 species of flowering plants in California. In plants primarily pollinated by butterflies and moths, nectar contained significant amounts of amino acids in addition to sugars. However, those pollinated by bees and birds, contained low amounts of amino acids. Propose hypotheses to account for these differences.
3. The explosive radiation of flowering plants and insect pollinators coincided in evolutionary time, beginning about 225 million years ago. Propose alternative hypotheses to explain this pattern, and evidence that would support the hypotheses.
4. Below are data on the proportion of fruits (A) and the number of seeds (B) produced by yellow and white flowered wild radish plants (Stanton et al. 1986). Pollinating bees and butterflies strongly prefer yellow-flowered to white flowered plants.

1. Do the data indicate that reporductive success of yellow and white flowered plants differ in this system? Explain.
2. Is pollen in short supply in this system? Explain your reasoning.

 

3. Are ovules in short supply? Explain your reasoning.
4. Would you predict that given enough generations, either of the flower types would increase relative to the other in this population? Explain.

1. Below are data on the different types of flowers visited on consecutive foraging trips by a bumblebee (bombus vagans) when (a) there are no other bees present, and when (b) other bees use the "favorite" flowers (Heinrich 1979).

1. What can you infer about pollination by the bumblebee from (A)? What implications does this pattern have for a species of flowering plant used by the bumblebee?
2. From the perspective of the plant, what features of individuals and populations of a plant visited by the bumblebee would ensure the highest rates of pollination (consider only graph (A) )?
3. Propose mechanisms to explain the pattern of visitation in graph (A).
4. Use graphs (A) and (B), what can be said of the effect of other bumblebees on pollination in this system.
5. Re-consider question b) in light of the data present in graph (B) and propose experiments/observations to test your hypotheses.